Zodiacal light
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The zodiacal light is a faint, roughly triangular 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.
The zodiacal light is produced by sunlight reflecting off dust particles which are present throughout much of the solar system and, 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. 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 5 miles from its neighbors. The gegenschein may be due to the fact that particles directly opposite Earth from the sun would be in full phase.
Sunlight absorbed by the dust particles is re-emitted as infrared radiation. This reradiation causes the particles to spiral slowly into Sun (Poynting-Robertson effect), 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 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.
See also
External Links
- A Brief History of Observations (http://zodiacal-light.hit.bg/zl_files/observations_text.html)
- "Zodiacal Light and the Gegenschein", an essay by J. E. Littleton (http://www.as.wvu.edu/~jel/skywatch/skw9810h.html)
Reference
- Reach, W. T. (1997). "The structured zodiacal light: IRAS, COBE, and ISO observations" (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1997ASPC..124...33R&db_key=AST|). Diffuse Infrared Radiation and the IRTS. ASP Conference Series. 124, 33-40
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