:: wikimiki.org ::

1 E11 M

1 E11 m

To help compare distances at different orders of magnitude this page lists lengths starting at 10¹¹ metres (100 Gm or 100 million kilometres or 0.7 astronomical units). Distances shorter than 10¹¹ m
- 108 million km — 0.7 AU — Distance between Venus and the Sun
- 150 million km — 1.0 AU — Distance between the Earth and the Sun
- 228 million km — 1.5 AU — Distance between Mars and the Sun
- 290 million km — 1.9 AU — Minimum diameter of Betelgeuse
- 480 million km — 3.2 AU — Maximum diameter of Betelgeuse
- 591 million km — 4.0 AU — Minimum distance between the Earth and Jupiter
- 624 million km — 4.2 AU — Diameter of Antares
- 780 million km — 5.2 AU — Distance between Jupiter and the Sun
- 965 million km — 6.4 AU — Maximum distance between the Earth and Jupiter Distances longer than 10¹² m

Orders of magnitude

An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. The ratios most commonly used are 1000, 10, 2, 1024 or e (Euler's number, a transcendental number approximately equal to 2.71828182846 that is used as the base for natural logarithms). Usually, orders of magnitude refers to a series of powers of ten; this article discusses the decimal scale. Orders of magnitude are generally used to make very approximate comparisons. If two numbers differ by one order of magnitude, one is about ten times larger than the other. If they differ by two orders of magnitude, they differ by a factor of about 100. Two numbers of the same order of magnitude have roughly the same scale: the larger value is less than ten times the smaller value. The order of magnitude of a number is, intuitively speaking, the number of powers of 10 contained in the number. More precisely, the order of magnitude of a number can be defined in terms of the decimal logarithm, usually as the integer part of the logarithm. For example, 4,000,000 has a logarithm of 6.602; its order of magnitude is 6. Thus, an order of magnitude is an approximate position on a logarithmic scale. An order of magnitude estimate of a variable whose precise value is unknown is an estimate rounded to the nearest power of ten. For example, an order of magnitude estimate for a variable between about 3 billion and 30 billion (such as the human population of the Earth) is 10 billion. An order of magnitude estimate is sometimes also called a zeroth order approximation. The pages in the table at right contain lists of items that are of the same order of magnitude in various units of measurement. This is useful for getting an intuitive sense of the comparative scale of familiar objects. SI units are used together with SI prefixes, which were devised with orders of magnitude in mind.

Extremely large numbers

For extremely large numbers, a generalized order of magnitude can be based on their double logarithm or super-logarithm. Rounding these downward to an integer gives categories between very "round numbers", rounding them to the nearest integer and applying the inverse function gives the "nearest" round number. The first gives rise to the categories :..., 1.023-1.26, 1.26-10, 10-1e10, 1e10-1e100, 1e100-1e1000, etc. (the first two mentioned, and the extension to the left, may not be very useful, the two just demonstrate how the sequence mathematically continues to the left). The second gives rise to the categories :negative numbers, 0-1, 1-10, 10-1e10, 1e10-10^1e10, 10^1e10-10^^4, 10^^4-10^^5, etc. (see tetration). The "midpoints" which determine which round number is nearer are in the first case: :1.076, 2.071, 1453, 4.20e31, 1.69e316,... and, depending on the interpolation method, in the second case :-.301, .5, 3.162, 1453, 1e1453, 10^1e1453, 10^^2@1e1453,... (See notation of extremely large numbers.) For extremely small numbers (in the sense of close to zero) neither method is suitable directly, but of course the generalized order of magnitude of the reciprocal can be considered. Similar to the logarithmic scale one can have a double logarithmic and super-logarithmic scale. The intervals above all have the same length on them, with the "midpoints" actually midway. More generally, a point midway between two points corresponds to the generalised f-mean with f(x) the corresponding function log log x or slog x. In the case of log log x, this mean of two numbers (e.g. 2 and 16 giving 4) does not depend on the base of the logarithm, just like in the case of log x (geometric mean, 2 and 8 giving 4), but unlike in the case of log log log x (4 and 65536 giving 16 if the base is 2, but different otherwise).

External links

- [http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/index.html Powers of 10], a graphic animated illustration that starts with a view of the Milky Way at 10²³ meters and ends with subatomic particles at 10^-16 meters.
- [http://www.alcyone.com/max/physics/orders/metre.html Orders of Magnitude - Distance]
- [http://www.vendian.org/envelope/TemporaryURL/what_is_oom.html What is Order of Magnitude?]
- ko:규모의 비교 ja:数量の比較

Metre

:This article is about the unit of length. For other uses of metre or meter, see meter (disambiguation). The metre (Commonwealth English) or meter (American English) (symbol: m) is the SI base unit of length. It is defined as the length of the path travelled by light in absolute vacuum during a time interval of 1/299,792,458 of a second. Adding SI prefixes to metre creates multiples and submultiples; for example kilometre (1000 metres; kilo- = 1000) and millimetre (one thousandth of a metre; milli- = 1 / 1 000).

Conversions

1 metre is equivalent to:
- exactly 1/0.9144 yards (approximately 1.0936 yards)
- exactly 1/0.3048 feet (approximately 3.2808 feet)
- exactly 10000/254 inches (approximately 39.370 inches)

History

The word metre is from the Greek metron (μετρον), "a measure" via the French mètre. Its first recorded usage in English is from 1797. In the 18th century, there were two favoured approaches to the definition of the standard unit of length. One suggested defining the metre as the length of a pendulum with a half-period of one second. The other suggested defining the metre as one ten-millionth of the length of the earth's meridian along a quadrant (one-fourth the polar circumference of the earth). In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because of the slight variation of the force of gravity over the surface of the earth, which affects the period of a pendulum. In 1793, France adopted the metre, with this definition, as its official unit of length. Although it was later determined that the first prototype metre bar was short by a fifth of a millimetre due to miscalculation of the flattening of the earth, this length became the standard. So, the circumference of the Earth through the poles is approximately forty million metres. Earth in a vacuum.]] In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France. This new organisation would preserve the new prototype metre and kilogram when constructed, distribute national metric prototypes, and would maintain comparisons between them and non-metric measurement standards. This organisation created a new prototype bar in 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice. In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of distance. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new SI system as equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889. To further reduce uncertainty, the seventeenth CGPM of 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of time and the speed of light: :The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. Note that this definition exactly fixes the speed of light in a vacuum at 299,792,458 metres per second. Definitions based on the physical properties of light are more precise and reproducible because the properties of light are considered to be universally constant.

Timeline of definition

- 1790 May 8 — The French National Assembly decides that the length of the new metre would be equal to the length of a pendulum with a half-period of one second.
- 1791 March 30 — The French National Assembly accepts the proposal by the French Academy of Sciences that the new definition for the metre be equal to one ten-millionth of the length of the earth's meridian along a quadrant (one-fourth the polar circumference of the earth).
- 1795 — Provisional metre bar constructed of brass.
- 1799 December 10 — The French National Assembly specifies that the platinum metre bar, constructed on 23 June 1799 and deposited in the National Archives, as the final standard.
- 1889 September 28 — The first CGPM defines the length as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium, measured at the melting point of ice.
- 1927 October 6 — The seventh CGPM adjusts the definition of the length to be the distance, at 0 °C, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmosphere of pressure and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other.
- 1960 October 20 — The eleventh CGPM defines the length to be equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the 2p¹⁰ and 5d⁵ quantum levels of the krypton-86 atom.
- 1983 October 21 — The seventeenth CGPM defines the length to be distance travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

External links

- [http://www.unitconversion.org/unit_converter/length.html?unit=meter&value=1 Length Converter: convert metre to other units, such as yard, mile, and so on]
- [http://physics.nist.gov/cuu/Units/meter.html History of the metre at the U.S. National Institute of Standards and Technology (NIST)]
- [http://www.mel.nist.gov/div821/museum/timeline.htm Timeline of history of the metre at the NIST]
- [http://www1.bipm.org/en/scientific/length/ Bureau International des Poids et Measures - Lengths] Category:SI base units Category:Units of length ko:미터 ms:Meter ja:メートル simple:Metre th:เมตร

Gigametre

Conversions

1 metre is equivalent to:
- exactly 1/0.9144 yards (approximately 1.0936 yards)
- exactly 1/0.3048 feet (approximately 3.2808 feet)
- exactly 10000/254 inches (approximately 39.370 inches)

History

Timeline of definition

External links

Kilometre

A kilometre (American spelling: kilometer), symbol: km is a unit of length in the metric system equal to 1000 metres (from the Greek words χίλια (khilia) = thousand and μέτρο (metro) = count/measure). It is approximately equal to 0.621 miles, 1094 yards or 3281 feet. Slang terms for kilometre include "klick" (sometimes spelt "click" or "klik") and "kay" (or "k"). All these slang terms can also refer to kilometres per hour.

Metric system

:Main articles: Metric system and Metre Like the kilometre, all units of length in the metric system are based on the metre, by adding an SI prefix that stands for a power of ten, such as hecto for one hundred to form hectometre (= 0.1 kilometre) or mega for one million to form megametre (= 1,000 kilometre). The metre is not only the basis for all units of length in the metric system, but also of units of area (the square metre) and volume (the cubic metre). This extends to the kilometre, so one can have square and cubic kilometres. Unicode has symbols for "km" (㎞), for square kilometre (㎢) and for cubic kilometre (㎦); however, they are useful only in CJK texts, where they are equal in size to one Chinese character.

Pronunciation

In theory, the pronunciation of the word kilometre should have the stress placed on the first syllable, in line with other metric prefixes (as in kilogram, kilojoule and, analogous, kilobyte). However, pronunciation with the stress on the second syllable is usual in English.

Astronomical unit

The astronomical unit (AU or au or a.u. or sometimes ua) is a unit of distance, approximately equal to the mean distance between Earth and Sun. The currently accepted value of the AU is 149 597 870 691 ± 30 metres (about 150 million kilometres or 93 million miles). The symbol "ua" is recommended by the Bureau International des Poids et Mesures [http://www.bipm.org/en/si/si_brochure/chapter4/table7.html], but in the United States and other anglophone countries the reverse usage is more common. The International Astronomical Union recommends "au" [http://www.iau.org/IAU/Activities/nomenclature/units.html] and international standard ISO 31-1 uses "AU".

The distance

Earth's orbit is not a circle but an ellipse; originally, the AU was defined as the length of the semimajor axis of said orbit. For greater precision, the International Astronomical Union in 1976 defined the AU as the distance from the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.256 898 3 days (a Gaussian year). More accurately, it is the distance such that the heliocentric gravitational constant (the product GM_☉) is equal to (0.017 202 098 95)² AU³/d². At the time the AU was introduced, its actual value was very poorly known, but planetary distances in terms of AU could be determined from heliocentric geometry and Kepler's laws of planetary motion. The value of the AU was first estimated by Jean Richer and Giovanni Domenico Cassini in 1672. By measuring the parallax of Mars from two locations on the Earth, they arrived at a figure of about 140 million kilometers. The first good measurement on the distance between Earth and the Sun was made by Eratosthenes in around 200 BC. By studying lunar eclipses, his result was 804 000 000 stadia. If we use the common Attic stadion this translates to roughly 150 million km. A somewhat more accurate estimate can be obtained by observing the transit of Venus. This method was devised by Edmond Halley, and applied to the transits of Venus observed in 1761 and 1769, and then again in 1874 and 1882. Another method involved determining the constant of aberration, and Simon Newcomb gave great weight to this method when deriving his widely accepted value of 8.80" for the solar parallax (close to the modern value of 8.794 148"). The discovery of the near-Earth asteroid 433 Eros and its passage near the Earth in 1900–1901 allowed a considerable improvement in parallax measurement. More recently very precise measurements have been carried out by radar and by telemetry from space probes. While the value of the astronomical unit is now known to great precision, the value of the mass of the Sun is not, because of uncertainty in the value of the gravitational constant. Because the gravitational constant is known to only five or six significant digits while the positions of the planets are known to 11 or 12 digits, calculations in celestial mechanics are typically performed in solar masses and astronomical units rather than in kilograms and kilometres. This approach makes all results dependent on the gravitational constant. A conversion to SI units would separate the results from the gravitational constant, at the cost of introducing additional uncertainty by assigning a specific value to that unknown constant. It is known that the mass of the Sun is very slowly decreasing, and therefore the orbital period of a body at a given distance is increasing. This implies that the AU is getting smaller (by about one centimetre per year) over time.

Examples

The distances are approximate mean distances. It has to be taken into consideration that the distances between celestial bodies change in time due to their orbits and other factors.
- The Earth is 1.00 ± 0.02 AU from the Sun.
- The Moon is 0.0026 ± 0.0001 AU from the Earth.
- Mars is 1.52 ± 0.14 AU from the Sun.
- Jupiter is 5.20 ± 0.05 AU from the Sun.
- Pluto is 39.5 ± 9.8 AU from the Sun.
- 90377 Sedna's orbit ranges between 76 and 942 AU from the Sun; Sedna is currently (2005) about 90 AU from the Sun.
- As of November 2005, Voyager 1 (the farthest human-made object) is 97 AU from the Sun.
- The mean diameter of the Solar system, including the Oort cloud, is approximately 10⁵ AU.
- Proxima Centauri (the nearest star) is ~268 000 AU away from the Sun.
- The mean diameter of Betelgeuse is 2.57 AU.
- The distance from the Sun to the centre of the Milky Way is approximately 1.7×10⁹ AU. Some conversion factors:
- 1 AU = 149 597 870.691 ± 0.030 km ≈ 92 955 807 miles ≈ 8.317 light minutes ≈ 499 light-seconds
- 1 light-second ≈ 0.002 AU
- 1 light-minute ≈ 0.120 AU
- 1 light-hour ≈ 7.214 AU
- 1 light-day ≈ 173 AU
- 1 light-year ≈ 63 241 AU
- 1 pc ≈ 206 265 AU

References

- E. Myles Standish. "Report of the IAU WGAS Sub-group on Numerical Standards". In Highlights of Astronomy, I. Appenzeller, ed. Dordrecht: Kluwer Academic Publishers, 1995. (Complete report available online: [http://ssd.jpl.nasa.gov/iau-comm4/iausgnsrpt.ps PostScript]. Tables from the report also available: [http://ssd.jpl.nasa.gov/astro_constants.html Astrodynamic Constants and Parameters])
- D. D. McCarthy ed., IERS Conventions (1996), IERS Technical Note 21, Observatoire de Paris, July 1996

External links

- [http://physics.nist.gov/cuu/Units/outside.html Units outside the SI] (at the NIST web site)
- [http://www.iau.org/IAU/Activities/nomenclature/units.html Recommendations concerning Units] (at the IAU web site)
- [http://home.comcast.net/~pdnoerd/SMassLoss.html Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System] (a discussion of the relation between the AU and other quantities)
- [http://www.ex.ac.uk/trol/scol/ccleng.htm Conversion Calculator for Units of LENGTH] Category:Celestial mechanics Category:Astronomical units of length ko:천문 단위 ja:天文単位 th:หน่วยดาราศาสตร์ zh-min-nan:Thian-bûn tan-ūi

Astronomical unit

The distance

Examples

References

External links

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

Astronomical unit

The distance

Examples

References

External links

Mars/Planet

Mars, the fourth planet from the Sun in our solar system, is named after the Roman god of war Mars (Ares in Greek mythology), because of its apparent red color. This feature also earned it the nickname "The Red Planet". Mars has two moons, Phobos and Deimos, which are small and oddly-shaped, possibly being captured asteroids. The prefix areo- refers to Mars in the same way geo- refers to Earth—for example, areology versus geology. (However, areology is also used to refer to the study of Mars as a whole rather than just the geological processes of the planet.) The astronomical symbol for Mars is a circle with an arrow pointing northeast (Unicode: ♂). This symbol is a stylized representation of the shield and spear of the god Mars, and in biology it is used as a sign for the male sex. The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as the fire star, 火星, a naming based on the ancient Chinese mythological cycle of Five Elements.

Mythology

Mars has been obvious to skygazers since prehistoric times. It was known by the Egyptians as "Her Deschel" or "the Red One." Among the Babylonians Mars was known as "Nergal" or "the Star of Death." The Romans were the ones to give Mars its modern name, after their god of war.

Physical characteristics

The red, fiery appearance of Mars is caused by iron oxide (rust) on its surface. Mars has only a quarter the surface area of the Earth and only one-tenth the mass, though its surface area is approximately equal to that of the Earth's dry land because Mars lacks oceans. The solar day (or sol) on Mars is very close to Earth's day: 24 hours, 39 minutes, and 35.244 seconds.

Atmosphere

Mars' atmosphere is thin: the air pressure on the surface is only 750 pascals, about 0.75% of the average on Earth. However, the scale height of the atmosphere is about 11 km, somewhat higher than Earth's 6 km. The atmosphere on Mars is 95% carbon dioxide, 3% nitrogen, 1.6% argon, and contains traces of oxygen and water. The atmosphere quite dusty, giving the Martian sky a tawny color when seen from the surface; data from the Mars Exploration Rovers indicates the suspended dust particles are roughly 1.5 microns across. In 2003, methane was apparently discovered in the atmosphere by Earth-based telescopes and possibly confirmed in March 2004 by the Mars Express Orbiter; present measurements state an average methane concentration of about 11±4 ppb by volume (see reference). The thin atmosphere cannot hold heat and is the cause of the lower temperatures on Mars. The maximum temperature is roughly 20℃ (68℉). The presence of methane on Mars would be very intriguing, since as an unstable gas it indicates that there must be (or have been within the last few hundred years) a source of the gas on the planet. Volcanic activity, comet impacts, and the existence of life in the form of microorganisms such as methanogens are among possible but as yet unproven sources. The methane appears to occur in patches, which suggests that it is being rapidly broken down before it has time to become uniformly distributed in the atmosphere, and so it is presumably also continually being released to the atmosphere. Plans are now being made to look for other companion gases that may suggest which sources are most likely; in the Earth's oceans biological methane production tends to be accompanied by ethane, while volcanic methane is accompanied by sulfur dioxide. Other aspects of the Martian atmosphere vary significantly. In the winter months when the poles are in continual darkness, the surface gets so cold that as much as 25% of the entire atmosphere condenses out into meters thick slabs of CO₂ ice (dry ice). When the poles are again exposed to sunlight the CO₂ ice sublimates, creating enormous winds that sweep off the poles as fast as 250 mph. These seasonal actions transport large amounts of dust and water vapor giving rise to Earth-like frost and large cirrus clouds. These clouds of water-ice were photographed by the Opportunity rover in 2004.[http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20041213a/merb_sol290_clouds-B313R1_br.jpg] Recently, evidence has been discovered suggesting that Mars may be warming in the short term[http://news.bbc.co.uk/2/hi/science/nature/4266474.stm]; however, it is now cooler than it was in the 1970s.[http://catdynamics.blogspot.com/2005/09/climate-science-mars-and-politics.html]

Geology

Opportunity The surface of Mars is thought to be primarily composed of basalt, based upon the Martian meteorite collection and orbital observations. There is some evidence that some portion of the Martian surface might be more silica-rich than typical basalt, perhaps similar to andesitic rocks on Earth, though these observations may also be explained by silica glass. Much of the surface is deeply covered by dust as fine as talcum powder. Observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft have revealed that parts of the planet's crust has been magnetized. This magnetization has been compared to alternating bands found on the ocean floors of Earth. One interesting theory, published in 1999 and reexamined in October 2005 in a publication by the same group, is that these bands could be evidence of the past operation of plate tectonics on Mars. However, this has yet to be proven [http://photojournal.jpl.nasa.gov/catalog/PIA02008] or widely accepted and remains an area of active research. plate tectonics Amongst the findings from the Opportunity rover is the presence of hematite on Mars in the form of small spheres on the Meridiani Planum. The spheres are only a few millimeters in diameter and are believed to have formed as rock deposits under watery conditions billions of years ago. Other minerals have also been found containing forms of sulfur, iron or bromine such as jarosite. This and other evidence led a group of 50 scientists to conclude in the December 9, 2004 edition of the journal Science that "Liquid water was once intermittently present at the Martian surface at Meridiani, and at times it saturated the subsurface. Because liquid water is a key prerequisite for life, we infer conditions at Meridiani may have been habitable for some period of time in Martian history". On the opposite side of the planet the mineral goethite, which (unlike hematite) forms only in the presence of water, along with other evidence of water, has also been found by the Spirit rover in the "Columbia Hills". In 1996, researchers studying a meteorite (ALH84001) believed to have originated from Mars reported features which they attributed to microfossils left by life on Mars. As of 2005, this interpretation remains controversial with no consensus having emerged.
Topography
As of 2005 As of 2005 The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. The surface of Mars as seen from Earth is consequently divided into two kinds of areas, with differing albedo. The paler plains covered with dust and sand rich in reddish iron oxides were once thought of as Martian 'continents' and given names like Arabia Terra (land of Arabia) or Amazonis Planitia (Amazonian plain). The dark features were thought to be seas, hence their names Mare Erythraeum, Mare Sirenum and Aurorae Sinus. The largest dark feature seen from Earth is Syrtis Major. Syrtis Major Mars has polar ice caps that contain frozen water and carbon dioxide that change with the Martian seasons — the carbon dioxide ice sublimates in summer it uncovers an underlying surface of layered water ice and dust. The polar carbon dioxide "hood" then forms again in winter. The supposedly-extinct shield volcano, Olympus Mons (Mount Olympus), is at 26 km the highest mountain in the solar system. It is in a vast upland region called Tharsis, which contains several large volcanos. See list of mountains on Mars. Mars also has the solar system's largest canyon system, Valles Marineris or the Mariner Valley, which is 4000 km long and 7 km deep. Mars is also scarred by a number of impact craters. The largest of these is the Hellas impact basin, covered with light red sand. See list of craters on Mars. The difference between Mars' highest and lowest points is nearly 31 km (from the top of Olympus Mons at an altitude of 26 km to the bottom of the Hellas impact basin at an altitude of 4 km below the datum). In comparison, the difference between Earth's highest and lowest points (Mount Everest and the Mariana Trench) is only 19.7 km. Combined with the planets' different radii, this means Mars is nearly three times "rougher" than Earth. The International Astronomical Union's Working Group for Planetary System Nomenclature is responsible for naming Martian surface features. Other notes: Zero elevation: Since Mars has no oceans and hence no 'sea level', a zero-elevation surface or mean gravity surface must be selected. The datum for Mars is defined by the fourth-degree and fourth-order spherical harmonic gravity field, with the zero altitude defined by the 610.5 Pa (6.105 mbar) atmospheric pressure surface (approximately 0.6% of Earth's) at a temperature of 273.16 K. This pressure and temperature correspond to the triple point of water. Zero meridian: Mars' equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's, by choice of an arbitrary point which was accepted by later observers. The German astronomers Wilhelm Beer and Johann Heinrich Mädler selected a small circular feature as a reference point when they produced the first systematic chart of Mars features in 1830-32. In 1877, their choice was adopted as the prime meridian by the Italian astronomer Giovanni Schiaparelli when he began work on his notable maps of Mars. After the spacecraft Mariner 9 provided extensive imagery of Mars in 1972, a small crater (later called Airy-0), located in the Sinus Meridiani ('Middle Bay' or 'Meridian Bay') along the line of Beer and Mädler, was chosen by Merton Davies of the RAND Corporation to provide a more precise definition of 0.0° longitude when he established a planetographic control point network. RAND Corporation
Canals
Mars has an important place in human imagination due to the belief by some that life existed on Mars. These beliefs are due mainly to observations by many in the 19th century popularized by Percival Lowell and Giovanni Schiaparelli. Schiaparelli called these observed features canali, meaning channels in Italian. This was popularly mistranslated as 'canals', and the myth of the Martian canals began. They were apparently artificial linear features on the surface that were asserted to be canals, and due to seasonal changes in the brightness of some areas that were thought to be caused by vegetation growth. This gave rise to many stories concerning Martians. The linear features are now known to be mostly non-existent or, in some cases, dry ancient watercourses. The color changes have been ascribed to dust storms.
Ice lakes
many stories On 29 July 2005, the BBC reported that a visible ice lake had been discovered in a crater in the north polar region of Mars[http://news.bbc.co.uk/1/hi/sci/tech/4727847.stm]. Images of the crater, taken by the High Resolution Stereo Camera on board the European Space Agency's Mars Express spacecraft, clearly show a broad sheet of ice in the bottom of an unnamed crater located on Vastitas Borealis, a broad plain that covers much of Mars' far northern latitudes, at approximately 70.5° North and 103° East. The crater is 35 km (23 mi) wide and about 2 km (1.2 mi) deep. The BBC report however, appears to have either intentionally sensationalized or unintentionally mis-interpreted the original HRSC/Mars Express feature[http://www.esa.int/SPECIALS/Mars_Express/SEMGKA808BE_0.html], which makes no claim or insinuation that this is a "lake". Like many thousands of other places on Mars, this ice sheet is a thin layer of frost that has condensed onto dark, cold sand dunes (about 200 m high) making their way across the bottom of the crater. The only thing remarkable about this feature is that it is far enough north to maintain at least some frost throughout the year.
The moons of Mars
Mars has two tiny natural moons, Phobos and Deimos, which orbit very close to the planet and are

1 E11 m

See also

Orders of magnitude

Extremely large numbers

See also

External links

Metre

Conversions

History

Timeline of definition

See also

External links

Gigametre

Conversions

History

Timeline of definition

See also

External links

Kilometre

Metric system

Pronunciation

See also

Astronomical unit

The distance

Examples

See also

References

External links

Astronomical unit

The distance

Examples

See also

References

External links

Sun

General information

Structure

Core

Radiation zone

Convection zone

Photosphere

Temperature minimum

Chromosphere

Corona

Theoretical problems

Solar neutrino problem

Coronal heating problem

Faint young sun problem

Magnetic field

Position of the Sun through the year

Solar space missions

History and future of the Sun

Human understanding of the Sun

The Sun as a power source

Sun and eye damage

External links

References

Astronomical unit

The distance

Examples

See also

References

External links

Mars/Planet

Mythology

Physical characteristics

Atmosphere

Geology

Topography

Canals

Ice lakes

The moons of Mars