ROCK STORY

Saturday, 22 December 2012

LIGHT

Visible light (commonly referred to simply as light) is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight.^[1] Visible light has a wavelength in the range of about 380 nanometres to about 740 nm – between the invisible infrared, with longer wavelengths and the invisible ultraviolet, with shorter wavelengths.
Primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarisation, while its speed in a vacuum, 299,792,458 meters per second (about 300,000 kilometers per second), is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in vacuum.
In common with all types of EMR, visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.
In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.^[2]^[3] This article focuses on visible light. See the electromagnetic radiation article for the general term.

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Speed of visible light

Main article: Speed of light

The speed of light in a vacuum is defined to be exactly 299,792,458 m/s (approximately 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation are believed to move at exactly this same speed in vacuum.
Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit.^[4] However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s.
Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating cog wheel was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau was able to calculate the speed of light as 313,000,000 m/s.
Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s.
The effective velocity of light in various transparent substances containing ordinary matter, is less than in vacuum. For example the speed of light in water is about 3/4 of that in vacuum. However, the slowing process in matter is thought to result not from actual slowing of particles of light, but rather from their absorption and re-emission from charged particles in matter.
As an extreme example of the nature of light-slowing in matter, two independent teams of physicists were able to bring light to a "complete standstill" by passing it through a Bose-Einstein Condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other at the Harvard-Smithsonian Center for Astrophysics, also in Cambridge.^[5] However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrary later time, as stimulated by a second laser pulse. During the time it had "stopped" it had ceased to be light.

Electromagnetic spectrum and visible light

Main article: Electromagnetic spectrum

Electromagnetic spectrum with light highlighted

Generally, EM radiation, or EMR (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region that we perceive as light, ultraviolet, X-rays and gamma rays.
The behaviour of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EMR interacts with single atoms and molecules, its behaviour depends on the amount of energy per quantum it carries.
EMR in the visible light region consists of quanta (called photons) that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which lead to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humansinfrared because its photons no longer have enough individual energy to cause a lasting molecular change (a change in conformation) in the visual molecule retinal in the human retina. This change triggers the sensation of vision.
There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. Infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, and this is how living animals detect it.
Above the range of visible light, ultraviolet light becomes invisible to humans mostly because it is absorbed by the tissues of the eye and in particular the lens. Humans with natural eye lenses removed, as well as many animals with eyes that do not require lenses (such as insects and shrimp) are able to directly detect ultraviolet visually, by quantum photon-absorption mechanisms, in much the same chemical way that normal humans detect visible light.

Optics

Main article: Optics

The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows and the aurora borealis offer many clues as to the nature of light.

Refraction

Main article: Refraction

An example of refraction of light. The straw appears bent, because of refraction of light as it enters liquid from air.

A cloud illuminated by sunlight

Refraction is the bending of light rays when passing through a surface between one transparent material and another. It is described by Snell's Law:

$n_1\sin\theta_1 = n_2\sin\theta_2\ .$

where $\theta_1$ is the angle between the ray and the surface normal in the first medium, $\theta_2$ is the angle between the ray and the surface normal in the second medium, and n₁ and n₂ are the indices of refraction, n = 1 in a vacuum and n > 1 in a transparent substance.
When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal (or rather normal) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as refraction.
The refractive quality of lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses, spectacles, contact lenses, microscopes and refracting telescopes are all examples of this manipulation.

Light sources

Units and measures

Main articles: Photometry (optics) and Radiometry

Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardised model of human brightness perception. Photometry is useful, for example, to quantify Illumination (lighting) intended for human use. The SI units for both systems are summarised in the following tables.

Table 1. SI radiometry units

Quantity	Symbol^{[nb 1]}	SI unit	Symbol	Dimension	Notes
Radiant energy	Q_e^{[nb 2]}	joule	J	M⋅L²⋅T⁻²	energy
Radiant flux	Φ_e^{[nb 2]}	watt	W	M⋅L²⋅T⁻³	radiant energy per unit time, also called radiant power.
Spectral power	Φ_eλ^{[nb 2]}^{[nb 3]}	watt per metre	W⋅m⁻¹	M⋅L⋅T⁻³	radiant power per wavelength.
Radiant intensity	I_e	watt per steradian	W⋅sr⁻¹	M⋅L²⋅T⁻³	power per unit solid angle.
Spectral intensity	I_eλ^{[nb 3]}	watt per steradian per metre	W⋅sr⁻¹⋅m⁻¹	M⋅L⋅T⁻³	radiant intensity per wavelength.
Radiance	L_e	watt per steradian per square metre	W⋅sr⁻¹⋅m⁻²	M⋅T⁻³	power per unit solid angle per unit projected source area. confusingly called "intensity" in some other fields of study.
Spectral radiance	L_eλ^{[nb 3]} or L_eν^{[nb 4]}	watt per steradian per metre³ or watt per steradian per square metre per hertz	W⋅sr⁻¹⋅m⁻³ or W⋅sr⁻¹⋅m⁻²⋅Hz⁻¹	M⋅L⁻¹⋅T⁻³ or M⋅T⁻²	commonly measured in W⋅sr⁻¹⋅m⁻²⋅nm⁻¹ with surface area and either wavelength or frequency.
Irradiance	E_e^{[nb 2]}	watt per square metre	W⋅m⁻²	M⋅T⁻³	power incident on a surface, also called radiant flux density. sometimes confusingly called "intensity" as well.
Spectral irradiance	E_eλ^{[nb 3]} or E_eν^{[nb 4]}	watt per metre³ or watt per square metre per hertz	W⋅m⁻³ or W⋅m⁻²⋅Hz⁻¹	M⋅L⁻¹⋅T⁻³ or M⋅T⁻²	commonly measured in W⋅m⁻²⋅nm⁻¹ or 10⁻²²W⋅m⁻²⋅Hz⁻¹, known as solar flux unit.^{[nb 5]}
Radiant exitance / Radiant emittance	M_e^{[nb 2]}	watt per square metre	W⋅m⁻²	M⋅T⁻³	power emitted from a surface.
Spectral radiant exitance / Spectral radiant emittance	M_eλ^{[nb 3]} or M_eν^{[nb 4]}	watt per metre³ or watt per square metre per hertz	W⋅m⁻³ or W⋅m⁻²⋅Hz⁻¹	M⋅L⁻¹⋅T⁻³ or M⋅T⁻²	power emitted from a surface per wavelength or frequency.
Radiosity	J_e or J_eλ^{[nb 3]}	watt per square metre	W⋅m⁻²	M⋅T⁻³	emitted plus reflected power leaving a surface.
Radiant exposure	H_e	joule per square metre	J⋅m⁻²	M⋅T⁻²
Radiant energy density	ω_e	joule per metre³	J⋅m⁻³	M⋅L⁻¹⋅T⁻²
See also: SI ·Radiometry ·Photometry · (Compare)

Table 2. SI photometry units

Quantity	Symbol^{[nb 6]}	SI unit	Symbol	Dimension	Notes
Luminous energy	Q_v ^{[nb 7]}	lumen second	lm⋅s	T⋅J ^{[nb 8]}	units are sometimes called talbots
Luminous flux	Φ_v ^{[nb 7]}	lumen (= cd⋅sr)	lm	J	also called luminous power
Luminous intensity	I_v	candela (= lm/sr)	cd	J	an SI base unit, luminous flux per unit solid angle
Luminance	L_v	candela per square metre	cd/m²	L⁻²⋅J	units are sometimes called nits
Illuminance	E_v	lux (= lm/m²)	lx	L⁻²⋅J	used for light incident on a surface
Luminous emittance	M_v	lux (= lm/m²)	lx	L⁻²⋅J	used for light emitted from a surface
Luminous exposure	H_v	lux second	lx⋅s	L⁻²⋅T⋅J
Luminous energy density	ω_v	lumen second per metre³	lm⋅s⋅m⁻³	L⁻³⋅T⋅J
Luminous efficacy	η ^{[nb 7]}	lumen per watt	lm/W	M⁻¹⋅L⁻²⋅T³⋅J	ratio of luminous flux to radiant flux
Luminous efficiency	V			1	also called luminous coefficient
See also: SI ·Photometry ·Radiometry · (Compare)

The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. The cone cells in the human eye are of three types which respond differently across the visible spectrum, and the cumulative response peaks at a wavelength of around 555 nm. Therefore, two sources of light which produce the same intensity (W/m²) of visible light do not necessarily appear equally bright. The photometry units are designed to take this into account, and therefore are a better representation of how "bright" a light appears to be than raw intensity. They relate to raw power by a quantity called luminous efficacy, and are used for purposes like determining how to best achieve sufficient illumination for various tasks in indoor and outdoor settings. The illumination measured by a photocell sensor does not necessarily correspond to what is perceived by the human eye, and without filters which may be costly, photocells and charge-coupled devices (CCD) tend to respond to some infrared, ultraviolet or both.

Light pressure

Main article: Radiation pressure

Light exerts physical pressure on objects in its path, a phenomenon which can be deduced by Maxwell's equations, but can be more easily explained by the particle nature of light: photons strike and transfer their momentum. Light pressure is equal to the power of the light beam divided by c, the speed of light. Due to the magnitude of c, the effect of light pressure is negligible for everyday objects. For example, a one-milliwatt laser pointer exerts a force of about 3.3 piconewtons on the object being illuminated; thus, one could lift a U. S. penny with laser pointers, but doing so would require about 30 billion 1-mW laser pointers.^[8] However, in nanometer-scale applications such as NEMS, the effect of light pressure is more significant, and exploiting light pressure to drive NEMS mechanisms and to flip nanometer-scale physical switches in integrated circuits is an active area of research.^[9]
At larger scales, light pressure can cause asteroids to spin faster,^[10] acting on their irregular shapes as on the vanes of a windmill. The possibility to make solar sails that would accelerate spaceships in space is also under investigation.^[11]^[12]
Although the motion of the Crookes radiometer was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum.^[13] This should not be confused with the Nichols radiometer, in which the (slight) motion caused by torque (though not enough for full rotation against friction) is directly caused by light pressure.^[14]

Historical theories about light, in chronological order

Classical Greece and Hellenism

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In the fifth century BC, Empedocles postulated that everything was composed of four elements; fire, air, earth and water. He believed that Aphrodite made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.
In about 300 BC, Euclid wrote Optica, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asks how one sees the stars immediately, if one closes one's eyes, then opens them at night. Of course if the beam from the eye travels infinitely fast this is not a problem.
In 55 BC, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote:
"The light & heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove." – On the nature of the Universe
Despite being similar to later particle theories, Lucretius's views were not generally accepted.
Ptolemy (c. 2nd century) wrote about the refraction of light in his book Optics.^[15]

Classical India

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In ancient India, the Hindu schools of Samkhya and Vaisheshika, from around the early centuries CE developed theories on light. According to the Samkhya school, light is one of the five fundamental "subtle" elements (tanmatra) out of which emerge the gross elements. The atomicity of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.
On the other hand, the Vaisheshika school gives an atomic theory of the physical world on the non-atomic ground of ether, space and time. (See Indian atomism.) The basic atoms are those of earth (prthivi), water (pani), fire (agni), and air (vayu) Light rays are taken to be a stream of high velocity of tejas (fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the tejas atoms.^{[citation needed]} The Vishnu Purana refers to sunlight as "the seven rays of the sun".^{[citation needed]}
The Indian Buddhists, such as Dignāga in the 5th century and Dharmakirti in the 7th century, developed a type of atomism that is a philosophy about reality being composed of atomic entities that are momentary flashes of light or energy. They viewed light as being an atomic entity equivalent to energy.^{[citation needed]}

Descartes

René Descartes (1596–1650) held that light was a mechanical property of the luminous body, rejecting the "forms" of Ibn al-Haytham and Witelo as well as the "species" of Bacon, Grosseteste, and Kepler.^[16] In 1637 he published a theory of the refraction of light that assumed, incorrectly, that light travelled faster in a denser medium than in a less dense medium. Descartes arrived at this conclusion by analogy with the behaviour of sound waves.^{[citation needed]} Although Descartes was incorrect about the relative speeds, he was correct in assuming that light behaved like a wave and in concluding that refraction could be explained by the speed of light in different media.
Descartes is not the first to use the mechanical analogies but because he clearly asserts that light is only a mechanical property of the luminous body and the transmitting medium, Descartes' theory of light is regarded as the start of modern physical optics.^[17]

Particle theory

Main article: Corpuscular theory of light

Pierre Gassendi.

Pierre Gassendi (1592–1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether.
Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to hold sway during the 18th century. The particle theory of light led Laplace to argue that a body could be so massive that light could not escape from it. In other words it would become what is now called a black hole. Laplace withdrew his suggestion later, after a wave theory of light became firmly established as the model for light (as has been explained, neither a particle or wave theory is fully correct). A translation of Newton's essay on light appears in The large scale structure of space-time, by Stephen Hawking and George F. R. Ellis.

Wave theory

In the 1660s, Robert Hooke published a wave theory of light. Christiaan Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the Luminiferous ether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.

Thomas Young's sketch of the two-slit experiment showing the diffraction of light. Young's experiments supported the theory that light consists of waves.

The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by Thomas Young), and that light could be polarised, if it were a transverse wave. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colours were caused by different wavelengths of light, and explained colour vision in terms of three-coloured receptors in the eye.
Another supporter of the wave theory was Leonhard Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory.
Later, Augustin-Jean Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory. By the year 1821, Fresnel was able to show via mathematical methods that polarisation could be explained only by the wave theory of light and only if light was entirely transverse, with no longitudinal vibration whatsoever.
The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminiferous aether was proposed, but its existence was cast into strong doubt in the late nineteenth century by the Michelson-Morley experiment.
Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850.^[18] His result supported the wave theory, and the classical particle theory was finally abandoned, only to partly re-emerge in the 20th century.

Quantum theory

In 1900 Max Planck, attempting to explain black body radiation suggested that although light was a wave, these waves could gain or lose energy only in finite amounts related to their frequency. Planck called these "lumps" of light energy "quanta" (from a Latin word for "how much." In 1905, Albert Einstein used the idea of light quanta to explain the photoelectric effect, and suggested that these light quanta had a "real" existence. In 1923 Arthur Holly Compton showed that the wavelength shift seen when low intensity X-rays scattered from electrons (so called Compton scattering) could be explained by a particle-theory of X-rays, but not a wave theory. In 1926 Gilbert N. Lewis named these liqht quanta particles photons.
Eventually the modern theory of quantum quantum mechanics came to picture light as (in some sense) both a particle and a wave, and (in another sense), as a phenomenon which is neither a particle or a wave (which actually are macroscopic phenomena, such as baseballs or ocean waves). Instead, modern physics sees light as something that can be described sometimes with mathematics appropriate to one type of macroscopic metaphor (particles), and sometimes another macroscopic metaphor (water waves), but is actually something that cannot be fully imagined. As in the case for radio waves and the X-rays involved in Compton scattering, physicists have noted that electromagnetic radiation tends to behave more like a classical wave at lower frequencies, but more like a classical particle at higher frequencies, but never completely loses all qualities of one or the other. Visible light, which occupies a middle ground in frequency, can easily be shown in experiments to be describable using either a wave or particle model, or sometimes both.

Electromagnetic theory as explanation for all types of visible light and all EM radiation

Main article: Electromagnetic radiation

A linearly polarised light wave frozen in time and showing the two oscillating components of light; an electric field and a magnetic field perpendicular to each other and to the direction of motion (a transverse wave).

In 1845, Michael Faraday discovered that the plane of polarisation of linearly polarised light is rotated when the light rays travel along the magnetic field direction in the presence of a transparent dielectric, an effect now known as Faraday rotation.^[19] This was the first evidence that light was related to electromagnetism. In 1846 he speculated that light might be some form of disturbance propagating along magnetic field lines.^[20] Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the ether.
Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in On Physical Lines of Force. In 1873, he published A Treatise on Electricity and Magnetism, which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as Maxwell's equations. Soon after, Heinrich Hertz confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory, and demonstrating that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction, and interference. Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging, and wireless communications.
In the quantum theory, photons are seen as wave packets of the waves described in the classical theory of Maxwell. The quantum theory was needed to explain effects even with visual light that Maxwell's classical theory could not (such as spectral lines).

Tuesday, 20 November 2012

CONSTELLATION

In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms (which themselves are generally referred to in non-technical language as "constellations"), which are patterns formed by prominent stars within apparent proximity to one another on Earth's night sky.

There are also numerous historical constellations not recognized by the IAU, or constellations recognized in regional traditions of astronomy or astrology, such as Chinese, Hindu and Australian Aboriginal.

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[edit] Terminology

The Late Latin term constellātiō can be translated as "set with stars". The term was first used in astrology, of asterisms that supposedly exerted influence, attested in Ammianus (4th century). In English the term was used from the 14th century, also in astrology, of conjunctions of planets. The modern astronomical sense of "area of the celestial sphere around a specific asterism" dates to the mid 16th century.
Colloquial usage does not distinguish the senses of "asterism" and "area surrounding an asterism". The modern system of constellations used in astronomy focuses primarily on constellations as grid-like segments of the celestial sphere rather than as patterns, while the term for a star-pattern is asterism. For example, the asterism known as the Big Dipper corresponds to the seven brightest stars of the larger IAU constellation of Ursa Major.
The term circumpolar constellation is used for any constellation that, from a particular latitude on Earth, never sets below the horizon. From the north pole, all constellations north of the celestial equator are circumpolar constellations. In the northern latitudes, the informal term equatorial constellation has sometimes used for constellations that lie to the south of the circumpolar constellations.^[1] Depending on the definition, equatorial constellations can include those that lie entirely between declinations 45° north and 45° south,^[2] or those that pass overhead between the tropics of Cancer and Capricorn. They generally include all constellations that intersect the celestial equator.

[edit] History

Further information: Former constellations and Star Names: Their Lore and Meaning

The current list of 88 constellations recognised by the International Astronomical Union since 1922 is based on the 48 listed by Ptolemy in his Almagest in the 2nd century.^[3]^[4] Ptolemy's catalogue is informed by Eudoxus of Cnidus, a Greek astronomer of the 4th century BC who introduced earlier Babylonian astronomy to the Hellenistic culture. Of the 48 constellations listed by Ptolemy, thirty can be shown to have a much longer history, reaching back into at least the Late Bronze Age. This concerns the zodiacal constellations in particular.

[edit] Ancient Near East

See also: Babylonian star catalogues and MUL.APIN

The oldest catalogues of stars and constellations are from Old Babylonian astronomy, beginning in the Middle Bronze Age. The numerous Sumerian names in these catalogues suggest that they build on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age. The classical Zodiac is a product of a revision of the Old Babylonian system in later Neo-Babylonian astronomy 6th century BC. Knowledge of the Neo-Babylonian zodiac is also reflected in the Hebrew Bible. E. W. Bullinger interpreted the creatures appearing in the books of Ezekiel (and thence in Revelation) as the middle signs of the four quarters of the Zodiac,^[5]^[6] with the Lion as Leo, the Bull is Taurus, the Man representing Aquarius and the Eagle standing in for Scorpio.^[7] The biblical Book of Job (dating to the 6th to 4th century BC) also makes reference to a number of constellations, including עיש `Ayish "bier", כסיל Kĕciyl "fool" and כימה Kiymah "heap" (Job 9:9, 38:31-32), rendered as "Arcturus, Orion and Pleiades" by the KJV, but `Ayish "the bier" actually corresponding to Ursa Major.^[8] The term Mazzaroth מַזָּרֹות, a hapax legomenon in Job 38:32, may be the Hebrew word for the zodiacal constellations.
The Greeks adopted the Babylonian system in the 4th century BC. A total of twenty Ptolemaic constellations are directly continued from the Ancient Near East. Another ten have the same stars but different names.^[9]

[edit] Graeco-Roman

^{[citation needed]}
There is only limited information on indigenous Greek constellations. Some evidence is found in Hesiod.^{[clarification needed]} Greek astronomy essentially adopted the older Babylonian system in the Hellenistic era, first introduced to Greece by Eudoxus of Cnidus in the 4th century BC. The original work of Eudoxus is lost, but it survives as a versification by Aratus, dating to the 3rd century BC. The most complete existing works dealing with the mythical origins of the constellations are by the Hellenistic writer termed pseudo-Eratosthenes and an early Roman writer styled pseudo-Hyginus.
The basis of western astronomy as taught during Late Antiquity and until the Early Modern period is the Almagest by Ptolemy, written in the 2nd century. Indian astronomy is also based on Hellenistic tradition, via transmission by the Indo-Greek kingdoms.

[edit] Classical Chinese constellations

Main article: Chinese constellation

Further information: Treatise on Astrology of the Kaiyuan Era

In classical Chinese astronomy, the northern sky is divided geometrically, into five "enclosures" and twenty-eight mansions along the ecliptic, grouped into Four Symbols of seven asterisms each. The 28 lunar mansions are one of the most important and also the most ancient structures in the Chinese sky, attested from the 5th century BC. Parallels to the earliest Babylonian (Sumerian) star catalogues suggest that the ancient Chinese system did not arise independently from that of the Ancient Near East.^[10] Classical Chinese astronomy is recorded in the Han period and appears in the form of three schools, which are attributed to astronomers of the Zhanguo period. The constellations of the three schools were conflated into a single system by Chen Zhuo, an astronomer of the 3rd century (Three Kingdoms period). Chen Zhuo's work has been lost, but information on his system of constellations survives in Tang period records, notably by Qutan Xida. The oldest extant Chinese star chart dates to the Tang period and was preserved as part of the Dunhuang Manuscripts. Native Chinese astronomy flourished during the Song Dynasty, and during the Yuan Dynasty became increasingly influenced by medieval Islamic astronomy.^[11]

[edit] Early Modern era

The constellations around the South Pole were not observable from north of the equator, by Babylonians, Greeks, Chinese or Arabs.
The modern constellations in this region were defined during the age of exploration, notably by Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman at the end of sixteenth century. They were depicted by Johann Bayer in his star atlas Uranometria of 1603. Several more were created by Nicolas Louis de Lacaille in his star catalogue, published in 1756.
Some modern proposals for new constellations were not successful; an example is Quadrans, eponymous of the Quadrantid meteors, now divided between Boötes and Draco. The classical constellation of Argo Navis was broken up into several different constellations, for the convenience of stellar cartographers.
By the end of the Ming Dynasty, Xu Guangqi introduced 23 asterisms of the southern sky based on the knowledge of western star charts.^[12] These asterisms have since been incorporated into the traditional Chinese star maps.

[edit] IAU constellations

In 1922, Henry Norris Russell aided the IAU in dividing the celestial sphere into 88 official constellations.^[13] Where possible, these modern constellations usually share the names of their Graeco-Roman predecessors, such as Orion, Leo or Scorpius. The aim of this system is area-mapping, i.e. the division of the celestial sphere into contiguous fields.^[14] Out of the 88 modern constellations, 36 lie predominantly in the northern sky, and the other 52 predominantly in the southern.
In 1930, the boundaries between the 88 constellations were devised by Eugène Delporte along vertical and horizontal lines of right ascension and declination.^[15] However, the data he used originated back to epoch B1875.0, which was when Benjamin A. Gould first made the proposal to designate boundaries for the celestial sphere, a suggestion upon which Delporte would base his work. The consequence of this early date is that due to the precession of the equinoxes, the borders on a modern star map, such as epoch J2000, are already somewhat skewed and no longer perfectly vertical or horizontal.^[16] This effect will increase over the years and centuries to come.

[edit] Asterisms

Much of the dark space between stars, as seen in the sky of the image above, is due to the human eye's low light sensitivity. Other images (like the Hubble Deep Field – not pictured) detect far more stars.

Main article: Asterism (astronomy)

The stars of the main asterism within a constellation are usually given Greek letters in their order of brightness, the so-called Bayer designation introduced by Johann Bayer in 1603. A total of 1,564 stars are so identified, out of approximately 10,000 stars visible to the naked eye.^[17]
The brightest stars, usually the stars that make up the constellation's eponymous asterism, also retain proper names, often from Arabic. For example, the "Little Dipper" asterism of the constellation Ursa Minor has ten stars with Bayer designation, α UMi to π UMi. Of these ten stars, seven have a proper name, viz. Polaris (α UMi), Kochab (β UMi), Pherkad (γ UMi), Yildun (δ UMi), Urodelus (ε UMi), Ahfa al Farkadain (ζ UMi) and Anwar al Farkadain (η UMi).
The stars within an asterism rarely have any substantial astrophysical relationship to each other, and their apparent proximity when viewed from Earth disguises the fact that they are far apart, some being much farther from Earth than others. However, there are some exceptions: many of the stars in the constellation of Ursa Major (including most of the Big Dipper) are genuinely close to one another, travel through the galaxy with similar velocities, and are likely to have formed together as part of a cluster that is slowly dispersing. These stars form the Ursa Major moving group.

[edit] Dark cloud constellations

The "Emu in the sky," a constellation defined by dark clouds rather than the stars. An IAU interpretation would recognise Crux (the Southern Cross) above the emu's head and Scorpius on the left. The head of the emu is the Coalsack.

Further information: Great Rift (astronomy)

The Great Rift, a series of dark patches in the Milky Way, is more visible and striking in the southern hemisphere than in the northern. It vividly stands out when conditions are otherwise so dark that the Milky Way's central region casts shadows on the ground. Some cultures have discerned shapes in these patches and have given names to these "dark cloud constellations." Members of the Inca civilization identified various dark areas or dark nebulae in the Milky Way as animals, and associated their appearance with the seasonal rains.^[18] Australian Aboriginal astronomy also describes dark cloud constellations, the most famous being the "emu in the sky