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2060 Chiron

2060 Chiron

2060 Chiron (IPA: ) is an object in the outer solar system with an orbit between those of Saturn and Uranus and a radius of 71±5 km [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004A%26A...413.1163G&db_key=AST&high=3ec9955c7527328]. Although it was initially classified as an asteroid, later dispute arose as to whether it was an asteroid or actually a comet. It was discovered in 1977 by Charles T. Kowal and named after Chiron of Greek legend. Chiron should not be confused with the moon of Pluto named Charon, discovered in 1978. In 1988 it was found that Chiron was undergoing an outburst in brightness (by about one magnitude), which is behaviour typical of comets but not asteroids. Further observations in 1989 showed that Chiron had developed a cometary coma. At the time of its discovery, Chiron was close to aphelion, whereas the observations showing a coma were done closer to perihelion, perhaps explaining why no cometary behavior had been seen earlier. Chiron is officially both a comet and an asteroid, more proof of the very fuzzy dividing line. As a comet, its name is 95P/Chiron. There are two other asteroids that are also listed as comets: 4015 Wilson-Harrington and 7968 Elst-Pizarro. Chiron is now classified as a centaur, the first of a class of objects orbiting between the outer planets. Centaurs are not in stable orbits and will eventually be removed by the giant planets. It has been calculated that in 1664 BC Chiron approached Saturn to within approx. 16 million kilometres; only 3 million km. further away than Saturn's largest outer moon Phoebe, and within the orbital radii of many of Saturn's newly discovered minor satellites. Chiron is probably a refugee from the Kuiper belt. Other centaurs such as 5145 Pholus are being observed for possible cometary behavior, but none has been seen so far. Its discovery was publicized enough in the popular press that a school in astrology emerged assigning it a great importance.

References

- PMID 10688775
- Patrick Moore Guinness book of Astronomy ISBN 0851123759 Chiron Chiron ja:キロン (小惑星)

International Phonetic Alphabet

: "IPA" redirects here. For other uses, see IPA (disambiguation). The NATO phonetic alphabet has also informally been called the International Phonetic Alphabet. The International Phonetic Alphabet (IPA) is a system of phonetic notation devised by linguists to accurately and uniquely represent each of the wide variety of sounds (phones or phonemes) used in spoken human language. It is intended as a notational standard for the phonemic and phonetic representation of all spoken languages. For a treatment of the English language using the IPA, see International Phonetic Alphabet for English, for a brief chart, see IPA chart for English.
History

Description
The general principle of the IPA is to provide a separate symbol for each speech segment, avoiding letter combinations (digraphs) such as sh and th in English orthography, and avoiding ambiguity such as that of c in English.
The principle of formation
The IPA is what MacMahon (1996) has termed a "selective" phonetic alphabet. It aims to provide a separate symbol for every contrastive (that is, phonemic) sound occurring in human language. For instance, a flap and a tap are two different articulations, but since no language has (yet) been found to make a phonemic distinction between them, the IPA does not provide them with dedicated symbols. Instead, it provides a single symbol, , that covers both. For non-contrastive (that is, phonetic or subphonemic) details of these sounds, the IPA relies on diacritics, which are optional. Thus there is a certain level of flexibility in representing a language with the IPA.
The principles behind the used symbols
The letters chosen for the IPA are generally drawn from the Latin and Greek alphabets, or are modifications of Latin or Greek letters. There are also a few letters derived from Latin punctuation, such as the glottal stop (originally an apostrophe, but later given the form of a "gelded" question mark to have the visual impact of the other consonants), and one, , although Latin in form, was inspired by Arabic ﻉ. In contrast, the old Latin-derived symbols for the clicks have been abandoned in favor of the iconic Khoisanist symbols, such as . The sound-values of the consonants from the Latin alphabet correspond to usage in French and Italian, which are close to those of most other European languages as well: , , , (hard) , , , , , (unvoiced) , , , . English values are used for , , and , The vowels from the Latin alphabet (, , , , ) correspond to the vowels of Spanish and are similar to Italian. is like the vowel in piece, like rule, etc. The other symbols from the Latin alphabet (, , , , , and ) correspond to sounds these letters represent in other languages. has the Germanic value, English y in yoke. has the Scandinavian and Old English value (Finnish y, German y or ü, French u, Dutch u). Letters that share a particular modification sometimes correspond to a similar type of sound. For example, all the retroflex consonants have the same symbol as the equivalent alveolar consonant, with the addition of a rightward pointing hook at the bottom. Although there is some correspondence between modified letters, generally the IPA does not have a systematic "featural" relationship between graphic shape and articulation. For instance, there is not a consistent relationship between lowercase letters and their small capital counterparts, nor are all labial consonants linked through a common character design. Diacritic marks can be combined with IPA letters to transcribe modified phonetic values or secondary articulations. There are also special symbols for suprasegmental features such as stress and tone.
Types of transcriptions
The International Phonetic Association recommends that a phonetic transcription should be enclosed in square brackets ("[ ]"). A transcription that specifically denotes only phonological contrasts may be enclosed in slashes ("/ /") instead. If one is in doubt, it is best to use brackets, for by setting off a transcription with slashes one makes a theoretical claim that every symbol within is phonemically contrastive for the language being transcribed. Phonetic transcriptions try to objectively capture the actual pronunciation of a word, whereas phonemic transcriptions are model dependent. For example, Noam Chomsky transcribed the English word night phonemically as /nixt/. In his model, the phoneme /x/ is often silent, but shows its presence by “lengthening” the preceding vowel. The preceding vowel in this case is the phoneme /i/, which is pronounced [aj] when long. So phonemic /nixt/ is equivalent to phonetic [najt], but only if you share Chomsky's belief that historical sounds such as the gh in night may remain in a word long after they have ceased to be pronounced. For phonetic transcriptions, there is flexibility in how closely sounds may be transcribed. A transcription that gives only a basic idea of the sounds of a language in the broadest terms is called a "broad transcription"; in some cases this may be equivalent to a phonemic transcription (only without any theoretical claims). A close transcription, indicating precise details of the sounds, is called a "narrow transcription". These are not binary choices, but the ends of a continuum, with many possibilities in between. All are enclosed in brackets. For example, in some dialects the English word pretzel in a narrow transcription would be , which notes several phonetic features that may not be evident even to a native speaker. An example of a broader transcription is , which only indicates some of the easier to hear features. A yet broader transcription would be . Here every symbol represents an unambiguous speech sound, but without making any claims as to their status in the language. There are also several possibilities in how to transcribe this word phonemically, but here the differences are not of precision, but of analysis. For example, pretzel could be or . The special symbol for English r is not used, for it is not meaningful to distinguish it from a rolled r. The differences in the letter e reflect claims as to what the essential difference is between the vowels of pretzel and pray; there are half a dozen ideas in the literature as to what this may be. The second transcription claims that there are two vowels in the word, even if they can't both be heard, while the first claims there is only one. Occasionally a transcription will be enclosed in pipes ("| |"). This goes beyond phonology into morphological analysis. For example, the words pets and beds could be transcribed phonetically as and (in a fairly narrow transcription), and phonemically as and . Because /s/ and /z/ are separate phonemes in English (unlike Spanish, for example), they receive separate symbols in the phonemic analysis. However, you probably recognize that underneath this, they represent the same plural ending. This can be indicated with the pipe notation. If you believe the plural ending is essentially an s, as English spelling would suggest, the words can be transcribed and . If, as most linguists would probably suggest, it is essentially a z, these would be and . To avoid confusion with IPA symbols, it may be desirable to specify when native orthography is being used, so that, for example, the English word jet is not read as "yet". This is done with angle brackets or chevrons: . It is also common to italicize such words, but the chevrons indicate specifically that they are in the original language's orthography, and not in English transliteration.
Consonants (pulmonic)

Single articulation
Closeup of the main pulmonic consonant section of the IPA chart The pulmonic consonant table, which includes most consonants, is arranged in rows that designate manner of articulation and columns that designate place of articulation. The main chart only includes consonants with a single place of articulation. Notes:
- Asterisks (
- ) mark reported sounds that do not (yet) have official IPA symbols. See the articles for ad hoc symbols found in the literature.
- Daggers (†) mark IPA symbols that do not yet have official Unicode support. Since May 2005, this is the case of the labiodental flap, symbolized by a right-hook v: labiodental flap ([http://std.dkuug.dk/jtc1/sc2/wg2/docs/N2945.pdf Proposal to add this symbol to Unicode])
- In rows where some symbols appear in pairs (the obstruents), the symbol to the right represents a voiced consonant (except for breathy-voiced ). However, cannot be voiced. In the other rows (the sonorants), the single symbol represents a voiced consonant.
- Although there is a single symbol for the coronal places of articulation for all consonants but fricatives, when dealing with a particular language, the symbols are treated as specifically alveolar, post-alveolar, etc., as appropriate for that language.
- Shaded areas indicate articulations judged to be impossible.
- The symbols represent either voiced fricatives or approximants.
- It is primarily the shape of the tongue rather than its position that distinguishes the fricatives , , and .
- The labiodental nasal is not known to exist as a phoneme in any language.
Coarticulation
Closeup of the co-articulated consonant section of the IPA chart
Notes:
- is described as a "simultaneous and ". However, this analysis is disputed. See the article for discussion.
- To be complete, this chart should also include the semi-palatalized postalveolar (palato-alveolar) fricatives and .
- The miscellaneous portion of the chart, as published by the IPA, includes additional symbols that would have been included in the main consonant chart were it not for difficulties in typesetting on a printed page. In this article, which does not suffer from such problems, they have been included in the main chart above.
Consonants (non-pulmonic)
Closeup of the non-pulmonic consonant section of the IPA chart Notes:
- All clicks are doubly articulated and require two symbols: a velar or uvular stop, plus a symbol for the release: , etc. When the dorsal articulation is omitted, a may usually be assumed.
- Symbols for the voiceless implosives are no longer supported by the IPA. Instead, the voiced equivalent is used with a voiceless diacritic: , etc.
- Although not confirmed from any language, and therefore not "explicitly recognized" by the IPA, a retroflex implosive, , is supported in the Unicode Phonetic Extensions Supplement, added in version 4.1 of the Unicode Standard, or can be created as a composite .
- The ejective symbol is often seen for glottalized but pulmonic sonorants, such as , but these are more properly transcribed as creaky ().
Vowels
Closeup of the vowel chart of the IPA Notes:
- Where symbols appear in pairs, the one to the right represents a rounded vowel, as does (at least prototypically). All others are unrounded.
- is not confirmed as a distinct phoneme in any language.
- is officially a front vowel, but there is little distinction between front and central open vowels, and is frequently used for an open central vowel.
Affricates and double articulation
Affricates and doubly articulated stops are represented by two symbols joined by a tie bar, either above or below the symbols. The six commonest affricates are optionally represented by ligatures, though this is no longer official IPA usage, due to the great number of ligatures that would be required to represent all affricates this way. A third affricate transcription sometimes seen uses the superscript notation for a consonant release, for example for , paralleling ~ . The symbols for the palatal plosives, are often used as a convenience for or similar affricates, even in official IPA publications, so they must be interpreted with care. Image of the six common affricate ligatures and their official IPA equivalents Note:
- If your browser uses Arial Unicode MS to display IPA characters, the following incorrectly formed sequences may look better due to a bug in that font: .
Extended IPA
The Extended IPA was designed for disordered speech. However, some of the symbols (especially diacritics, below) are occasionally used for transcribing normal speech as well. View a pdf file [http://www2.arts.gla.ac.uk/IPA/ExtIPAChart97.pdf here]. The last symbol may be used with the alveolar click for , a combined alveolar and sublaminal click or "cluck-click".
Suprasegmentals
Closeup of the suprasegmental section of the IPA chart
Intonation

Tone
IPA allows for the use of either tone diacritics or tone letters to indicate tones. Note:
- With regard to tone diacritics, Unicode encodes marks for some contour tones, but not all. In Unicode version 4.1, only hacek (rising) and circumflex (falling) diacritics were encoded. Subsequent versions may also include six additional diacritics for contour tones, such as the macron-acute and the grave-acute-grave ligatures. (See an image here.) Note that contour tone diacritics are not encoded as sequences of level tone diacritics in Unicode.
- With regard to tone letters, Unicode does not have separate encodings for contour tones. Instead, sequences of level tone letters are used, with proper display dependent on the font, usually by means of OpenType font rendition: or . (These are probably not displaying correctly in your browser. See the image for a sample of how they should appear.) Since few fonts support combination tone letters (see the external links for one that is free), a common solution is to use the old system of superscript numerals from '1' to '5', for example [e⁵³, e³¹²]. However, this depends on local linguistic tradition, with '5' generally being high and '1' being low for Asian languages, but '1' being high and '5' low for African languages. An old IPA convention sometimes still seen is to use sub-diacritics for low contour tones: for low-falling and low-rising.
- The upstep and downstep modifiers are superscript arrows. Unicode version 4.1 does not encode these, though subsequent versions will. The arrows for upstep and downstep should not be confused with the full-height arrows, which are used to indicate airflow direction.
Diacritics
Closeup of the diacritic section of the IPA chart
Sub-diacritics may be placed above a symbol with a descender, i.e. . The dotless i, <ı>, is used when the dot would interfere with the diacritic. Other IPA symbols may appear as diacritics to represent phonetic detail: (fricative release), (breathy voice), (glottal onset), (epenthetic schwa), o (diphthongization). Notes: #Some linguists restrict this breathy-voice diacritic to sonorants, and transcribe obstruents as . #With aspirated voiced consonants, the aspiration is also voiced. Many linguists prefer one of the diacritics dedicated to breathy voice. The state of the glottis can be finely transcribed with diacritics. A series of alveolar plosives ranging from an open to a closed glottis phonation are:
Extended IPA diacritics
The letters and diacritics of the ExtIPA The ExtIPA has widened the use of some of the regular IPA diacritics, such as for pre-aspiration, or for a linguolabial sibilant, as well as adding some new ones. Some of the ExtIPA diacritics can be used for non-disordered speech as well, for example for the unusual airstream mechanisms of Damin. One modification is the use of subscript parentheses around the phonation diacritics to indicate partial phonation; a single parenthesis at the left or right of the voicing indicates that it is partially phonated at the beginning or end of the segment. For example, is a partially voiced [s], shows partial initial voicing, and partial final voicing; also is a partially devoiced [z], shows partial initial devoicing, and partial final devoicing. These conventions may be convenient for representing various voice onset times. Phonation diacritics may also be prefixed or suffixed rather than placed directly under the segment to represent relative timing. For instance, is a pre-voiced [z], a post-voiced [z], and is an [a] with a creaky offglide. Other ExtIPA diacritics are, In addition to these symbols, a subscript < or > indicates that an articulation is laterally offset to the left or right, and a double exclamation mark indicates 'ventricular' phonation, though it is not clear how this differs from 'harsh' phonation.
Prosodic notation
The ExtIPA also makes use of musical notation for the tempo and dynamics of connected speech. These are subscripted on the insides of a notation that indicates that they are comments on the prosody. Pauses are indicated with periods or numbers inside parentheses.
Obsolete and nonstandard symbols

How to transcribe sounds that don't have symbols in the IPA charts
The remaining blank cells on the IPA chart can be filled without too much difficulty if the need arises. Some ad hoc symbols have appeared in the literature, for example for the lateral flaps and voiceless lateral fricatives, the epiglottal trill, and the labiodental plosives. Diacritics can supply much of the remainder, which would indeed be appropriate if the sounds were allophones. For example, the Spanish bilabial approximant is commonly written as a lowered fricative, . Similarly, voiced lateral fricatives would be written as raised lateral approximants, . A few languages such as Banda have a bilabial flap as the preferred allophone of what is elsewhere a labiodental flap. It has been suggested that this be written with the labiodental flap symbol and the advanced diacritic, . Similarly, a labiodental trill would be written (bilabial trill and the dental sign). Palatal and uvular taps, if they exist, and the epiglottal tap could be written as extra-short plosives, . A retroflex trill can be written as a retracted , just as retroflex fricatives sometimes are. The remaining consonants, the uvular laterals and the palatal trill, while not strictly impossible, are very difficult to pronounce and are unlikely to occur even as allophones in the world's languages. The vowels are similary manageable by using diacritics for raising, lowering, fronting, backing, centering, and mid-centering. For example, the unrounded equivalent of can be transcribed as mid-centered , and the rounded equivalent of [æ] as raised . True mid vowels are lowered , while centered are near-close and open central vowels, respectively. The vowels that aren't representable in this scheme are the compressed vowels, which would require a dedicated diacritic.
Names of the symbols
It is often desirable to distinguish an IPA symbol from the sound it is intended to represent, since there is not a one-to-one correspondance between symbol and sound in broad transcription. The symbol's names and phonetic descriptions are described in the Handbook of the International Phonetic Association. The symbols also have nonce names in the Unicode standard. In some cases, the Unicode names and the IPA names do not agree. For example, IPA calls "epsilon", but Unicode calls it "small letter open E".
The letters
The traditional names of the Latin and Greek letters are used for unmodified symbols. In Unicode, some of the symbols of Greek origin have Latin forms for use in IPA; the others use the symbols from the Greek section. Examples: Note #The Latin "upsilon" is frequently called "horseshoe u" in order to distinguish it from the Greek upsilon. Historically, it derives from a Latin small capital U. The IPA standard includes some small capital letters, such as , although it is common to refer to these symbols as simply "capital" or "cap" letters, because the IPA standard does not include any full-size capital letters. A few letters have the forms of cursive or script letters. Examples: Note #The "looptail G" 10 px is not strictly an IPA character, but is an acceptable alternative. #In form and origin, but not in name, this is the Greek upsilon. Ligatures are called precisely that, although some have alternate names. Examples: Many letters are turned, or rotated 180 degrees. Examples: The symbol can be described as a turned cee, but it is almost always referred to as open o, which described both its articulation and its shape. The symbol is often also called "caret" or "wedge" for it similarity to that diacritic. A few letters are reversed (flipped on a vertical axis): reversed E, reversed epsilon, reversed glottal stop [often called by its Arabic name, ayin]. One letter is inverted (flipped on a horizontal axis): inverted R. ( could also be called an inverted double-u, but turned double-u is more common.) When a horizontal stroke is added, it is called a bar: barred H, barred o, reversed barred glottal stop or barred ayin, barred dotless J or barred gelded J [apparently never 'turned F'], double-barred pipe, etc. One letter instead has a slash through it: slashed O. The implosives have hook tops: hook-top B, as does hook-top H. Such an extension at the bottom of a letter is called a tail. It may be specified as left or right depending on which direction it turns: right-tail N, right-tail turned R, left-tail N [note that has its own traditional name, engma], left-tail em, tail Z [or just retroflex Z], etc. When the tail loops over itself, it's called curly: curly-tail jay, curly-tail C. There are also a few unique modifications: belted L, closed reversed epsilon [there was once also a closed omega], right-leg turned M, turned long-leg R [there was once also a long-leg R], double pipe, and the obsolete stretched C. Several non-English letters have traditional names: C cedilla, eth (also spelled edh), engma, schwa, exclamation mark, pipe. Other symbols are unique to the IPA, and have developed their own quirky names: fish-hook R, ram's horns, bull's eye, esh [apparently never 'stretched ess'], ezh [sometimes also yogh], hook-top heng. The is usually called by the sound it represents, glottal stop. This is not normally a problem, because this symbol is seldom used to represent anything else. However, to specify the symbol itself, it is sometimes called a gelded question mark.
The diacritic marks
Diacritics with traditional names: : acute, macron, grave, circumflex, caron, wedge, or háček, diaeresis or umlaut, breve, (superscript) tilde, plus variants such as subscript tilde, superimposed tilde, etc. Non-traditional diacritics: : seagull, hook, over-cross, corner, bridge, inverted bridge, square, under-ring, over-ring, left half-ring, right half-ring, plus, under-bar, arch, subscript wedge, up tack, down tack, left tack, right tack, tie bar, under-dot, under-stroke. Diacritics are alternately named after their function: The bridge is also called the dental sign, the under-stroke the syllabicity sign, etc.
Comparison to other phonetic notation
The IPA is not the only phonetic transcription system in use. The other common Latin-based system is the Americanist phonetic notation, devised for representing American languages, but used by some US linguists as an alternate to the IPA. There are also sets of symbols specific to Slavic, Indic, Finno-Ugric, and Caucasian linguistics, as well as other regional specialies. The differences between these alphabets and IPA are relatively small, although often the special characters of the IPA are abandoned in favour of diacritics or digraphs. Other alphabets, such as Hangul, may have their own phonetic extensions. There also exist featural phonetic transcription systems, such as Alexander Bell's Visible Speech and its derivatives. There is an extended version of the IPA for disordered speech (extIPA), which has been included in this article, and another set of symbols used for voice quality (VoQS). There are also many personal or idiosyncratic extensions, such as Luciano Canepari's ^canIPA. Since the IPA uses symbols that are outside the ASCII character set, several systems have been developed that map the IPA symbols to ASCII characters. Two notable systems are Kirshenbaum and SAMPA (or X-SAMPA). These systems are often used in electronic media, although their usage has been declining with the development of computer technology, specifically because of spreading support for Unicode. See also: Unicode and HTML
See also

- International Phonetic Alphabet for English explains those IPA symbols used to represent the phonemes of English.
- IPA chart for English: simplifed version.
- TIPA provides IPA support for LaTeX.
- SAMPA, X-SAMPA and Kirshenbaum are other methods of mapping IPA designations into ASCII.
- List of phonetics topics
- Uralic Phonetic Alphabet (UPA)
External links

- [http://www2.arts.gla.ac.uk/IPA/ipa.html Official home page of the IPA]
Free IPA font downloads

- [http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&item_id=DoulosSILfont Doulos SIL], a Times IPA font that supports tone letters, the new labiodental flap, and many non-standard phonetic symbols, but only in roman typeface.
- [http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&item_id=Gentium Gentium], a highly legible international (Latin, Greek, Cyrillic) font in roman and italic typefaces that includes the IPA, but not yet tone letters or the new labiodental flap.
- [http://www.travelphrases.info/gallery/Test_IPA.html Test page] for installed fonts. Includes alternate variants and tone letters.
Keyboards

- [http://www.linguiste.org/phonetics/ipa/chart/keyboard/ Online keyboard]
- [http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&item_id=ipa-sil_keyboard IPA-SIL keyboard layout for Mac OS X] for Unicode IPA input
- [http://wikisophia.org/wiki/Wikitex#Tipa WikiTeX] supports editing IPA sequences directly in Wiki articles.
Sound files

- [http://hctv.humnet.ucla.edu/departments/linguistics/VowelsandConsonants/index.html Peter Ladefoged's Course in Phonetics (with sound files)]
- [http://hctv.humnet.ucla.edu/departments/linguistics/VowelsandConsonants/course/chapter1/chapter1.html Pronounceable IPA chart]
- [http://hctv.humnet.ucla.edu/departments/linguistics/VowelsandConsonants/vowels/contents.html An introduction to the sounds of languages]
- [http://web.uvic.ca/ling/resources/ipa/ipa-lab.htm IPA Lab] Chart with sound files at University of Victoria. (Works with QuickTime.)
- [http://www.paulmeier.com/ipa/charts.html Flash version of IPA charts, with sound samples]
- [http://www.ling.hf.ntnu.no/ipa/full/ Another set of IPA sound samples]
Charts

- [http://www2.arts.gla.ac.uk/IPA/fullchart.html IPA chart source]
- [http://www.linguiste.org/phonetics/ipa/chart/ IPA Chart] in Unicode and XHTML/CSS ----
- [http://web.uvic.ca/ling/resources/ipa/charts/IPANumberChart96.pdf IPA number chart], at University of Victoria.
Unicode
Official Unicode PDF files:
- [http://www.unicode.org/charts/PDF/U0250.pdf Unicode chart for main IPA letters]
- [http://www.unicode.org/charts/PDF/U02B0.pdf Unicode chart for IPA modifier letters]
- [http://www.unicode.org/charts/PDF/U0300.pdf Unicode chart including IPA diacritics] ----
- [http://www.phon.ucl.ac.uk/home/wells/ipa-unicode.htm International Phonetic Alphabet in Unicode]
- [http://tlt.its.psu.edu/suggestions/international/bylanguage/ipachart.html Unicode-HTML codes for IPA symbols:] Tables of symbol names and HTML codes at PennState.
Personal extensions of the IPA

- [http://venus.unive.it/canipa/ ^canIPA] : Luciano Canepari's system (500 base symbols)
References

- Albright, Robert W. (1958). The International Phonetic Alphabet: Its background and development. International journal of American linguistics (Vol. 24, No. 1, Part 3); Indiana University research center in anthropology, folklore, and linguistics, publ. 7. Baltimore. (Doctoral dissertation, Standford University, 1953).
- Ball, Martin J.; Esling, John H.; & Dickson, B. Craig. (1995). The VoQS system for the transcription of voice quality. Journal of the International Phonetic Alphabet, 25 (2), 71-80.
- Canepari, Luciano. (2005a). "A Handbook of Phonetics: ‹Natural› Phonetics." München: Lincom Europa, pp. 518. [https://ssl.kundenserver.de/s83009615.einsundeinsshop.de/sess/utn1541a7584d7471b/shopdata/0002_New+titles/product_details.shopscript ISBN 3-8958-480-3] (hb).
- Canepari, Luciano. (2005b) "A Handbook of Pronunciation: English, Italian, French, German, Spanish, Portuguese, Russian, Arabic, Hindi, Chinese, Japanese, Esperanto." München: Lincom Europa, pp. 436. [https://ssl.kundenserver.de/s83009615.einsundeinsshop.de/sess/utn1541a7584d7471b/shopdata/0002_New+titles/product_details.shopscript ISBN 3-89586-481-1] (hb).
- Duckworth, M.; Allen, G.; Hardca

Solar system

The solar system comprises our Sun and the retinue of celestial objects gravitationally bound to it. Traditionally, this is said to consist of the Sun, nine planets and their 158 currently known moons; however, a large number of other objects, including asteroids, meteoroids, planetoids, comets, and interplanetary dust, orbit the Sun as well. Although the term "solar system" is frequently applied to other star systems and the planetary systems which may comprise them, it should strictly refer to our system specifically: the word "solar" is derived from the Sun's Latin name, Sol (and the term sometimes appears as Solar System). When talking about another stellar system (or planetary system), including the star(s) and bodies associated with them through gravity, it is usual to shorten it to "the system" (e.g. "the Alpha Centauri system" or "the 51 Pegasi system").

Structure and layout of the solar system

The Sun (astronomical symbol ☉) is a main sequence G2 star that contains 99.86% of the system's known mass. Its two largest orbiting bodies, Jupiter and Saturn, account for 91% of the remainder (The Oort Cloud might hold a substantial percentage, but as yet its existence is unconfirmed). In broad terms, the charted regions of our solar system consist of the Sun and its planetary system: the eight bodies in relatively unique orbits (commonly called planets or major planets) and two belts of smaller objects (which can be called minor planets, planetoids, meteoroids, planetesimals or, in the case of Pluto, planets). Objects in orbit round the Sun all lie within the same shallow plane, called the ecliptic, and all orbit in the same direction. Many are in turn orbited by moons, and the largest are encircled by planetary rings of dust and other particles. The major planets are, in order, Mercury (☿), Venus (♀), Earth (♁), Mars (♂), Jupiter (♃), Saturn (♄), Uranus (♅/10px), Neptune (♆), and Pluto (♇), though Pluto's status has been thrown into question by the discovery of (see below). Eight of the nine planets are named after or derived from gods and goddesses from Greco-Roman mythology; Earth, a Germanic word, is known in many Romance languages as Terra, the Roman goddess of the Earth. Distances within the solar system are measured most often in astronomical units, or AU. 1 AU is the distance between the Earth and the Sun, or 149 598 000 kilometers. Pluto is roughly 38 AU from the Sun, while Jupiter lies at roughly 5.2 AU. For very large distances within the solar system, such as regions beyond Pluto or the orbital circumferences of planets, the terameter (Tm, one milliard kilometers) is sometimes used. Despite the fact that many diagrams (like the image at the top of this article), for practicality's sake, represent the solar system as having each orbit the same distance apart, in actuality the orbits are largely arranged geometrically, that is, each is roughly double the distance from the Sun as the one before it. Venus’s distance from the Sun is roughly double that of Mercury, Earth’s distance is roughly double that of Venus, Mars’s double that of Earth and so on. This relationship is roughly expressed in the Titius-Bode law, a mathematical formula for predicting the semi-major axes of planets in AU. In its simplest form, it is written :

a= 0.4 + 0.3\times k

where k=0,1,2,4,8,16,32,64,128. By this formulation, we would expect Mercury's orbit (k=0) to be 0.4 AU, and Mars's orbit (k=4) to be at 1.6 AU. In fact their orbits are 0.38 and 1.52 AU.Ceres, the largest asteroid, lies at k=8. This law is only a rough guide, and doesn't fit all of the planets (Neptune is far closer than predicted, though Pluto lies at Neptune's predicted orbit). As of now, there is no scientific explanation for why this law "works," and many claim it is merely a coincidence. Pluto

Origin and evolution of the solar system

The current hypothesis of solar system formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant. It states the solar system was formed from a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2-3 times the mass of the Sun. Over time, the nebula began to collapse, possiby due to disturbance by a nearby supernova. This explosion sent shock waves into space, which squeezed the nebula, pushing more and more matter inward until gravitational forces overcame its internal gas pressure and it also began to collapse. As the nebula collapsed, it decreased in size, which in turn caused it to spin faster to conserve angular momentum. And as the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning pancake shape with a bulge at the center. When the nebula further condensed, a protostar was formed in the middle. This system was heated by the friction of the rocks colliding into each other. Lighter elements such as hydrogen and helium evaporated out of the centre and migrated to the edges of the disc, thus concentrating the heavier elements to form dust and rocks in the centre. These heavier elements clumped together to form planetesimals and protoplanets. In the outer regions of this solar nebula, ice and volatile gases were able to survive, and as a result, the inner planets are rocky and the outer planets were massive enough to capture large amounts of lighter gases, such as hydrogen and helium. After 100 million years, the pressures and densities of hydrogen in the centre of the collapsed nebula became great enough for the protosun to sustain thermonuclear fusion reactions. As a result of this, hydrogen was converted to helium, and a great amount of heat was released. 4×¹H → ⁴He + neutrinos + photons During that time, the protostar turned into the Sun and the protoplanets and planetesimals were transformed into planets. All of the planets formed in a relatively short time of a few million years.

Regions of the solar system

protostar's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind) [http://quake.stanford.edu/~wso/gifs/HCS.html]. (click to enlarge) ]] According to their location, the objects in the solar system are divided into three zones: Zone I or the inner solar system, including terrestrial planets and the Main belt of asteroids; Zone II, including the giant planets, their satellites and the centaurs, and Zone III, or the outer solar system, comprising the area of the Trans-Neptunian objects including the Kuiper Belt, the Oort cloud, and the vast region in between.

Interplanetary medium

The environment in which the solar system resides is called the interplanetary medium. The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, which forms a very tenuous "atmosphere" (the heliosphere), permeating the interplanetary medium in all directions for at least ten billion (10) miles (16 Tm or 16 km) into space. Small quantities of dust are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. Some of the dust is likely interstellar dust from outside the solar system. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the Solar System, the heliospheric current sheet.

The inner planets

The four inner or terrestrial planets are characterised by their dense, rocky makeup. They formed in the hotter regions close to the Sun, where lighter and more volatile materials evaporated, leaving only those with high melting points, such as silicates, which form the planets' solid crusts and semi-liquid mantles, and iron, which forms their cores. All have impact craters and many possess tectonic surface features, such as rift valleys and volcanoes. The four inner planets are: volcanoes
- Mercury (0.39 AU from the Sun): The closest planet to the Sun is also the smallest and most atypical of the inner planets, having no atmosphere and, to date, no observed geological activity save that produced by impacts. Its relatively large iron core suggests that it was once a much larger world whose outer mantle was sheared off in early formation by the Sun’s gravity.
- Venus (0.72 AU): The first truly terrestrial planet, Venus, like the Earth, possesses a thick silicate mantle around an iron core, as well as a substantial atmosphere and evidence of one-time internal geological activity, such as volcanoes. It is much drier than Earth, and its atmosphere is 90 times as dense as Earth’s, however, and composed overwhelmingly of carbon dioxide with traces of sulfuric acid.
- Earth/Moon (1 AU): The largest of the inner planets, Earth is also the only one to demonstrate unequivocal evidence of ongoing geological activity. Its liquid hydrosphere, unique among the terrestrials, is probably the reason why Earth is also the only planet where multi-plate tectonics has been observed, since water acts as a lubricant for subduction. Its atmosphere is radically different from the other terrestrials, having been altered by the presence of life to contain 21 percent free oxygen. Its satellite, the Moon, is sometimes considered a terrestrial planet in a co-orbit with its partner, since its orbit around the Sun never actually loops back on itself when observed from above. The Moon possesses many of the features in common with other terrestrial planets, though it lacks an iron core.
- Mars (1.5 AU): Smaller than the Earth or Venus, Mars possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically active and [http://www.universetoday.com/am/publish/mars_volcanoes_active.html recent evidence] suggests it may have continued to be so until very recently. Mars possesses two tiny moons thought to be captured asteroids.

The asteroid belt

Asteroids are objects smaller than planets that mostly occupy the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun, and are composed in significant part of non-volatile minerals. The main belt contains tens of thousands (possibly millions) over 1 km across, though they can be as small as dust. Despite their large numbers, the total mass of the main asteroid belt is unlikely to be more than a thousandth that of the Earth. Asteroids with a diameter of less than 50 m are called meteoroids. The largest asteroid, Ceres, has a diameter of roughly 1000 km; large enough to be spherical, which would make it a planet by some definitions of the word. The asteroids are thought to be the remnants of a small terrestrial planet that failed to coalesce due to the gravitational interference of Jupiter. They are subdivided into asteroid groups and families based on their specific orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. Trojan asteroids are located in either of Jupiter's L₄ or L₅ points, though the term is also sometimes used for asteroids in any other planetary Lagrange point as well. The inner solar system is dusted with rogue asteroids, many of which cross the orbits of the inner planets.

The outer planets

The four outer planets, or gas giants, (sometimes called Jovian planets) are so large they collectively make up 99 percent of the mass known to orbit the Sun. Their large sizes and distance from the Sun meant they could hold on to much of the hydrogen and helium too light for the smaller and hotter terrestrial planets to retain.
- Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely hydrogen and helium is not very different from that of the Sun. Three of its 63 satellites, Ganymede, Io and Europa, share elements in common with the terrestrial planets, such as volcanism and internal heating. Jupiter has a faint, smoky ring.
- Saturn (9.5 AU), famous for its extensive ring system, shares many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 49 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan is the only satellite in the solar system with a substantial atmosphere.
- Uranus (19.6 AU) and Neptune (30 AU), while having many characteristics in common with the other gas giants, are nonetheless more similar to each other than they are to Jupiter or Saturn. They are both substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of “ices”, such as water, ammonia and methane. For this reason some astronomers suggested that they belong in their own category, “Uranian planets,” or “ice giants.” Both planets possess dark, insubstantial ring systems. Neptune’s largest moon Triton is geologically active. Centaurs are icy comet-like bodies that have less-eccentric orbits so that they remain in the region between Jupiter and Neptune. The first centaur to be discovered, 2060 Chiron, has been called a comet since it has been shown to develop a tail, or coma, just as comets do when they approach the sun.

The trans-Neptunian region

The area beyond Neptune, often referred to as the outer solar system or simply the "trans-Neptunian region", is still largely unexplored.

The Kuiper belt

This region's first formation, which actually begins inside the orbit of Neptune, is the Kuiper belt, a great ring of debris, similar to the asteroid belt but composed mainly of ice and far greater in extent, which lies between 30 to 50 AU from the Sun. This region is thought to be the place of origin for short-period comets, such as Halley's comet. Though there are estimated to be over 70,000 Kuiper belt objects with a diameter greater than 100 km, the total mass of the Kuiper belt is relatively low, perhaps equalling or just exceeding the mass of the Earth. Many Kuiper belt objects have orbits that take them outside the plane of the ecliptic.
- Pluto, the solar system's smallest planet, is considered to be part of the Kuiper Belt population. Like others in the belt, it has a relatively eccentric orbit inclined 17 degrees to the ecliptic and ranging from 29.7 AU from the Sun at perihelion to 49.5 AU at aphelion. It has a large moon (the largest in the solar system relative to its own size), called Charon, and, new observations suggest, two other, much smaller moons. Like the Earth/Moon, Pluto and Charon are often considered a double planet. A member of the traditional nine planets, Pluto's tiny mass (less than 1% of Earth's) and diameter have called this status into question. Kuiper belt objects with Pluto-like orbits are called Plutinos. Other Kuiper belt objects have resonant orbits and are grouped accordingly. The remaining Kuiper belt objects, in more "classical" orbits, are classified as Cubewanos. The Kuiper Belt has a very sharply defined edge. At around 49 AU, a sharp dropoff occurs in the number of objects observed. This dropoff is known as the "Kuiper Cliff", and as yet its cause is unknown. Some speculate that something must exist beyond the belt large enough to sweep up the remaining debris, perhaps as large as Earth or Mars. This view is still controversial, however.

The scattered disc

Overlapping the Kuiper belt but extending much further outwards is the scattered disc. Scattered disc objects are believed to have been originally native to the Kuiper belt, but were ejected into erratic orbits in the outer fringes. One particular scattered disc object, originally found in 2003 but confirmed two years later by Mike Brown, has renewed the old debate about what constitutes a planet since, though its size is not yet known, it is almost certainly larger than Pluto. It currently has no name, but has been given the provisional designation , and has been nicknamed "Xena" by its discoverers, after the television character. It has many similarities with Pluto: its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and is steeply inclined to the ecliptic plane, indeed, at 44 degrees, more so than any known object in the solar system. Like Pluto, it is believed to consist largely of rock and ice, and has a [http://www.gps.caltech.edu/%7Embrown/planetlila/moon/index.html moon]. Whether it and the largest Kuiper belt objects should be considered planets or whether instead Pluto should be reclassified as a minor planet has not yet been resolved.

A new region?

Sedna, the newly discovered Pluto-like object with a gigantic, highly elliptical 10,500-year orbit that takes it from about 76 to 928 AU, has too distant a perihelion to be a scattered member of the Kuiper Belt and could be the first in an entirely new population. is also believed to be a member of this population.

Comets

Comets are composed largely of volatile ices and have highly eccentric orbits, generally having a perihelion within the orbit of the inner planets and an aphelion far beyond Pluto. Short-period comets exist with apoapses closer than this, however, and old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids. Long period comets have orbits lasting thousands of years. Some comets with hyperbolic orbits may originate outside the solar system.

And beyond

The point at which the solar system ends and interstellar space begins is not precisely defined, since its outer boundaries are delineated by two separate forces: the solar wind and the Sun's gravity. gravity The heliosphere expands outward in a great bubble to about 95 AU, or three times the orbit of Pluto. The edge of this bubble is known as the termination shock; the point at which the solar wind collides with the opposing winds of the interstellar medium. Here the wind slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath that looks and behaves very much like a comet's tail; extending outward for a further 40 AU at its stellar-windward side, but tailing many times that distance in the opposite direction. The outer boundary of the sheath, the heliopause, is the point at which the solar wind finally terminates, and one enters the environment of interstellar space. Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way. But even at this point, we could not be said to have left the solar system, for the Sun's gravity will still hold sway even up to the Oort cloud, the great mass of icy objects, currently hypothetical, believed to be the source for all long-period comets and to surround our solar system like a shell from 50,000 to 100,000 AU beyond the Sun, or almost halfway to the next star system. The vast majority of the solar system, therefore, is completely unknown.

Age of the solar system

Scientists estimate that the solar system is 4.6 billion years old. To calculate this figure, they examine an unstable element, which is subject to radioactive decay. By observing how much this element has decayed, they can calculate how old this element is. The oldest rocks on earth are approximately 3.9 billion years old, however it is hard to find these rocks as the earth has been thoroughly resurfaced. To estimate the age of the solar system, scientists must find rocks from space, such as meteorites – which are formed during the early condensation of the solar nebula. The oldest meteorite was found to have an age of 4.6 billion years, hence the solar system must be around 4.6 billion years old.

Galactic orbit of the solar system

The solar system is part of the Milky Way galaxy, a spiral galaxy with a diameter of about 100,000 light years containing approximately 200 billion stars, of which our Sun is rather large and bright. (The vast majority of stars are red dwarfs; our Sun is placed near the middle of the Hertzsprung-Russell diagram, but stars larger and hotter than it are rare, whereas stars dimmer and cooler than it are very common, although we can observe only those few other red dwarfs that are very near our Sun in space). Estimates place the solar system at between 25,000 and 28,000 light years from the galactic center in the Orion Arm. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years. At the galactic location of the solar system, the escape velocity with regard to the gravity of the Milky Way is about 1000 km/s. The solar system appears to have a very unusual orbit. It is both extremely close to being circular, and at nearly the exact distance at which the orbital speed matches the speed of the compression waves that form the spiral arms. The solar system appears to have remained between spiral arms for most of the existence of life on Earth. The radiation from supernovae in spiral arms could theoretically sterilize planetary surfaces, preventing the formation of large animal life on land. By remaining out of the spiral arms, Earth may be unusually free to form large animal life on its surface.

Planetary system formation

For many years, our solar system had the only planetary system known, and so theories of planetary formation only had to explain one system to be plausible. The discovery in recent years of many extrasolar planets has uncovered systems very different to our own, and theories have had to be revised accordingly. Exoplanets have not been seen by astronomers yet, however we know they exist because of the gravitational tug the planets induce on the star, and hence making the star ‘wobble’. Astronomers can calculate how massive the planets are by observing how much the star wobbles. Exoplanets can also be observed more directly by their occultation of the stars' discs, which dims them slightly. In October, 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of a massive planet orbiting 51 Pegasi – a Sun-like star in the constellation Pegasus. This planet is about half as massive as Jupiter, and had an orbital period of 4.2 Earth days, due to its closeness to the star (0.05 AU). Since then, over 160 more planets have been identified. Many extrasolar planetary systems contain such a “hot Jupiter”: a planet comparable to or larger than Jupiter orbiting very close to the parent star, perhaps orbiting it in a matter of days. It has been hypothesised that while the giant planets in these systems formed in the same place as the gas giants in our system did, some sort of migration took place which resulted in the giant planet spiralling in towards the parent star. Any terrestrial planets which had previously existed would presumably either be destroyed or ejected from the system. There has also been some photographic evidence to suggest that regions in the Orion Nebula, which is 1500 light years from Earth, have star systems forming.

Discovery of the solar system

The planets out to Saturn were known to ancient astronomers, who observed the wandering of these objects against the apparently fixed pattern of stars. Venus and Mercury were each identified as single objects despite the difficulty of connecting "evening" and "morning stars". It was also identified that the two non-pointlike objects, the sun and the Moon, moved across the same fixed background. However knowledge of the nature of these celestial drifters was entirely speculative and largely incorrect. The nature and structure of the solar system were long misperceived, for at least two reasons:
- The Earth was considered stationary, and the motion of objects in the sky was therefore taken at face value: the sun was thought to orbit the Earth, for example (This conception of the universe, in which the Earth is at the center, is called the Geocentric model; geos means "Earth" in Greek).
- Many solar system objects and phenomena cannot be perceived at all without technical aid. Over the last several hundred years, conceptual and technological advances have helped us understand the solar system much better. The first and most fundamental of the conceptual advances was the Copernican Revolution, which proposed that the planets orbit the sun—models of the solar system with the sun in the center are called heliocentric (helios meaning "Sun" in Greek). Despite the name, the most striking (and then-controversial) Copernican realization was not that the sun was central but that the Earth was peripheral, orbital: planets had been considered merely points in the sky, but if the Earth itself was a planet, perhaps the other planets were, like Earth, huge solid spheres. Philosophically, there were a number of objections to heliocentrism:
- If the Earth is moving, what force keeps the air from flying off into space?
- The Earth is made of heavy rock. Heavy rock moves down. Down in a sphere means the centre. The planets are ephemeral and light, so they are above. How can Earth be a planet?
- If the Earth is mobile, then why do we not observe parallax in the stars (the stars appearing to shift in relation to further objects due to the change in position)? The subsequent invention of the telescope gave the principal technological advance on discovering the solar system, with Galileo's improved version of the telescope rapidly giving benefit in terms of discovering satellites of other planets, especially Jupiter's four major satellites. This showed that all objects in the universe did not orbit the Earth. However, perhaps Galileo's most important discovery was that the planet Venus has phases like the Moon, proving that it must orbit the Sun. Then, in 1687, Isaac Newton devised his law of universal gravitation which explained the force that both kept the Earth moving through the heavens and also kept the air from flying away. Finally, in 1838, astronomer Friedrich Wilhelm Bessel successfully measured the parallax of the star 61 Cygni, proving conclusively that the Earth was in motion.

Exploration of the solar system

Since the start of the space age, a great deal of exploration has been performed by unmanned space missions that have been organized and executed by various space agencies. The first probe to land on another solar system body was the Soviet Union's Luna 2 probe, which impacted on the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on Venus in 1965, Mars in 1976, the asteroid 433 Eros in 2001, and Saturn's moon Titan in 2005. Spacecraft have also made close approaches to other planets: Mariner 10 passed Mercury in 1973. The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980–1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far beyond Pluto's orbit, and astronomers anticipate that they will encounter the heliopause which defines the outer edge of the solar system in the next few years. Pluto remains the only planet not having been visited by a man-made spacecraft, though that will change with the launching of New Horizons by NASA in January 2006. It is scheduled to fly by Pluto in July 2015 and then make an extensive study of as many Kuiper Belt objects as it can. Through these unmanned missions, we have been able to get close-up photographs of most of the planets and, in the case of landers, perform tests of their soils and atmospheres. Manned exploration, meanwhile, has only taken human beings as far as the Moon, in the Apollo program. The last manned landing on the Moon took place in 1972, but the recent discovery of ice in deep craters in the polar regions of the Moon has prompted speculation that mankind may return to the Moon in the next decade or so. Manned missions to Mars have been eagerly anticipated by generations of space enthusiasts, and it was hoped that the first manned interplanetary flights would take place in the 1980s, after the successful Apollo program. Europe (ESA and EU) now plans manned Lunar and Mars missions as part of Aurora Exploration Programme endorsed in 2001. United States followed with similar programme called Vision for Space Exploration in 2004.

Attributes of major planets

All attributes below are measured relative to the Earth: Of the other objects, Ganymede has the largest mass (0.02). Note: Although is a minor planet, it is being considered as possibly being a major planet (the tenth in the solar system). See Planet (Table) for a more comprehensive table.

Attributes of the largest minor planets

The largest minor planets are smoothly rounded, like planets, because their gravity overcomes material strength that keeps smaller bodies in non-spherical shapes. Before the discovery of 2060 Chiron and the trans-Neptunian objects, the term "minor planet" was a synonym for asteroid, but many people now prefer to restrict the use of "asteroid" to refer to rocky bodies of the inner solar system. Most trans-Neptunian objects are icy, like comets, although those we can detect at that distance are much larger than comets. Several asteroids, in the strict sense, are large enough to be spherical. The largest known trans-Neptunian objects are much larger than the large asteroids. (Natural satellites of major planets also range smoothly from small non-spherical objects to large spherical ones, and the largest are larger than 1 Ceres, the largest asteroid). All attributes below are measured relative to the Earth:

Other facts

The total surface area of the solar system's objects that have solid surfaces and a diameter greater than 1 km is ~1.7 km² —about 11 times the area of the Earth's land masses. It has been suggested that the Sun may be part of a binary star system, with a distant companion named Nemesis. Nemesis was proposed to explain some timing regularities of the great extinctions of life on Earth. The hypothesis says that Nemesis creates periodical perturbations in the Oort cloud of comets surrounding the solar system, causing a "comet shower". Some of them hit Earth, causing destruction of life. This hypothesis is no longer taken seriously by most scientists, mostly because infrared surveys failed to spot any such object, which should have been very conspicuous at those wavelengths. The concept of the tenth planet has frequently been explored in science fiction works and conspiracy theories (see also Planet X, and hypothetical planet).

The solar system in small scales

Scaling down the size of the solar system makes it easier for students to grasp the relative distances. The enormous ratio of interplanetary distances to planetary diameters makes constructing a scale model of the solar system a challenging task. (For example, the distance between the Earth and the Sun is almost 12,000 times the diameter of the Earth.) Several places have built such models.

The solar system in astrology

External links

- [http://solarsystem.nasa.gov/index.cfm NASA's Solar System Exploration site]
- [http://space.jpl.nasa.gov NASA's Solar System Simulator]
- [http://www.jpl.nasa.gov/solar_system NASA/JPL Solar System main page]
- [http://members.aol.com/astroequation/ Astronomical Enigma] Mathematical Order in the orbits of the solar system.
- [http://www.solarviews.com Solarviews]
- [http://celestia.sourceforge.net Celestia] Free 3D realtime space-simulation (OpenGL)
- [http://www.nineplanets.org/ The Nine Planets] Comprehensive solar system site by Bill Arnett
- [http://www.krysstal.com/solarsys_planets.html Planetary data]
- [http://www.solstation.com/habitable.htm Stars and Habitable Planets]
- [http://www.michaelschultz.de/index_en.html Solar System] An interactive planets animation (145 zoom steps and time effects)
- [http://my.execpc.com/~culp/space/timeline.html Timeline of solar system exploration]
- [http://www.anzwers.org/free/universe/index.html An Atlas of the Universe]
- mirror matter [http://uk.arxiv.org/abs/astro-ph/0104251 planets] and other [http://uk.arxiv.org/abs/astro-ph/0110161 mirror objects] in the solar system?
- [http://www.solarsystem.org.uk/ The Virtual Solar System, including a scale model of the system]
- ko:태양계 ms:Sistem suria ja:太陽系 simple:Solar system th:ระบบสุริยะ zh-min-nan:Thài-iông-hē

Saturn (planet)

Saturn is the sixth planet from the Sun. It is a gas giant, the second-largest planet in the solar system after Jupiter. Saturn has a prominent system of rings, consisting of mostly ice particles with a smaller amount of rocky debris. It was named after the Roman god Saturn. Its symbol is a stylized representation of the god's sickle (Unicode: ♄). The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as the earth star, 土星, based on the Five Elements.

Physical characteristics

Saturn's shape is visibly flattened at the poles and bulging at the equator (an oblate spheroid); its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but to a lesser degree. Saturn is also the only one of the Solar System's planets less dense than water, with an average specific density of 0.69. This is only an average value, however; Saturn's upper atmosphere is less dense and its core is considerably more dense than water. Saturn's interior is similar to Jupiter's, having a rocky core at the center, a liquid metallic hydrogen layer above that, and a molecular hydrogen layer above that. Traces of various ices are also present. Saturn has a very hot interior, reaching 12000 K at the core, and it radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen. Kelvin-Helmholtz mechanism Saturn's atmosphere exhibits a banded pattern similar to Jupiter's (in fact, the nomenclature is the same), but Saturn's bands are much fainter and they're also much wider near the equator. Saturn's winds are among the Solar System's fastest; Voyager data indicates peak easterly winds of 500 m/s (1116 mph).(1) Saturn's finer cloud patterns were not observed until the Voyager flybys. Since then, however, Earth-based telescopy has improved to the point where regular observations can be made. Saturn's usually-bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter; in 1990 the Hubble Space Telescope observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters and in 1994 another, smaller storm was observed. The 1990 storm was an example of a Great White Spot, a unique but short-lived Saturnian phenomenon with a roughly 30-year periodicity. Previous Great White Spots were observed in 1876, 1903, 1933, and 1960, with the 1933 storm being the most famous. The careful study of these episodes reveal interesting patterns; if it holds another storm will occur in ~2020.(2) Astronomers using infrared imaging have shown that Saturn has a warm polar vortex, and is the only planet in the solar system known to do so. (1) [http://www.solarviews.com/eng/vgrsat.htm Voyager Saturn Science Summary] (2) Patrick Moore, ed., The 1993 Yearbook of Astronomy, Mark Kidger, "The 1990 Great White Spot of Saturn", 176-215, (New York: W.W. Norton & Company, 1992).

Rotational behavior

Since Saturn does not rotate on its axis at a uniform rate, two rotation periods have been assigned to it, like in Jupiter's case: System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is System II. System III, based on radio emissions from the planet, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close in value to System II, it has largely superseded it. While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased slightly, to approximately 10 h 45 m 45 s (± 36 s). [http://www.nasa.gov/mission_pages/cassini/media/cassini-062804.html] The cause of the change is unknown.

Planetary rings

Saturn is probably best known for its planetary rings, which make it one of the most visually remarkable objects in the solar system.

History

The rings were first observed by Galileo Galilei in 1610 with his telescope, but he clearly did not know what to make of them. He wrote to the Grand Duke of Tuscany that "Saturn is not alone but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one [Saturn itself] is about three times the size of the lateral ones [the edges of the rings]." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish, and then in 1613 they reappeared again, further confusing Galileo. The riddle of the rings was not solved until 1655 by Christiaan Huygens, using a telescope much more powerful than the ones available to Galileo in his time. In 1675, Giovanni Domenico Cassini determined that Saturn's ring was actually composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division.

Physical characteristics

The rings can be viewed using a quite modest modern telescope or with a good pair of binoculars. They extend from 6,630 km to 120,700 km above Saturn's equator, and are composed of silica rock, iron oxide, and ice particles ranging in size from specks of dust to the size of a small automobile. There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by Édouard Roche in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces (see Roche limit). A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material that Saturn formed out of. This theory is not widely accepted today, since Saturn's rings are thought to be unstable over periods of millions of years and therefore of relatively recent origin. While the largest gaps in the rings, such as the Cassini division and Encke division, could be seen from Earth, the Voyager spacecrafts discovered the rings to have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise from the gravitational pull of Saturn's many moons in several different ways. Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be undiscovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites such as Prometheus and Pandora. Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini division in this manner. Still more structure in the rings actually consists of spiral waves raised by the moons' periodic gravitational perturbations. Data from the Cassini space probe indicates that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas (O₂) and is thought to be a product of the disintegration of water ice from the rings into its components, oxygen and hydrogen. [http://news.bbc.co.uk/1/hi/sci/tech/4640641.stm]

Dark side of the rings

Compare images from the Cassini spacecraft taken in March and October 2004, and a Pioneer 11 picture from 1979: The side of Saturn's rings that is lit by the Sun looks very different to the backlit side, which is darker overall and appears almost black in the thick B ring. From Earth, we cannot appreciate this because the Earth cannot view Saturn from an angle that displays the backlit side of the rings, and our only views of it are from spacecraft. In 2004, the Cassini spacecraft revealed the first views of the backlit side in 25 years.

Spokes of the rings

1979.]] Until 1980, the structure of the rings of Saturn was explained exclusively as the action of gravitational forces. The Voyager spacecraft found radial features in the B ring, called spokes, which could not be explained in this manner, as their persistence and rotation around the rings were not consistent with orbital mechanics. The spokes appear dark against the lit side of the rings, and light when seen against the unlit side. It is assumed that they are connected to electromagnetic interactions, as they rotate almost synchronously with the magnetosphere of Saturn. However, the precise mechanism behind the spokes is still unknown. magnetosphere.]] Twenty-five years later, Cassini observed the spokes again. They appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as Saturn comes closer to equinox. The spokes were not visible when Cassini arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe spoke formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and the spokes reappeared in images taken September 5, 2005.

Natural satellites

2005 Saturn has a large number of moons. The precise figure will never be certain as the orbiting chunks of ice in Saturn's rings are all technically moons, and it is difficult to draw a distinction between a large ring particle and a tiny moon. Seven of the moons are massive enough to have collapsed into a spheroid under their own gravitation. These are compared to Earth's moon in the table below. Saturn's most noteworthy moon is Titan, the only moon in the solar system to have a dense atmosphere. Due to the tidal forces of Saturn, the moons are currently not at the same position as they were when they were first formed (for a timeline of discovery dates, see Timeline of natural satellites).

Exploration of Saturn

Timeline of natural satellites

Pioneer 11 flyby

Saturn was first visited by Pioneer 11 in September 1979. It flew within 20,000 km of the planet's cloudtops. Low-resolution images were acquired of the planet and few of its moons. Resolution was not good enough to discern surface features, however. The spacecraft also studied the rings; among the discoveries were the thin F-ring and the fact that dark gaps in the rings are bright when viewed towards the Sun, or in other words, they are not empty of material. It also measured the temperature of Titan. [http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PN10&11.html]

Voyager flybys

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, rings, and the satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan greatly increasing our knowledge of the atmosphere of the moon. However, it also proved that Titan's atmosphere is impenetrable in visible wavelengths, so no surface details were seen. The flyby also changed spacecraft's trajectory out from the plane of the solar system. Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's camera stuck and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus. The probes discovered and confirmed several new satellites orbiting near or within the planet's rings. They also discovered the small Maxwell and Keeler gaps.

Cassini orbiter

On July 1, 2004, the Cassini-Huygens spacecraft performed the SOI (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn. Before the SOI, Cassini had already studied the system extensively. In June 2004, it had conducted a close flyby of Phoebe sending back high-resolution images and data. The orbiter completed two Titan flybys before releasing the Huygens probe on December 25, 2004. Huygens descended onto the surface of Titan on January 14, 2005, sending a flood of data during the atmospheric descent and after the landing. As of 2005, Cassini is conducting multiple flybys of Titan and icy satellites. The primary mission ends in 2008 when the spacecraft has completed 74 orbits around the planet. :For the latest information and news releases, see [http://saturn.jpl.nasa.gov Cassini website].

Best viewing of Saturn

2008 While it is a rewarding target for observation for most of the time it is visible in the sky, Saturn and its rings are best seen when the planet is at or near opposition (the configuration of a planet when it is at an elongation of 180° and thus appears opposite the Sun in the sky.) In the opposition on January 13, 2005, Saturn appeared at its brightest until 2031, mostly due to a favourable orientation of the rings relative to the Earth. Saturn appears to the naked eye in the night sky as a bright, yellowish star varying usually between magnitude +1 and 0 and takes approximately 29 and a half years to make a complete circuit of the ecliptic against the background constellations of the zodiac. Optical aid (a large pair of binoculars or a telescope) magnifying at least 20X is required to clearly resolve Saturn's rings for most people.

Appearance

Saturn in fiction and film

Saturn is a popular setting for science fiction novels and films, although the planet tends to be used as a pretty backdrop rather than as an important part of the plot.
- In Voltaire's Micromégas (1752), the eponymous hero arrives at Saturn first (Uranus and Neptune were unknown then). Saturn's citizens are « only a thousand fathoms high », have 72 senses and live for about 15,000 years. Micromégas forms a close friendship with the secretary of the Academy of Saturn, who accompanies him to Earth.
- The unwitting adventurers in Jules Verne's Off on a Comet (1877) pass within 415,000,000 miles of Saturn while riding on a comet. The book describes Saturn as having 8 satellites and 3 rings. It contains a black and white illustration showing what night might look like from the surface of the planet. The rings are brightly illuminated by the sun, and an elliptical shadow is cast on them by the planet. The drawing shows the surface of Saturn as a rocky, desolate, solid surface.
- In H. P. Lovecraft's Cthulhu Mythos (1928–), Saturn was known as Cykranosh in the Hyperborean Era, both Tsathoggua and Atlach-Nacha came to Earth from there, and Tsathoggua's paternal uncle Hziulquoigmnzhah still resides there.
- In Isaac Asimov's short story The Martian Way (1952), Martian colonists use a chunk of ice from Saturn's rings to bring water to the dry world.
- Kurt Vonnegut's novel The Sirens of Titan (1959) is partly set on Titan, Saturn's best known moon.
- In the Star Trek universe (1966–), Saturn is used for the Starfleet Academy Flight Range.
- In Arthur C. Clarke's novel version of 2001: A Space Odyssey (1968), a spacecraft visits the Saturnian system. Clarke's later novel Imperial Earth (1976) takes place partially at a human colony on Titan.
- Douglas Trumbull's film Silent Running (1972) features an ark-like spacecraft traveling through the Saturnian system.
- In the sixth book of the Yoko Tsuno comic book series (Les Trois soleils de Vinéa, 1976), a small part of the action takes place on a Vinean space station in orbit around Saturn. Saturn's moon Titan is also briefly mentioned and depicted. Other Saturnian moons are visible but not named.
- The film Saturn 3 (1980) is mostly set on one of Saturn's moons, but also features a journey through the planet's rings.
- The science fiction anime series The Super Dimension Fortress Macross (1982–1983) has one episode that takes place in Saturn's rings, and the beginning of the movie adaptation The Super Dimension Fortress Macross: Do You Remember Love? takes place near the moon Titan and Saturn's rings.
- An episode of the cartoon series Transformers from 1985, "The God Gambit," reveals that humanoid aliens have a thriving civilization on the moon Titan. In a later episode from 1986, "Money is Everything," which takes place in the year 2006, Titan has been terraformed by humans.
- Warhammer 40,000's universe (1987) places the headquarters of the Grey Knights and Ordo Malleus in Saturn's moons, owing to their defensive capability.
- Tim Burton's film Beetlejuice (1988) is partly set on a fictional Saturn, populated by giant sandworms.
- The Citadel research and mining space station, setting of the computer game System Shock (1994), is in orbit of Saturn for most of the game.
- Stephen Baxter's novel Titan (1997) is focused on the moon Titan, but contains vivid depictions of a journey through the Saturnian system.
- In Michael McCollum's novel The Clouds of Saturn (1998), SparrowHawk pilots Larson Sands and Halley Trevanon fight against the Northern Alliance during a time when the Sun has flared out of control and boiled Earth's oceans away.
- In the sci-fi anime Cowboy Bebop (1998), in the year 2068 a war was fought on Titan.
- In the anime Bishoujo Senshi Sailor Moon, Sailor Saturn is a guardian representing the planet. Her birth is thought to bring destruction to the world, as she's known as the sailor of death and rebirth. On her forehead is the planet's symbol.
- Ben Bova's novel Saturn (2003) is about a spacecraft traveling toward the planet, although Saturn itself does not figure greatly in the story.

Saturn in various cultures

Chinese and Japanese culture designate the planet Saturn as "Earth Star." This is based on Five Elements which was traditionally used to classify natural elements. In Hebrew, Saturn is called 'Shabbathai'. Its Angel is Cassiel. Its Intelligence, or beneficial spirit, is Agiel (layga), and its spirit (darker aspect) is Zazel (lzaz). See: Kabbalah.

External links

- [http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html NASA's Saturn fact sheet]
- [http://saturn.jpl.nasa.gov/home/index.cfm NASA's Cassini mission to Saturn]
- [http://hubblesite.org/newscenter/newsdesk/archive/releases/2001/15/image/a Change of seasons on Saturn]
- [http://www.affs.org/html/studies_on_the_rings_of_saturn.html Theoretical description of the rings of Saturn]
- [http://www.vias.org/spacetrip/saturn_1.html A Trip Into Space] Description and photos of Saturn
(moon navigator) | Saturn | Pan | ...
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Asteroid

:This page is about the astronomical body Asteroid. For the arcade game, see Asteroids. An asteroid is a small, solid object in our Solar System, orbiting the Sun. An asteroid is an example of a minor planet (or plan