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Zodiacal Light

The term zodiacal light is used to describe a faint, glowing band of light that occasionally appears near the eastern or western horizon, which is caused by reflection of sunlight from tiny dust particles in the solar system.

Our solar system can be pictured as a huge disk, with the planets, their moons, and asteroids mostly orbiting in or near the same plane. If you look at the sky some evening when the Moon and two or three planets are visible, you will see they fall more or less in a line across the sky; this imaginary line, which is the plane of Earth's orbit projected against the sky, is typically called the ecliptic, but also is sometimes called the zodiac.

Planets and moons are not the only things in the solar system; they're just the most visible. The ecliptic is filled with tiny particles of dust left over from the formation of the solar system. The solar system formed from a vast, rotating cloud of gas and dust termed the solar nebula. As this cloud contracted, its rotation rate increased. The faster rotation flattened the cloud into a disk, much as a blob of pizza dough becomes a disk when the baker tosses it spinning into the air. Most of this spinning disk of gas and dust condensed into the conglomerations of matter that are now the Sun and planets, but some was left behind, floating free in the space between the planets.

Today, this remnant dust is still there. It is far too sparse and cold to be directly detectable, but it does reflect light. As the Sun's radiation streams through the solar system, a small amount of it is reflected by these dust particles. Some is reflected toward Earth.

As a result, we occasionally see a thin band of light, called the zodiacal light, extending upward from the eastern or western horizon along the plane of the ecliptic, or zodiac. The light appears in a band because the dust particles reflecting it lie mostly in the plane of the ecliptic. The effect is much the same as when we shine a flashlight beam through a cloud of chalk dust in the air: we can see the faint, reflected light from the particles. The zodiacal light typically appears in deep dusk after sunset or before sunrise, because it is at those times of night that the Sun is favorably placed to illuminate the intraplanetary dust. Zodiacal light is also easier to see in the fall and winter in the northern hemisphere, when the zodiac is highest in the sky and the plane of the ecliptic is oriented more nearly perpendicular to the horizon than in summer. The ghostly zodiacal light is not visible every night, and it is easily washed out by moonlight or the glow of a nearby town or city. But from a dark viewing site under the right conditions, it is easily visible and makes a splendid addition to the array of wonders above.

Jeffrey Hall

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