Zodiacal Light

Oct 11, 2018

During moonless periods in September and October, the steep morning ecliptic favors the appearance of the zodiacal light in the eastern sky for about half an hour before dawn.

The glow is sunlight reflected from interplanetary particles drifting in the plane of our solar system. During this week, look east, below the stars of Leo (the Lion), for a broad wedge of faint light rising from the horizon and centered on the ecliptic. The ecliptic passes directly through the bright star Regulus in Leo. Don’t confuse the zodiacal light with distant light pollution, or the Milky Way, which is sitting further to the southeast.

In the mid-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. The zodiacal light appears as a column, brighter at the horizon, tilted at the angle of the ecliptic. The light scattered from extremely small dust particles is strongly forward scattering, although the zodiacal light actually extends all the way around the sky, hence it is brightest when observing at a small angle with the Sun. This is why it is most clearly visible near sunrise or sunset, when the sun is blocked, but the dust particles nearest the line of sight to the sun are not. The dust band that causes the zodiacal light is uniform across the whole ecliptic.

The dust further from the ecliptic is almost undetectable except when viewed at a small angle with the sun. Thus, it is possible to see more of the width at small angles toward the sun, and it appears wider near the horizon, closer to the sun under the horizon.

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