Zodiacal Light

StarDate: September 20, 2009

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The sky can fool you sometimes.

If you're an early riser -- and away from city lights -- you just might see an example over the next few days. Look toward the east an hour or two before sunrise for a ghostly pyramid of light rising from the horizon. It may look like the first blush of twilight, but it's not. Instead, it's the zodiacal light -- a glow that comes from far beyond Earth.

Twilight is produced by Earth's atmosphere. Sunlight reflects off of air molecules, plus dust grains and other solid particles, to make the sky brighter long before the Sun itself hoves into view.

The zodiacal light is also produced by sunlight reflecting off of dust grains. But these grains are scattered throughout the solar system. Some of them probably were stripped from icy comets that passed near the Sun, while others are the ground-up debris from collisions between rocky asteroids.

The dust grains are concentrated in the plane of the solar system, so they appear along the zodiac -- the stretch of constellations that the Sun traverses -- hence the name "zodiacal light." Astronomers call this path the ecliptic.

This is one of the best times of year to see the zodiacal light because the ecliptic extends almost straight up into the sky in the pre-dawn hours. That lets the light of the zodiac climb the sky like a ghostly pyramid -- a pyramid of light from beyond Earth.

Tomorrow: marking a change of seasons.

Script by Damond Benningfield, Copyright 2009

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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