Zodiacal Light

Dust is a common feature of the solar system. It is found even in the very high temperature (~2,000,000K) solar corona, where its presence is revealed through the reflection of the spectrum of the solar photosphere. The dust is located predominantly in the ecliptic plane. Sunlight reflected from this dust is visible just after sunset or before sunrise along a tapered cone pointing upward from the horizon, along the ecliptic. It is called zodiacal light. It is best seen at the times of year when the ecliptic rises most steeply from the horizon. This means early evening in spring, and before dawn in the fall. Figure 5.17 illustrates the angle of visibility. It usually is not well seen from high-latitude sites, because of the angle of the ecliptic in autumn, and the lack of night sky in the summer. At high latitudes, it also competes with aurorae and air glow, a faint, diffuse emission from excited high-altitude atoms and ions.

A related phenomenon is the gegenschein or "counter-glow." This phenomenon, which is usually difficult to see except at exceptionally dark and high-altitude sites, is a faint ecliptic ecliptic

Figure 5.17. The springtime visibility (in the Northern Hemisphere) of zodiacal light. Drawing by E.F. Milone.

patch of light, opposite the Sun in the sky. It is believed that the gegenschein too has its origin in solar system dust, but here we see the dust at "full" phase, scattering sunlight directly back from the Sun. Its angular size is ~6° x 9°, and it varies in size by ~10%. It has been suggested that the Mesomerican "spider path to heaven" refers either to the zodiacal light or to the gegenschein (see §12.15).

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