Zodiacal light
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The zodiacal light is a faint, roughly triangular, whitish glow seen in the night sky which appears to extend up from the vicinity of the sun along the ecliptic or zodiac. In mid-northern latitudes, the zodiacal light is best observed in the western sky in the spring after the evening twilight has completely disappeared, or in the eastern sky in the autumn just before the morning twilight appears. It is so faint that it is completely masked by either moonlight or light pollution. The zodiacal light decreases in intensity with distance from the Sun, but on very dark nights it has been observed in a band completely around the ecliptic. In fact, the zodiacal light covers the entire sky, being responsible for 60% of the total skylight on a moonless night. There is also a very faint, but still slightly increased, oval glow directly opposite the Sun which is known as the gegenschein.
This phenomenon was first investigated by the astronomer Giovanni Domenico Cassini in 1683 and first explained by Nicolas Fatio de Duillier in 1684.
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[edit] Formation
The zodiacal light is produced by sunlight reflecting off dust particles which are present in the solar system and known as cosmic dust. Consequently, its spectrum is the same as the solar spectrum. The material producing the zodiacal light is located in a lens-shaped volume of space centered on the sun and extending well out beyond the orbit of Earth. This material is known as the interplanetary dust cloud. Since most of the material is located near the plane of the solar system, the zodiacal light is seen along the ecliptic. The amount of material needed to produce the observed zodiacal light is amazingly small. If it were in the form of 1 mm particles, each with the same albedo (reflecting power) as Earth's moon, each particle would be 8 km from its neighbors. The gegenschein may be due to the fact that particles directly opposite the sun as seen from Earth would be in full phase.
The Poynting-Robertson effect causes the particles to spiral slowly into the Sun, thus requiring a continuous source of new particles to maintain the zodiacal cloud. Cometary dust and dust generated by collisions among the asteroids are believed to be mostly responsible for the maintenance of the dust cloud producing the zodiacal light and the gegenschein. In recent years, observations by a variety of spacecraft have shown significant structure in the zodiacal light including dust bands associated with debris from particular asteroid families and several cometary trails.
[edit] Historical explanations
Previously, it was believed that zodiacal light was just the atmosphere of the sun. According to the 1728 Cyclopaedia:
- "The zodiacal light is nothing but the solar atmosphere, a rare and subtile fluid, either luminous by itself, or made so by the rays of the sun surrounding its globe; but in a greater quantity, and more extensively, about its equator, than any other."
[edit] Importance to Islam
The Prophet Muhammed is known to have described zodiacal light in reference to the timing of the five daily prayers, calling it the "false dawn," or "al Fajr al Khaadib." Muslim oral tradition preserves numerous sayings, or hadith, in which Muhammed describes the difference between the light of false dawn, appearing in the sky long after sunset, and the light of the first band of horizontal light at sunrise, the true dawn. Practitioners of Islam use the Prophet's descriptions of zodiacal light to avoid errors in determining the timing of daily prayers.
Such practical descriptions and applications of astronomical observations were vital to the golden age of Islamic astronomy.
[edit] See also
[edit] References
- Reach, W. T. (1997). "The structured zodiacal light: IRAS, COBE, and ISO observations". Diffuse Infrared Radiation and the IRTS. ASP Conference Series. 124, 33-40
- This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain.
- Examples of zodiacal light descriptions in Islamic tradition, and their application: http://islamqa.com/index.php?ref=26763&ln=eng&txt=false%20fajr
[edit] External links
- A Brief History of Observations
- "Zodiacal Light and the Gegenschein", an essay by J. E. Littleton
- APOD for September 25, 2007
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