Definitions

zodiacal light

zodiacal light

zodiacal light or zodiacal band, a faint band of light sometimes seen in the western sky just after sunset in the spring, extending up from the horizon at the point where the sun has just set, or in the eastern sky just before sunrise in the autumn. The light is so faint as to be obscured by moonlight. It is caused by the reflection and scattering of sunlight by a sparse band of tiny dust particles that appears to be an extension of the solar corona, stretching out beyond the orbit of Jupiter. Concentrated in the plane of the ecliptic, the faint light is best seen in the region of the sky called the zodiac. Near the equator the zodiacal light sometimes seems to stretch completely across the sky. It was first investigated and explained by the astronomer Gian Domenico Cassini about 1690.

See also gegenschein.

Band of very faint light in the night sky. It is thought to be sunlight reflected from interplanetary dust grains lying mostly in the plane of the zodiac, or ecliptic. Seen in the west after twilight and in the east before dawn, it is most clearly visible in the tropics, where the ecliptic is approximately perpendicular to the horizon. In midnorthern latitudes it is best seen evenings in February and March and mornings in September and October (vice versa in midsouthern latitudes). The light can be followed visually to a point about 90° from the Sun. It continues to the region opposite the Sun, where a slight enhancement, the gegenschein, is visible.

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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.

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 because 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.

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."

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.

Trivia

Recently, after a 30-year break from his studies to play guitar with multi-platinum rock group Queen, guitarist Brian May published his doctoral thesis on zodiacal light, entitled, "A Survey of Radial Velocities in the Zodiacal Dust Cloud." He subsequently received his Ph.D in Applied Astrophysics from the Imperial College of London.

See also

References

  • Reach, W. T. (1997). structured zodiacal light: IRAS, COBE, and ISO observations" Diffuse Infrared Radiation and the IRTS. ASP Conference Series. 124, 33–40
  • Examples of zodiacal light descriptions in Islamic tradition, and their application: http://islamqa.com/index.php?ref=26763&ln=eng&txt=false%20fajr

External links

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