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Sundial

Sundial

:This article pertains to the astronomical instrument. For the psychedelic rock band, see Sun Dial. Sun Dial Sun Dial A sundial measures time by the position of the sun. The most commonly seen designs, such as the 'ordinary' or standard garden sundial, cast a shadow on a flat surface marked with the hours of the day. As the position of the sun changes, the time indicated by the shadow changes. However, sundials can be designed for any surface where a fixed object casts a predictable shadow. Most sundial designs indicate apparent solar time. Minor design variations can measure standard and daylight saving time, as well. Sundials are known from ancient Egypt, and were developed further by other cultures, including the Greeks and Romans. The mathematician and astronomer Theodosius of Bithynia (ca. 160 BC-ca. 100 BC) is said to have invented a universal sundial that could be used anywhere on Earth. The French astronomer Oronce Fine constructed a sundial of ivory in 1524. The Italian astronomer Giovanni Padovani published a treatise on the sundial in 1570, in which he included instructions for the manufacture and laying out of mural (vertical) and horizontal sundials.

Installation of standard sundials

1570 Tilting the style or gnomon of a standard sundial is the only practical way to install a mass-produced garden sundial so that it will keep time. Some mass-produced garden sundials are improperly designed, and unable to keep time. Many sundials are made to be used at 45 degrees north. A sundial can be adjusted to another latitude by tilting it so its style or gnomon(s) is (are) parallel the Earth's axis of rotation. That is, the end of a gnomon should point at the north celestial pole in the northern hemisphere, or the south celestial pole in the southern hemisphere. A sundial can be rotated around its style or gnomon (which must still point at the celestial pole) a maximum of 7.5 degrees to the east or west to adjust to the local standard time zone (time zones are 360 degrees/24 hours = 15 degrees wide). Tilt the sundial so that it is oriented as if it were at the longitude of the center of your local time zone. To correct for daylight saving time, a face needs two sets of numerals or a correction table, and must be adjusted for longitude from the center of the time zone. The admittedly informal standard is to have numerals in hot colors for summer, and in cool colors for winter. Twisting the face of the sundial will not work because sundials (except at the north and south pole) do not have equal hour angles. Ordinary sundials do not correct apparent solar time to clock time. There is a 15 minute variation through the year, known as the equation of time, because the Earth's orbit is slightly elliptical and its axis is tilted relative to the plane of its orbit. A quality sundial will include a permanently-mounted table or graph giving this correction for at least each month of the year. Some more-complex sundials have curved hour-lines, curved gnomons or other arrangements to directly display the clock time.

Design & principles of operation

Terminology

The 'shadow-maker' of the sundial is called a gnomon. The sun casts a shadow from the gnomon to a surface called the dial face or dial plate (often shortened to face). Most sundials indicate time on the dial face by the shadow of a line in space called the style[http://www.sundialsoc.org.uk/glossary/alpha.htm#S]. On a standard garden sundial, this line is the top edge of the gnomon. The style should be parallel the Earth's axis of rotation. In common speech, sometimes style refers to the entire gnomon. Some sundials indicate both the time and the date by the shadow of a particular point on the gnomon. That point is called the nodus. The nodus may be the tip of a gnomon with an arbitrary (usually horizontal or vertical) orientation[http://www.sundialsoc.org.uk/glossary/alpha.htm#N]. A few sundials have both a style and a nodus, with the nodus in the form of a small sphere or a notch on a polar-pointing gnomon, or simply the tip of the gnomon. In general, the best material for a face is a very light color to give a high contrast with the shadow. The numerals should be dark, visible on the unshaded portion of the face. The gnomon should be sturdy, preferably metal, because gnomons are usually thin, and can break easily. The traditional luxury materials are a white marble face, with markings inlaid in black marble. Traditional styles are thick bronze to prevent corrosion. It is traditional for a sundial to have a motto.

Equatorial or Equinoctial[http://www.sundials.co.uk/tbequ.htm] sundial

left The simplest sundial is a disk mounted on a bar. The bar must be parallel to the Earth's axis of rotation. The disk forms a plane parallel to the plane of the Earth's equator. The disk is marked so that one edge of the shadow of the bar shows the time as the Earth rotates. Usually noon will be at the bottom of the disk, 6AM on the western edge, and 6PM on the eastern edge. In the winter, the north side of the disk will be shaded, and hard to read. In the summer, the south side will be shaded. In the above design, the bar is the style. The disk in the above design is called the face. In the summer, the north end of the bar is the nodus, but in the winter, the south end of the bar is the nodus. left A series of cocentric circles can be drawn on the face which plot the path of the shadow of the nodus on specific days, thus the dial can be used as a calendar as well as clock. The style shows the time and the nodus the date. One disadvantage of this design is that with a solid face, near the equinox, when sun is just on the celestial equator, the dial is hard to read.

Garden sundial

equinox The classic garden sundial uses the same principle, except the lines of the disk are projected, using trigonometry, onto a face that is parallel to the ground. The advantage of the garden sundial is that it keeps time all year, and its face is never completely shaded in the daytime (as vertical sundials are). For use in a public area, this sundial can be made visible by placing it in a square, or making the face of frosted glass, elevated high in the air, and visible from underneath. The top edge of the gnomon is parallel with the axis of the Earth's rotation. The shadow will cross time markings on the face.The markings of each edge are aligned with the edge of the gnomon that produces the shadow. The angle of the face markings from the root of the gnomon (the substyle) are calculated from the formula face-angle = arctan(sin(latitude)
- tan(hour-angle)). The angle of the style (gnomon)= 90 - latitude. (See Logo programming language for a sample program to draw a garden sundial)

Vertical sundials

Logo programming language Logo programming language Although they are rare in modern life, sundials on vertical south-facing walls (north-facing in the southern hemisphere) are a traditional ancient convenience. They are easy to see from large distances and inexpensive to arrange. One sturdy method is to paint the sundial on the wall, and construct the gnomon as a tripod of metal bars. Fancy sundials used to have faces of inlaid stone. A problem is that vertical sundials only keep time for the part of the year in which the sun illuminates the wall. They are very similar to garden sundials. The formula for a south-facing sundial face is face-angle = arctan(cos(latitude)
- tan(hour-angle)). The angle of the style (gnomon)= latitude. It used to be traditional to place four sundials on the roof or sides of a tower to provide the time. In this way, the time was available to all for the entire year. In principle, sundials can be placed on any surface, at any angle, given the correct trigonometric projection of the face. For example, sundials on roofs are harder to calculate but quite practical.

Portable sundials, for navigation and time

During the middle ages advanced yet portable astronomical instruments were developed.

Diptych sundial

One popular portable sundial design was called a diptych. It consisted of two small flat faces, joined by a hinge. Diptychs usually folded into little flat boxes suitable for a pocket. The gnomon was a string between the two faces. When the string was tight, the two faces formed both a vertical and horizontal sundial. The best material was white ivory, inlaid with black lacquer markings. The best gnomons were black braided silk, linen or hemp. By making the two sundials have different angles to the string (and thus different projections), a diptych can be self-aligning. When both faces show the same time, the diptych shows the local apparent solar time. Additionally, the hinge will be level, and point north (in the northern hemisphere), and the diptych will be angled so the gnomon is parallel to the Earth's axis of rotation. At solar noon, sunrise and sunset, the latitude adjustment of the diptych can't affect the time of either sundial, but at 9am and 3pm, each degree of latitude error (from holding the sundial at the wrong angle) creates four minutes of difference between the two faces. This means that a diptych can also act as a compass and even measure latitude. Some diptychs included a small scale and a plumb-bob to read the latitude. Some others included a compass rose to measure angles to geographic features. Large (meter-sized) diptychs may have been used for navigation in ancient times. navigation

Early 18th Century portable sundials

This form of sundial was about 8 cm diameter and made of brass. It had a brass lid, not shown, to protect it when travelling. Several features enabled precision to be achieved. It had an iron compass needle so that North could be accurately set. The scale division is to 5 minutes. This dial was made in Dublin in 1742 by Gabriel Stokes a mathematical instrument maker.

Elevation sundial

Astrolabes were used as sundials, as well as for calendrical observations, navigation and astronomy. An even smaller design was the ring. It had a small handle, or was a fob or the decoration of a necklace. When held by its handle, a hole would cast a shadow on the inside of the ring, telling the time by markings on the inside. The user had to know if it was morning or evening. Usually the hole was mounted in a sliding lockable piece of metal, which was adjusted to correct date. In recent times, U.S. Special Forces have taken to engraving a simple sundial on their knife-blade. It works even when a watch fails. knife knife

Precision sundials (heliochronometers)

A precision sundial, called a heliochronometer, corrects apparent solar time to mean solar time or another standard time. Heliochronometers usually indicate the minutes to within 1 minute of Universal Time[http://www.sunlitdesign.com/infosearch/sundialaccuracy.htm].

Equatorial bow sundial

The classic shape for a heliochronometer is an equatorial bow sundial. A bar, slot or stretched wire parallel to the earth's axis forms the style. The face is a semicircle with markings on the inner surface. This pattern, built a couple of meters wide out of temperature-invariant steel invar, was used to keep the trains running on time in France before World War I. One of the simplest sundials that reads clock time is an equatorial bow with a gnomon shaped like two vases[http://www.wsanford.com/~wsanford/exo/sundials/ca/claremont/info.html]. The vase-shape directly shades the hour line in the correct place as the year passes, and the sun changes elevation. The most precise sundials ever made are monumental equatorial bows constructed of masonry, part of the Yantra mandir (Jaipur), in India, built as part of a set of astronomical instruments.

Precision noonmarks

In some older houses, particularly farmhouses, a noon-mark can be found carved into a floor or windowsill. Such marks act as sundials to indicate local noon, and they provided a simple and accurate time reference for households that did not possess accurate clocks. In modern times, some Oriental countries' post offices have set their clocks from a precision noon-mark. These in turn provided the times for the rest of the society. The typical noon-mark sundial was a lens set above an analemmatic plate. The plate has an engraved figure-eight shape. When the edge of the sun's image touches the part of the shape for the current month, it is noon!

Ancient Greek sundials

The ancient Greeks used a type of sundial sometimes referred to as pelekinon (axe-like, apparently because shape of the hour and day lines suggest the ancient double-headed ax pelekus). The gnomon was a rod or pole upright in a horizontal face or half-spherical face. The shadow of the tip of the rod sweeps out hyperbolic curves on a flat face, or circles on a spherical face. The advantage of these dials is that they can be marked to tell the exact time for all times of year.

Analemmatic sundials

Analemmatic sundials correct solar time to mean solar time or another standard time. These usually have hour lines shaped like "figure eights" (analemmas) according to the equation of time. This compensates for the slight eccentricity in the Earth's orbit that causes a 15 minute variation from mean solar time. Very accurate dials of this type fit nicely in a public square, using a ball at the tip of a flagpole as the nodus, with the face painted on or inlaid in the pavement. Correction: The two lines above aren't correct. An analemmatic sundial has nothing to do with correction from solar time to standard time. The description below is correct. Correction added october 2005. A fun, less accurate version of the sundial is to lay out the hour marks on concrete, and then let the user stand in a square marked with the month. The month squares are arranged to correct the sundial for the time of year. The user's head then forms the gnomon of the dial. If the sundial is molded into the concrete, it is almost perfectly immune to vandalism, as well as truly fun and reasonably accurate. The geometrical construction of an analemmatic sundial is simple. First imagine an equatorial sundial floating in the air: a vertical bar directed towards the pole and a ring in the plane perpendicular to the bar. Label the lowest point of the ring "12", and the other hour marks as usual. At a certain time and date, the shadow of a certain point A on the bar (which falls here or there depending on the time of year) falls on a certain point B of the ring (which depends on the hour, and the position in the Earth's orbit). Now draw the point B' in the ground just below B and the point A' just below A. Now if you stand at A' your shadow will point at B', because the sun is somewhere in the plane A B A' B'. In middle latitudes, the ellipse with the hour-marks should be about six meters wide, so the shadow of the head of the beholder will fall near it most of the time. Article of interest: [http://pass.maths.org.uk/issue11/features/sundials/index.html “Analemmatic sundials: How to build one and why they work” by C.J. Budd and C.J. Sangwin]

Reflection sundials

Isaac Newton invented a sundial for a south-facing window. He placed a tiny mirror on the windowsill, and painted the sundial's face in a mirror-image pelekinon on the ceiling and walls. The mirror formed the gnomon by reflecting a spot of light. This provides a large, accurate, perfectly correctable sundial with minimal material, and no wasted space at all. This design could easily be made analemmatic.

Analog calculating sundials

A last, interesting variation accurately keeps clock time, while still resembling a conventional garden sundial. It is a horizontal sundial with a face cut on a cardioid (a sort of heart-shape). A cardioid is the shape that connects the intersections between the solar-time marks of a conventional sundial, and the equal-angles of a true clock-time face. The place where the shadow crosses the cardioid's edge is the place where clock time can be read on the underlying clock-time dial. The sundial is adjusted for daylight saving time by rotating the underlying equal-angle clock-time face. The sun-time face does not move.

Digital sundials

A digital sundial uses light and shadow to 'write' the time in numerals (or even words), rather than marking time with position. One such design uses two parallel masks to screen sunlight into patterns appropriate for the time of day.

Reference

Sundials: Their Theory and Construction, Albert E. Waugh, Dover Publications, Inc., 1973, ISBN 0-486-22947-5.

See also


- Sundial Bridge at Turtle Bay — a single spar cable stayed bridge that casts a shadow into a garden where the time may be read.
- Scottish sundial — the ancient renaissance sundials of Scotland.

External links


- [http://www.liverpoolmuseums.org.uk/nof/sun/ Sunbeams and Sundials] Guide to the sun, the seasons and sundials
- [http://members.aon.at/sundials/index_e.htm Sundials in and around Austria (Europe)]
- [http://au23.troja.mff.cuni.cz/~mira/sh/sh.php?lang=1 Sundials in the Czech Republic and Slovakia (Europe)]
- Sundials on the Internet, the leading information site on sundials: http://www.sundials.co.uk
- A virtual sundial: http://www.quns.cam.ac.uk/Queens/virtualdial/VirtDial.html
- [http://www.biol.rug.nl/maes/zonnewijzers/welcome-e.htm Sundials]
- [http://www.scottishsundials.co.uk Scottish Sundials - by Location, Type and Date]
- [http://www.digitalsundial.com/patent.html Digital sundial patent, with description of related designs]. The patent was filed June 1995 [http://community.middlebury.edu/~schar/sundial/patent.html].
- [http://www.treveris.com/sonnenuhren Sundials in the oldest city of Germany: Trier]
- [http://planetary.org/rrgtm/marsdial/ MarsDial] Sundials carried on Mars rovers Spirit and Opportunity
- [http://www.infraroth.de/cgi-bin/slinks.pl Sundial links] - collection of sundial links
- The [http://www.sundialsoc.org.uk/ British Sundial Society] has a [http://www.sundialsoc.org.uk/glossary/chronology/chronology.htm history of sundials] and a [http://www.sundialsoc.org.uk/glossary/alpha.htm glossary of sundial terminology]
- [http://www.mysundial.ca/tsp/tsp.html The Sundial Primer] Learn about many different kinds of sundials and how to make them. You will also find a number of different paper sundial kits that are fun to make. Category:Clocks ja:日時計 simple:Sundial

Psychedelic Rock

Psychedelic music is a musical genre inspired by or attempting to replicate the mind-altering experience of drugs such as cannabis, psilocybin, mescaline, and especially LSD. It is not rigorously defined, and is sometimes interpreted to include everything from Acid Rock and Flower Power music to Hard Rock. However, an inner core of the genre that came to public attention in 1967 can be recognized by characteristic features such as modal melodies; esoteric lyrics often describing dreams, visions, or hallucinations; longer songs and lengthy instrumental solos; and recently invented "trippy" electronic effects such as distortion, reverb, and reversed, delayed and/or phased sounds. The album that brought psychedelic rock into pop culture was The Beatles's Sgt. Pepper's Lonely Hearts Club Band. While the first musicians to be influenced by psychedelic drugs were in the jazz and folk scenes, the first use of the term "psychedelic" in popular music was by the "acid-folk" group The Holy Modal Rounders in 1964. The first use of the word "psychedelic" in a rock music context is usually credited to the 13th Floor Elevators, and the earliest known appearance of this usage of the word in print is in the title of their 1966 album The Psychedelic Sounds of the 13th Floor Elevators. The psychedelic sound itself had been around at least a year earlier in the live music of the Grateful Dead and Pink Floyd, and Donovan's hit Sunshine Superman. The genre reached its maximum popularity in 1967 and then quickly tapered off, though a number of bands continued and there has been a revival since the 1980s.

History

The Beatnik counterculture included ideas of changed consciousness, shared with writers like Timothy Leary and Aldous Huxley, whose book The Doors of Perception explored the idea of drugs stripping away barriers to thought and exposing unfiltered reality. Certain drugs including LSD were not illegal, and one of those experimenting was Ken Kesey who took part as a medical guinea pig in experiments with "psychomimetic" drugs in the late 1950s, and went on to gather like-minded people calling themselves the Merry Pranksters whose attempts to spread the message of psychedelic drugs developed into the Acid Tests of the mid 1960s and Psychedelia in art and music. In 1962 British rock embarked on a frenetic race of ideas that spread back to the U.S. with the British Invasion. The folk music scene also experimented with outside influences. In the tradition of Jazz and blues music many musicians began to take drugs, and include drug references in their songs. In 1965 Bob Dylan was influenced by the Beatles to bring in electric rock instrumentation in his album Bringing It All Back Home, but The Byrds beat him to it with a jangling electric hit single version of a track from the album with hints of psychedelia, Mr. Tambourine Man.

U.S.A. in the 60s

Psychedelia began in the United States folk scene, with the Holy Modal Rounders introducing the term in 1964. From a background including folk and jug band music and influenced by The Byrds, The Grateful Dead went electric and fell in with Ken Kesey's LSD fuelled Merry Pranksters. The Dead played to light shows at the Prankster's Acid Tests, with pulsing images being projected over the group in what became a widespread practice, and developed Acid rock which they played at the Trips Festival of January 1966 along with Big Brother & the Holding Company. Soon the Fillmore was providing a regular venue for "San Francisco Sound" groups like Quicksilver Messenger Service, another former jug band Country Joe and the Fish, and the folk based Jefferson Airplane, whose debut album was recorded at the end of 1965 and later produced "White Rabbit" and "Somebody to Love". The Byrds also contributed to psychedelia in 1966 with Eight Miles High, a song with odd vocal harmonies and an extended guitar solo that guitarist Roger McGuinn states was inspired by Raga and John Coltrane, and later that year the 13th Floor Elevators titled their album The Psychedelic Sounds of the 13th Floor Elevators. The music increasingly became involved in opposition to the Vietnam War. In 1965 members of Rick And The Ravens and The Psychedelic Rangers came together with Jim Morrison to form the Doors. Other bands in the vanguard included Spirit, SRC, the United States of America, Sly and the Family Stone, Love, Blue Cheer, The Amboy Dukes and Jefferson Airplane in the vanguard. January 1967 brought the first album from The Doors and their major hit single Light My Fire, and they went on to have a series of successes. The Grateful Dead made a number of records attempting to capture something of their live sound, with less commercial success. Initially, the the Beach Boys, with their squeaky-clean image, seemed unlikely as psychedelic types. Their music, however, grew more psychedelic and experimental, perhaps due in part to writer/producer/arranger Brian Wilson's increased drug usage and burgeoning mental illness. In 1966, responding to the Beatles' innovations, they produced their album Pet Sounds and later that year had a massive hit with the psychedelic single "Good Vibrations". Wilson's magnum-opus SMiLE (which was never finished, and was remade by Wilson with a new band in 2004) also shows this growing experimentation. There were also less well known psychedelic bands in outlying regions, such as the 13th Floor Elevators and Bubble Puppy working out of Texas, and the Third Bardo in New York City, a group which had a brief revival in the 1990s. Another band that is still widely unknown was the Docks of Dover. The influence was also felt in black music, where record labels such as Motown dabbled for a while with psychedelic soul, producing such hits as "Ball of Confusion (That's What the World is Today)" and "Psychedelic Shack" (by The Temptations), and "Reflections" (by Diana Ross & the Supremes), and the 11-minute-long "Time Has Come Today" by The Chambers Brothers, before falling out of favour.

Britain in the 60s

In the United Kingdom Donovan, going electric like Dylan, had a 1965 hit with Sunshine Superman, one of the very first overtly psychedelic pop records. Pink Floyd had been developing psychedelic rock with light shows since 1965 in the underground culture scene, and in 1966 the Soft Machine formed. In August 1966 The Beatles joined in the fun with their Revolver featuring psychedelia in "Tomorrow Never Knows" and in "Yellow Submarine" which combined these references with appeal to children and nostalgia, a formula repeated in "Strawberry Fields Forever" which would keep their music widely popular. From a blues rock background, the British supergroup Cream debuted in December. The Jimi Hendrix Experience with Noel Redding and Mitch Mitchell brought Jimi Hendrix fame in Britain, and later in his American homeland. Pink Floyd's "Arnold Layne" in March 1967 only hinted at their live sound, then after the Beatles' groundbreaking album Sgt. Pepper's Lonely Hearts Club Band ("Lucy in the Sky with Diamonds") was released in June, Pink Floyd showed their psychedelic sounds in The Piper at the Gates of Dawn and Cream did the same in Disraeli Gears. In the folk scene itself blues, drugs, jazz and eastern influences had featured since 1964 in the work of Davy Graham and Bert Jansch, and in 1967 the Incredible String Band's The 5000 Spirits or the Layers of the Onion developed this into full blown psychedelia. Other artists joining the psychedelic revolution included Eric Burdon (previously of The Animals), and The Small Faces. The Who's Sell Out had an early psychedelic track "I Can See for Miles", but the album concept was out of tune with the times, and it was their later album Tommy that established them in the scene. The Rolling Stones had drug references and psychedelic hints in their 1966 singles "19th Nervous Breakdown" and "Paint it Black", then the fully psychedelic Their Satanic Majesties Request ("In Another Land") suffered from the problems the group was having at the time. In 1968 Jumpin' Jack Flash and Beggars Banquet re-established them, but their disastrous concert at Altamont ended the dream on a downer.

The end of the 60s

A good number of the bands who pioneered psychedelic rock gave it up by the end of the 1960s. The increasingly hostile political environment and the embrace of amphetamines, heroin and cocaine by the underground led to a turn toward harsher music. At the same time, Bob Dylan released John Wesley Harding and the Band released Music from Big Pink, both albums that rejected psychedelia for a more roots-oriented approach. Many bands in England and America followed suit. Eric Clapton cites Music from Big Pink as a primary reason for quitting Cream, for example. The Grateful Dead also went back to basics and had major successes with Workingman's Dead and American Beauty in 1970, then continued to successfully develop their rambling live music and produce a long string of records over the next twenty-five years. The musicians and bands who continued to embrace psychedelia often went on to create progressive rock in the 1970s, which maintained the love of unusual sounds and extended solos but added jazz and classical influences to the mix. For example, progressive rock group Yes sprang out of three British psychedelic bands: Syn (featuring Chris Squire), Tomorrow (featuring Steve Howe) and Mabel Greer's Toy Shop (Jon Anderson). Also, psychedelic rock strongly influenced early heavy metal bands, Black Sabbath probably being the best example. Psychedelic rock, with its distorted guitar sound and adventurous compositions can be seen as an important bridge between heavy metal and earlier blues oriented rock. Alongside the progressive stream, space rock bands such as Hawkwind, Arthur Brown's Kingdom Come and Gong maintained a more explicitly psychedelic course into the 1970s.

More recent bands

Phish, active from the early 1980s, played psychedelic music with a strong jazz influence and a great deal of technical skill, utilizing elaborate modal melodies and complex rhythmic accompaniment. In the mid 1980s a Los Angeles-based movement named the Paisley Underground acknowledged a debt to the Byrds, incorporating psychedelia into a folky, jangle pop sound. The Bangles were the most successful band to emerge from this movement; amongst others involved were Green on Red, the Three O'Clock and Dream Syndicate. [http://www.loxleybeade.de Loxley Beade] from Darmstadt/Germany created a very own mixture of Psychedelic, Folk-Rock and oriental influences by using exotic instruments. A British counterpoint to the Paisley Underground was a number of post New Wave bands, most notably The Soft Boys and the solo albums of their singer Robyn Hitchcock, and The Teardrop Explodes and their vocalist Julian Cope. Hitchcock was heavily influenced by Syd Barrett and John Lennon, which accounts for part of the sound, though his famous flow-of-consciousness inter-song links in concerts is also responsible. In the mid 1980s, The Shamen began with a self-consciously psychedelic curriculum influenced by Barrett and Love, before re-orienting themselves towards rave. Other British dabblers in psychedelia include XTC and Martin Newell with The Cleaners from Venus and The Brotherhood of Lizards. Alternative rock groups also dabbled in psychedelia, most famously, Nirvana on their debut single, 'Love Buzz'. Recently the group Kula Shaker, under the leadership Crispian Mills, created much Indian-influenced psychedelic music such as their most recent album 'Peasants, pigs and Astronauts'. A number of bands such as Ozric Tentacles and the Welsh Gorky's Zygotic Mynci continue to play psychedelic music, in a tradition that goes back to the sixties via acts such as Steve Hillage, Gong and their assorted side projects. British bands Anomie and My Bloody Valentine are standard-bearers for British garage psychedelia, citing Pink Floyd and Hawkwind as their musical influences. Some electronic or electronic-influenced music now termed "ambient" or "trance" would have fallen within the category of psychedelia in the 1966 to 1990 period, such as Aphex Twin or Orbital. Stoner rock acts like Kyuss and their successors also carry forward the flag of explicitly psychedelic music into the present day. The Smashing Pumpkins fused psychedelic rock sounds with heavy metal, becoming a highly successful alternative rock act in the 1990s. Rising from the Japanese noise underground, Acid Mothers Temple mix the subtle, relaxing resonance of Blue Cheer and most obviously Grateful Dead's psychedelic sound, the thought provoking melodies of French folk, and concrete bursts of noise that run through music of Boredoms. Other recent endevours include Neutral Milk Hotel, The Apples (In Stereo), Of Montreal, and Olivia Tremor Control: all members of the now defunct Elephant 6 musical collective, formally headquartered in Athens, Georgia. In the Late 1990's and early 2000's a new Psychedelic Scene flourished in the Silverlake area of Los Angeles, California. Among the bands that stood out were the [http://www.brianjonestownmassacre.com/bandinfo.html]Brian Jonestown Massacre. The leader of this band is [http://profile.myspace.com/index.cfm?fuseaction=user.viewprofile&friendID=926016&Mytoken=ABD4FDD8-1297-CBFB-EB97AC12C7CC5CFA30226909]Anton Newcombe who can be credited for being the big influence on the Scene. Other bands to play the scene were Beachwood Sparks. [http://www.subpop.com/bands/beachwood_sparks/html/index.html]Beachwood Sparks' influences are the Byrds, Buffalo Springfield, Gram Parsons and his Flying Burrito Brothers group. Spinning off from the Beachwood Sparks is a band called the [http://www.thetyde.com/index.htm/]Tyde who frequented the scene often. The more well known of the Los Angeles scene were the groups known as the [http://www.bomp.com/Warlocks.html]Warlocks,and BRMC. The [http://www.thequarterafter.com/]Quarter After, another less known group is Byrds influenced and has toured with the Brian Jonestown Massacre. The groups of the Silverlake Scene have been mentioned in the recent documentary Dig! that documents the [http://www.brianjonestownmassacre.com/bandinfo.html]Brian Jonestown Massacre and the Dandy Warhols rivalry. In recent years there has been a intergration of heavier rock with psychedelia sounds and effects, such as the musical band Tool.

See also


- The Electric Kool-Aid Acid Test
- Psychedelic trance
- Psychedelic soul
- Psychedelic reggae For a comprehensive list see: List of psychedelic music artists

External links


- [http://www.toolband.com Tool Official Site]
- [http://www.spiritplants.org/phpbb/viewforum.php?f=4/ Spirit Plant Music Forum]
- [http://www.musiccitysf.com Music City SF]
- [http://www.rockhallsf.com San Francisco Rock and Roll Hall of Fame]
- [http://www.markmurex.com/ Psychedelic art and music] Category:American styles of music Category:British styles of music Category:Rock music genres Category:Psychedelia

Time

Attempting to understand Time has long been a prime occupation for philosophers, scientists and artists. There are widely divergent views about its meaning, hence it is difficult to provide an uncontroversial and clear definition of time. The Oxford English Dictionary defines it as "the indefinite continued progress of existence and events in the past, present, and future, regarded as a whole". Another standard dictionary definition is "a non-spatial linear continuum wherein events occur in an apparently irreversible order." This article looks at some of the main philosophical and scientific issues relating to time. The measurement of time has also occupied scientists and technologists, and was a prime motivation in astronomy. Time is also a matter of significant social importance, having economic value ("time is money") as well as personal value due to an awareness of the limited time in each day and in our lives. Units of time have been agreed upon to quantify the duration of events and the intervals between them. Regularly recurring events and objects with apparently periodic motion have long served as standards for units of time - such as the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum.

Philosophy of time

Main article: Philosophy of space and time; Ontology In ancient thought, Zeno's paradoxes challenged the conception of infinite divisibility, and eventually led to the development of calculus. Parmenides (of whom Zeno was a follower) believed that time, motion, and change were illusions, basing this on a rather interesting argument. More recently, McTaggart held a similar belief. Newton believed time and space form a container for events, which is as real as the objects it contains. In contrast, Leibniz believed that time and space are a conceptual apparatus describing the interrelations between events. Leibniz and others thought of time as a fundamental part of an abstract conceptual framework, together with space and number, within which we sequence events, quantify their duration, and compare the motions of objects. In this view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events. The bucket argument proved problematic for Leibniz, and his account fell into disfavour, at least amongst scientists, until the development of Mach's principle. Modern physics views the curvature of spacetime around an object as much a feature of that object as are its mass and volume. Immanuel Kant, in the Critique of Pure Reason, described time as an a priori notion that allows us (together with other a priori notions such as space) to comprehend sense experience. With Kant, neither space nor time are conceived as substances, but rather both are elements of a systematic framework necessarily structuring the experiences of any rational agent. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantify how far apart events occur. Nietzsche, inspired by the concept of eternal return in his book Thus Spoke Zarathustra, argued that time possesses a circular characteristic. Postulating an infinite past, "all things" must have come to pass therein; the same for an infinite future. In Existentialism, time is considered fundamental to the question of being, in particular by the philosopher Martin Heidegger.

Contemporary theses in the philosophy of time

In contempoary philosophy there has been a very active debate over the nature of time, especially in light of the big changes in physics since the 1920s. Contributors include Ned Markosian, Ted Sider, Quentin Smith, and L. Nathan Oaklander. Two major theses have been developed, along with some hybrids. There is no real consensus among philosophers about which, if any, is correct. The two major theories can be summed up as follows: 1. A-theory of time: Presentism: Oaklander writes: "[A] version of the pure A-theory, known as "", purports to avoid… the problem of change... According to presentism, only the present exists. Thus, it is not the case that, say, O is green and [then] O is red [if, for example, O is a tomato]." (Oaklander, L. Nathan. In Smith, Quentin, and Oaklander, L. Nathan. 1995. Time, Change, and Freedom. New York: Routledge. 2004, 27.) 2. B-theory of time: Eternalism: the following passage from L. Nathan Oaklander sums this up
…[T]ime [involves] events strung out along a series united to one another by the relations of earlier than, later and simultaneity… The events in the temporal series are fixed in that they never change their position relative to each other… It has become customary to call the entire series of events spread out along the time-line from earlier to later, the “B-series.” When viewed solely in terms of the B-series, time is thought of as static or unchanging for there is nothing about temporal relations between events that changes... Time not only has a static aspect, it also has a transitory aspect. In addition to conceiving of time in terms of events standing in temporal relations, we also conceive of time and the events in time as moving or passing from the far future to the near future, from the hear future to the present, and then from present they recede into the more and more distant past… When events are ordered in terms of the notions of past, present, or future they form what is called an “A-series.” It should be noted, of course, that the A- and B-series are not really “two” different series of events, but the same series ordered in two different ways. (Oaklander 2004,Page 69)

Time in physics

never change Main article: Time in physics Time is currently one of the few fundamental quantities (quantities which cannot be defined via other quantities because there is nothing more fundamental known at present). Thus, similar to definition of other fundamental quantities (like space and mass), time is defined via measurement. Currently, the standard time interval (called conventional second, or simply second) is defined as 9 192 631 770 oscillations of a hyperfine transition in the 133Cs atom. Prior to Albert Einstein's relativistic physics, time and space had been treated as distinct dimensions; Einstein linked time and space into spacetime. Einstein showed that people traveling at different speeds will measure different times for events and different distances between objects, though these differences are minute unless one is traveling at a speed close to that of light. Many subatomic particles exist for only a fixed fraction of a second in a lab relatively at rest, but some that travel close to the speed of light can be measured to travel further and survive longer than expected. According to the special theory of relativity, in the high-speed particle's frame of reference, it exists for the same amount of time as usual, and the distance it travels in that time is what would be expected for that velocity. Relative to a frame of reference at rest, time seems to "slow down" for the particle. Relative to the high-speed particle, distances seems to shorten. Even in Newtonian terms time may be considered the fourth dimension of motion; but Einstein showed how both temporal and spatial dimensions can be altered (or "warped") by high-speed motion. Einstein (The Meaning Of Relativity - 1968): "Two events taking place at the points A and B of a system K are simultaneous if they appear at the same instant when observed from the middle point, M, of the interval AB. Time is then defined as the ensemble of the indications of similar clocks, at rest relatively to K, which register the same simultaneously."

Measurement

Present day standards

The standard unit for time is the SI second, from which larger units are defined like the minute, hour, and day. Because they do not use the decimal system, and because of the occasional need for a leap-second, the minute, hour, and day are "non-SI" units, but are officially accepted for use with the International System. There are no fixed ratios between seconds (or days) on the one hand and months and years on the other hand -- months and years having significant variations in length. Despite its great social importance, the week is not mentioned even as a "non-SI" unit. ([http://www1.bipm.org/utils/en/pdf/si-brochure.pdf See external pdf file: The International System of Units].) The measurement of time is so critical to the functioning of our modern societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on atomic clocks around the world, known as International Atomic Time (TAI). This is the yardstick for other time scales including Coordinated Universal Time (UTC) which is the basis for civil time. The 60 base used for seconds, minutes and hours is all the remains of the ancient Phoenician counting base, using 60 as the equivalent of 10, or 100 in modern times. A 60 base is known as sexagesimal.

Chronology

Another form of time measurement consists of studying the past. Events in the past can be ordered in a sequence (creating a chronology), and be put into chronological groups (periodization). One of the most important systems of periodization is Geologic time, which is a system of periodizing the events that shaped the Earth and its life. Chronology, periodization, and interpretation of the past are together known as the study of history.

Psychology

Different people may judge identical lengths of time quite differently. Time can "fly"; that is, a long period of time can seem to go by very quickly. Likewise, time can seem to "drag," as in when one performs a boring task. The psychologist Jean Piaget called this form of time perception "lived time". Time appears to go fast when sleeping, or, to put it differently, time seems not to have passed while asleep. Time also appears to pass more quickly as one gets older. For example, a day for a child seems to last longer than a day for an adult. One possible reason for this is that with increasing age, each segment of time is an increasingly smaller percentage of the person's total experience. Altered states of consciousness are sometimes characterised by a different estimation of time. Some psychoactive substances--such as entheogens--may also dramatically alter a person's temporal judgement. In explaining his theory of relativity, Albert Einstein is often quoted as saying that although sitting next to a pretty girl for an hour feels like a minute, placing one's hand on a hot stove for a minute feels like an hour. This is intended to introduce the listener to the concept of the interval between two events being perceived differently by different observers.

Use of time

The use of time is an important issue in understanding human behaviour, education, and travel behaviour. The question concerns how time is allocated across a number of activities (such as time spent at home, at work, shopping, etc.). Time use changes with technology, as the television or the Internet created new opportunities to use time in different ways. However, some aspects of time use are relatively stable over long periods of time, such as the amount of time spent traveling to work, which despite major changes in transport, has been observed to be about 20-30 minutes one-way for a large number of cities over a long period of time. This has led to the disputed time budget hypothesis. Time management is the organization of tasks or events by first estimating how much time a task will take to be completed, when it must be completed, and then adjusting events that would interfere with its completion so that completion is reached in the appropriate amount of time. Calendars and day planners are common examples of time management tools. Arlie Russell Hochschild and Norbert Elias have written on the use of time from a sociological perspective.

See also


- Event
- Duration
- Change
- Rate
- Causality
- Present (time)
- Cycles and List of cycles

General units of time


- Second
- Minute
- Hour
- Day
- Week
- Fortnight
- Month
- Quarter
- Year
- Decade
- Century
- Millennium

Special units of time


- Geologic timescale
- Season
- Eon
- Era
- Period
- Epoch
- Stage
- Cosmological decade
- Tithi
- Fiscal year
- Ship's bells
- Half-life
- Periodization and list of time periods
- Unix epoch
- Swatch Internet Time
- Hexadecimal Time
- Shake (time) Light-year is the distance light can travel in an Earth year and so is a unit of distance rather than time.

Time measurement and horology


- Calendar
- Lunar calendar
- Solar calendar
- Chronometer
- Railroad chronometers
- Clock
- Water clock
- Hourglass
- Sundial
- Time zone
- Time scales and time standards
- Watch
- Network Time Protocol (NTP)

Theory and study of time


- Philosophy of physics
- Spacetime
- Time travel
- Exponential time
- Planck time
- Orders of magnitude (time)
- Eternity
- Peter Lynds
- A Brief History of Time
- Periodization
- Chronology
- History
- Time discipline
- Time management
- Wikibooks:English:Time
- Wheel of time
- Timescapes

References


- Oxford English Dictionary - [http://www.askoxford.com/concise_oed/time?view=uk]

External links

Perception of time


- [http://plato.stanford.edu/entries/time-experience/ The Experience and Perception of Time]
- [http://cogprints.ecs.soton.ac.uk/archive/00003125/ Subjective Perception of Time and a Progressive Present Moment: The Neurobiological Key to Unlocking Consciousness]
- [http://www.primitivism.com/time.htm Time and Its Discontents]
- [http://www.ericdigests.org/2003-5/time.htm Time and Learning]
- [http://mixingmemory.blogspot.com/2004/12/by-request-time-perception-i.html Time Perception I] and [http://mixingmemory.blogspot.com/2004/12/time-perception-ii-cognitive-factors.html II]
- [http://theorderoftime.org/ The Order of Time: Platform for an Alternative Time Consciousness]
- [http://www.chabad.org/article.asp?AID=74335 What is Time?] An elucidation of the Lubavitcher Rebbe's comments on the topic.

Physics


- [http://physics.nist.gov/GenInt/Time/world.html A walk through Time]
- [http://pages.britishlibrary.net/lobster/tmx Time Travel and Multi-Dimensionality]
- [http://arxiv.org/abs/physics/0310055 Time and classical and quantum mechanics: Indeterminacy vs. discontinuity]
- [http://www.sankey.ws/time.html Time as a universal consequence of quanta]

Timekeeping


- [http://tycho.usno.navy.mil/systime.html Different systems of measuring time]
- [http://physics.nist.gov/cuu/Units/outside.html non-SI units]
- [http://www1.bipm.org/en/scientific/tai/time_server.html UTC/TAI Timeserver]
- [http://tycho.usno.navy.mil/leapsec.html Leapsecond]
- [http://www.intuitor.com/hex/hexclock.html Hex Time]
- [http://www.florencetime.net Florencetime.net]
- [http://news.bbc.co.uk/2/hi/science/nature/3486160.stm BBC article on shortest time ever measured]
- [http://www.awi-net.org American Watchmakers-Clockmakers Institute]
- [http://www.timeanddate.com/worldclock/ The World Clock - Time Zones]

Miscellaneous


- [http://www.boost.org/doc/html/date_time.html Boost Date-Time Library -- Powerful C++ Library for date-time manipulation]
- [http://www.cyclesresearchinstitute.org/ Cycles Research Institute]
- [http://www.timeticker.com/ TimeTicker and the time tickers...]
- [http://www.welt-zeit-uhr.de/worldtime.php World Time and Zones]
- [http://www.timetools.co.uk Time Servers] NTP Time Servers provide accurate timing for computers and computer networks.

Further reading


-
- Peter Galison, Einstein's Clocks and Poincaré's Maps: Empires of Time (2003).
- [http://seizethedaylight.com Seize the Daylight: The Curious and Contentious Story of Daylight Saving Time by David Prerau] (Thunder’s Mouth Press; $23.00; ISBN 1-56025-655-9)
- ko:시간 ja:時間 simple:Time

Sun

:: For the astrological significance of the Sun, see Solar system in astrology. ::"Solar" redirects here; for the superhero by that name, see Solar (comics). The Sun (or Sol) is the star at the center of our Solar system. Earth orbits the Sun, as do many other bodies, including other planets, asteroids, meteoroids, comets and dust. Its heat and light support almost all life on Earth. The Sun is a ball of plasma with a mass of about 2 kg, which is somewhat higher than that of an average star. About 74% of its mass is hydrogen, with 25% helium and the rest made up of trace quantities of heavier elements. It is thought that the Sun is about 5 billion years old, and is about halfway through its main sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. In about 5 billion years time the Sun will become a white dwarf. Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over 106 K when its visible surface (the photosphere) has a temperature of just 6,000 K. Looking directly at the Sun can damage the retina and one's eyesight. See below for details.

General information

See below The Sun is classified as a main sequence star, which means it is in a state of "hydrostatic balance", neither contracting nor expanding, and is generating its energy through nuclear fusion of hydrogen nuclei into helium. The Sun has a spectral class of G2V, with the G2 meaning that its color is yellow and its spectrum contains spectral lines of ionized and neutral metals as well as very weak hydrogen lines [http://www.astro.uiuc.edu/~kaler/sow/spectra.html#classes], and the V signifying that it, like most stars, is a "dwarf" star on the main sequence[http://www.physics.uq.edu.au/people/ross/phys2080/spec/analyz.htm]. The Sun has a predicted main sequence lifetime of about 10 billion years. Its current age is thought to be about 4.5 billion years, a figure which is determined using computer models of stellar evolution, and nucleocosmochronology . The Sun orbits the center of the Milky Way galaxy at a distance of about 25,000 to 28,000 light-years from the galactic centre, completing one revolution in about 226 million years. The orbital speed is 217 km/s, equivalent to one light year every 1400 years, and one AU every 8 days. The astronomical symbol for the Sun is a circle with a point at its centre (Image:Sol.gif).

Structure

Image:Sol.gif The Sun is a near-perfect sphere, with an oblateness estimated at about 9 millionths, which means the polar diameter differs from the equatorial by about 10 km. This is because the centrifugal effect of the Sun's slow rotation is 18 million times weaker than its surface gravity (at the equator). Tidal effects from the planets do not significantly affect the shape of the Sun, although the Sun itself orbits the center of mass of the solar system, which is offset from the Sun's center mostly because of the large mass of Jupiter. The mass of the Sun is so comparatively great that the center of mass of the solar system is generally within the bounds of the Sun itself. The Sun does not have a definite boundary as rocky planets do, as the density of its gases drops off following an approximately exponential relationship with distance from the centre of the Sun. Nevertheless, the Sun has well defined interior structure, described below. The Sun's radius is measured from centre to the edges of the photosphere. The solar interior is not directly observable and the Sun itself is opaque to electromagnetic radiation. However, just as the study of the waves generated by earthquakes (seismology) can be used to study the interior structure of the Earth, helioseismology, the study of sound waves that travel through the Sun's interior, has also contributed greatly to our understanding of the Sun's structure . Computer modeling of the Sun is also used as a theoretical tool to investigate its deep layers.

Core

At the center of the Sun, where its density reaches up to 150,000 kg/m3 (150 times the density of water on Earth), thermonuclear reactions (nuclear fusion) convert hydrogen into helium, producing the energy that keeps the Sun in a state of equilibrium. About 8.9 protons (hydrogen nuclei) are converted to helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second or 383 yottawatts (9.15 tons of TNT per second). The core extends from the center of the Sun to about 0.2 solar radii, and is the only part of the Sun where an appreciable amount of heat is produced by fusion: the rest of the star is heated by energy that is transferred outward. All of the energy of the interior fusion must travel through the successive layers to the solar photosphere, before it escapes to space. The high-energy photons (gamma and X rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as constant absorption and re-emission at lower energies in the solar mantle (see below). Estimates of the "photon travel time" range from as much as 50 million years (Richard S. Lewis, The Illustrated Encyclopedia of the Universe, Harmony Books, New York, 1983, p. 65) to as little as 17,000 years [http://www.badastronomy.com/bitesize/solar_system/sun.html]. Upon reaching the surface after a final trip through the convective outer layer, the photons escape as visible light. Neutrinos are also released in the fusion reactions in the core, but unlike photons they very rarely interact with matter, and so almost all are able to escape the Sun immediately.

Radiation zone

From about 0.2 to about 0.7 solar radii, the material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward. In this zone, there is no thermal convection: while the material grows cooler with altitude, this temperature gradient is slower than the adiabatic lapse rate and hence cannot drive convection. Heat is transferred by ions of hydrogen and helium emitting photons, which travel a brief distance before being re-absorbed by other ions. Because of this, it can take a photon nearly 1,000,000 years to reach the photosphere.

Convection zone

photosphere From about 0.7 solar radii to 1.0 solar radii, the material in the Sun is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative zone. The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the solar granulation and supergranulation. The turbulent convection of this outer part of the solar interior gives rise to a 'small-scale' dynamo that produces magnetic north and south poles all over the surface of the Sun.

Photosphere

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere, sunlight is free to propagate into space and its energy escapes the Sun entirely. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 1023/m3 (this is about 1% of the particle density of Earth's atmosphere at sea level). The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays.

Temperature minimum

The coolest layer of the Sun is the temperature minimum region about 500 km above the photosphere. It is about 4,000 K. It is the only part of the Sun cool enough to support simple molecules such as carbon monoxide and water; all other parts of the Sun are hot enough to break chemical bonds.

Chromosphere

Above the visible surface of the Sun is a thin layer, about 2,000 km thick, that is dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chromos, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun.

Corona

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the solar system and heliosphere. The low corona, which is very near the surface of the Sun, has a particle density of 1011/m3 (Earth's atmosphere near sea level has a particle density of about 2x1025/m3). The temperature of the corona is several megakelvins.

Theoretical problems

Solar neutrino problem

megakelvin For some time it was thought that the number of neutrinos produced by the nuclear reactions in the Sun was only a third of the number predicted by theory, a result that was termed the solar neutrino problem. Several neutrino observatories were constructed, including the Sudbury Neutrino Observatory and Kamiokande to try to measure the solar neutrino flux. It has recently been found that neutrinos have rest mass, and can therefore transform into harder-to-detect varieties of neutrinos while en route from the Sun to Earth in a process known as neutrino oscillation . Thus, measurement and theory have been reconciled.

Coronal heating problem

The optical surface of the Sun (the photosphere) is known to have a temperature of about 6,000 K. Above it lies the solar corona with a temperature of one million kelvins. The high temperature of the corona suggests that it is heated by something other than the photosphere. It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere. Two main mechanisms have been proposed to explain coronal heating: Wave heating, in which sound, gravitational and magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other proposed mechanism is flare heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of solar flares and waves. , , , . Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfven waves have been found to dissipate or refract before reaching the corona (, ). In addition, Alfven waves do not easily dissipate in the corona . Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales , but this is still an open topic of investigation.

Faint young sun problem

Theoretical models of the sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75 percent as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop. However, the geologic record shows that the Earth has remained at a fairly constant temperature throughout its history. In fact, the young Earth was actually warmer than it is today. Some scientists have suggested that the young Earth's atmosphere contained much larger quantities of greenhouse gases such as carbon dioxide and/or ammonia than are present today . Others suggest that cosmic rays might strongly influence the Earth's climate, and that their flux was much higher in the early history of the solar system .

Magnetic field

cosmic ray's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind) [http://quake.stanford.edu/~wso/gifs/HCS.html]. (click to enlarge)]] All matter in the Sun is in the form of gas and plasma due to its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (28 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences. (See magnetic reconnection.) The solar activity cycle includes old magnetic fields being stripped off the Sun's surface starting from one pole and ending at the other. The magnetic field of the sun reverses once for each 11-year sunspot cycle. The influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium creates the largest structure in the Solar System, the Heliospheric current sheet. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth being over 100 times greater than originally anticipated. If space were a vacuum, then the Sun's 10-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10-11 tesla. But satellite observations show that it is about 100 times greater at around 10-9 tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g. the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an MHD dynamo.

Position of the Sun through the year

The path of the Sun across the sky varies throughout the year. The shape described by the Sun's position, considered at the same time each day for a complete year, is called the analemma, and resembles a figure 8, aligned along the North/South direction. The most obvious variation in the Sun's apparent position through the year is a North/South swing over 47 degrees of angle, due to the 23.5 degree tilt of the Earth, but there is an East/West component as well. The North/South swing in apparent angle is the main source of seasons on Earth.

Solar space missions

seasons using UV light from the He+ emission line at 30.4 nm. (Animation (980 kB MPEG))]] To obtain an uninterrupted view of the Sun, the European Space Agency and NASA cooperatively launched the Solar and Heliospheric Observatory (SOHO) on December 2, 1995. Originally a two-year mission, SOHO is now over ten years old (as of late 2005). It has proved so useful that a follow-on mission, the Solar Dynamics Observatory, is planned for launch in 2008. Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is much less well known. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. It returned to Earth in 2004 and is undergoing analysis, but it was damaged by crash-landing when its parachute failed to deploy on reentry to Earth's atmosphere.

History and future of the Sun

The Sun is thought to be a second-generation star, whose formation may have been triggered by shockwaves from a nearby supernova. This is suggested by a high abundance of heavy elements such as iron, gold and uranium in the solar system: the most plausible ways that these elements could be produced are by endothermic nuclear reactions during a supernova or by transmutation via neutron absorption inside a massive first generation star. Our Sun does not have enough mass to explode as a supernova, and its mass is below the Chandrasekhar limit. Instead, in 4-5 billion years it will enter its red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches about 3 K. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed. Following the red giant phase, giant thermal pulsations will cause the Sun to throw off its outer layers forming a planetary nebula. The Sun will then evolve into a white dwarf, slowly cooling over eons. This stellar evolution scenario is typical of low to medium mass stars.

Human understanding of the Sun

:see also sun worship sun worship mythology]] Mankind's most fundamental understanding of the Sun is as the luminous disk in the heavens whose presence above the horizon creates day, and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a deity or other supernatural phenomenon. One of the first people in the Western world to offer a scientific explanation for the sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the Peleponessus, and not the chariot of Helios. For teaching this heresy he was imprisoned by the authorities and sentenced to death (though later released through the intervention of Pericles). With respect to the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac. Thus, the Sun was considered by Greek astronomers to be one of the seven planets (Greek planetes "wanderer"), after which the seven days of the week are named in some languages.

The Sun as a power source

Sunlight — that is, light radiated from the surface of the Sun — is thought to be the main source of energy near the surface of Earth. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. It is about 1370 watts per square meter of area. Sunlight on the surface of Earth is attenuated by the Earth's atmosphere, so that less power arrives at the surface — closer to 1000 watts per directly exposed square meter in clear conditions. This energy can be harnessed through several natural and synthetic processes. Photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or do other useful work. The energy stored in petroleum is thought to have been converted from sunlight by photosynthesis in the distant past.

Sun and eye damage

Sunlight is very bright, and looking directly at the Sun is painful to the eyes. Looking directly at the Sun when it is high in the sky causes temporary bleaching of the photosensitive pigments in the retina, which makes phosphene visual artifacts and may cause temporary partial blindness. Direct viewing of the Sun with the naked eye delivers about 4 milliwatts of sunlight to the retina that is in the solar image, heating it up and potentially (though not normally) damaging it. Brief viewing of the full direct Sun with the naked eye is unpleasant but generally safe. Viewing the Sun through light-concentrating optics such as binoculars is hazardous without an attenuating (ND) filter to dim the sunlight. Suitable filters are available at welding supply shops and camera stores. Using a proper filter is very important as some improvised filters reduce visible light while passing either infrared or ultraviolet rays that can still damage the eye. Viewing the Sun through unfiltered 7x50 mm binoculars can deliver as much as 2.5 watts of sunlight into each eye, over 300 times more power than naked eye viewing. Even brief glances at the midday Sun through unfiltered binoculars can cause permanent blindness. During partial eclipses of the Sun, another hazardous condition exists because of the way the eye responds to bright light. The pupil is controlled by the total amount of light in the visual field, not by the brightest object in the field. During partial eclipses, most sunlight is blocked by the Moon passing directly in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the dim overall light, the pupil tends to dilate from about 2 mm to perhaps 6 mm diameter, increasing the eye's collecting area by a factor of nearly 10. Each retinal cell that is exposed to the partially-eclipsed solar image thus receives about ten times as much light as it would looking at the normal, non-eclipsed Sun. Viewing the partially eclipsed Sun with the naked eye can cause permanent localized damage to the retina, resulting in small, permanent blind spots for the viewer. This is an especially insidious hazard for inexperienced observers and for children, because there is no immediate perception of pain and it is tempting to stare at the spectacle of the eclipsing Sun, compounding any damage. During sunrise and sunset, sunlight is attenuated by a particularly long passage through Earth's atmosphere, and the direct Sun is sometimes faint enough to be viewed directly without discomfort or safely with binoculars. Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.

External links


- [http://sohowww.nascom.nasa.gov/data/realtime-images.html Current SOHO snapshots]
- [http://soi.stanford.edu/data/farside/index.html Far-Side Helioseismic Holography] from [http://www.stanford.edu Stanford]
- [http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html NASA Eclipse homepage]
- [http://sohowww.nascom.nasa.gov/ Nasa SOHO (Solar & Heliospheric Observatory) satellite] [http://sohowww.nascom.nasa.gov/explore/faq/sun.html FAQ]
- [http://soi.stanford.edu/results/sounds.html Solar Sounds] from [http://www.stanford.edu Stanford]
- [http://www.spaceweather.com Spaceweather.com]
- [http://scienceworld.wolfram.com/astronomy/Sun.html Eric Weisstein's World of Astronomy - Sun]
- [http://www.astro.uu.nl/~strous/AA/en/antwoorden/zonpositie.html The Position of the Sun]
- [http://www.lmsal.com/YPOP/FilmFestival/index.html A collection of solar movies]
- [http://www.solarphysics.kva.se/ The Institute for Solar Physics- Movies of Sunspots and spicules]
- [http://science.msfc.nasa.gov/ssl/pad/solar/default.htm NASA/Marshall Solar Physics website]
- [http://rredc.nrel.gov/solar/codesandalgorithms/spa Solar Position Algorithm] and [http://www.nrel.gov/docs/fy04osti/34302.pdf documentation] from the [http://www.nrel.gov National Renewable Energy Laboratory]
- [http://libnova.sourceforge.net/index.html libnova] - a celestial mechanics and astronomical calculation library

References

# Alfven, H., 1947, Monthly Notices of the Royal Astronomical Society., 107, 211 # # Biermann, L., 1946, Naturwissenschaffen, 33, 118 # Bonanno, A., Schlattl, H., Paternò, L. (2002), The age of the Sun and the relativistic corrections in the EOS, Astronomy and Astrophysics, v.390, p.1115-1118 # Carslaw, K.S., Harrison, R.G., Kirkby, J., 2002, Cosmic Rays, Clouds, and Climate, Science, 298, 1732-1737 # Kasting, J.F., Ackerman, T.P., 1986, Climatic Consequences of Very High Carbon Dioxide Levels in the Earth’s Early Atmosphere, Science, v. 234, p. 1383-1385 # Parker, E.N., 1958, Astrophysical Journal, 128, 644 # Parker, E.N., 1988, Astrophysical Journal, 330, 474 # Priest, E.R., 1982, Solar Magnetohydrodynamics (Dordrecht: Reidel), pp. 206-245 # Schlattl, H. (2001), Three-flavor oscillation solutions for the solar neutrino problem, Physical Review D, vol. 64, Issue 1 # Sturrock, P.A., & Uchida, Y., 1981, Astrophysical Journal., 246, 331 # Thompson, M.J. (2004), Solar interior: Helioseismology and the Sun's interior, Astronomy & Geophysics, v. 45, p. 4.21-4.25 Category:Yellow dwarfs Category:Space plasmas Category:Plasma physics als:Sonne zh-min-nan:Ji̍t-thâu ko:태양 ms:Matahari ja:太陽 simple:Sun th:ดวงอาทิตย์

Solar time

Solar time is based on the idea that when the sun reaches its highest point in the sky, it is noon. Apparent solar time is based on the apparent solar day, which is the interval between two successive returns of the Sun to the local meridian. Solar time can be measured by a sundial. The length of a solar day varies throughout the year for two reasons. First, the Earth's orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun and slower when it is farthest from the Sun (see Kepler's laws of planetary motion). Second, due to Earth's axial tilt, the Sun does not usually appear to move along Earth's celestial equator but usually appears to move at an angle to it during the year; thus, the sun appears to move either fast or slow depending on whether it appears to be far from or close to the equator (see tropical year). Consequently, apparent solar days are shorter in March (26–27) and September (12–13) than they are in June (18–19) or December (20–21). Mean solar time is artificial clock time adjusted via observations of the diurnal rotation of the fixed stars to agree with average apparent solar time. The length of a mean solar day is a constant 24 hours throughout the year even though the amount of daylight within it may vary. An apparent solar day may differ from a mean solar day (of 86,400 seconds) by as much as nearly 22 seconds shorter to nearly 29 seconds longer. Because many of these long or short days occur in succession, the difference builds up to as much as nearly 17 minutes early or a little over 14 minutes late. The difference between apparent solar time and mean solar time is called the equation of time. Many methods have been used to simulate mean solar time throughout history. The earliest were clepsydras or water clocks, used for almost four millennia from as early as the middle of the second millennium BC until the early second millennium. Before the middle of the first millennium BC, the water clocks were only adjusted to agree with the apparent solar day, thus were no better than the shadow cast by a gnomon (a vertical pole), except that they could be used at night. Nevertheless, it has always been known that the sun moves eastward relative to the fixed stars along the ecliptic. Thus since the middle of the first millennium BC, the diurnal rotation of the fixed stars has been used to determine mean solar time, against which clocks were compared to determine their error rate. Babylonian astronomers knew of the equation of time and were correcting for it as well as the different rotation rate of stars, sidereal time, to obtain a mean solar time much more accurate than their water clocks. This ideal mean solar time has been used ever since then to describe the motions of the planets, Moon, and Sun. Mechanical clocks did not achieve the accuracy of Earth's 'star clock' until the beginning of the twentieth century. Even though today's atomic clocks have a much more constant rate than the Earth, its star clock is still used to determine mean solar time. Since sometime in the late 1900s, Earth's rotation has been defined relative to an ensemble of extra-galactic radio sources and then converted to mean solar time by an adopted ratio. The difference between this calculated mean solar time and Coordinated Universal Time (UTC) is used to determine whether a leap second is needed.

See also


- Day
- LMT
- Time

External link


- [http://ptaff.ca/soleil/?lang=en_CA Sunrise and sunset and maximum sun altitude, all year long, anywhere]
- [http://theorderoftime.org/truetime/solartime.html Tempometer; calculation help to set a clock to the sun.]
- [http://www.theorderoftime.com/science/sun.html The Order of the Sun. Everything about consciously timing to the sun.] Category:Time scales ja:太陽時

Greeks

:For other uses of the name "Greek", see Greek (disambiguation) The Greeks are a nation and ethnic group, who have populated Greece from the 17th century BC until the present day.

Identity of the Greek people

17th century BC

Classical and Roman

Herodotus states that the Athenians declared, before the battle of Plataea, that they would not go over to Mardonius, because in the first place, they were bound to avenge the burning of the Acropolis; and, secondly, they would not betray their fellow Greeks, to whom they were bound by:
- A common language1 (the use of one of the dialects of the Greek language)
- Common blood2 (descent from Hellen, son of Deucalion)
- Common shrines, statues and sacrifices (practice of the ancient Greek religion)3 and
- Common habits and customs.  This notion that the Greeks had a common descent was then comparatively recent. As Thucydides observes, the name of Hellas spread from a valley in Thessaly to the Greek-speaking peoples after the formation of the text of Homer (the Panellenes of Il. 2.530 are the troops of Thessaly, contrasting with the Achaeans), not long before his own time. This places the idea in the Archaic period, when Greek-speakers discovered that the world was wider, wealthier, and more cultured than they had hitherto imagined. Homer's Trojan War is, indeed, a conflict among Greeks: the Trojans speak Greek, bear Greek names, and worship the Greek gods; and Priam is descended from Zeus (see Alaksandus). The Carians are the only people Homer considers barbarophonoi. Nor did the late and schematic myth of the sons of Hellen ever convince other mythographers to comply with it. Theseus is descended from Erechtheus, son of the Earth; Oedipus from the Phoenician Cadmus; Agamemnon from Phrygian Pelops; Heracles and Perseus from Egyptian Danaus. Whole cities were not descended from Hellen: Athens, Lemnos, and the Cretans were Pelasgian; and 1 Maccabees 12:21 attests that the Spartans are children of Abraham. The myth of Hellen combined into one group the smaller tribes that participated in the Delphic Amphictyon, such as the Aeolians, the Achaeans, and the Dorians. Traces of the older distinctions remained; Dorians were forbidden in the Parthenon; although the Spartan king Cleomenes I claimed this did not apply to him — as a descendant of Heracles, he was an Achaean. (As in this example, the Greeks almost always reckoned descent only through the male line.) So the exact nature of Greek identity has been an open question since ancient times. It has not become clearer with time: descent is at best a matter of tradition, and the Greeks have altered their language, religion, and customs since Herodotus. Nevertheless, there has been, in practice, a continuous Greek identity since ancient times, containing at le