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Harbinger of Spring: Zodiacal Light After Dusk



From latitudes outside the tropics, you don't often see zodiacal light after dusk unless it's late winter or early spring. In the Northern Hemisphere, folks at temperate latitudes can view the zodiacal light in the west after dusk in February, March and April, as long as the Moon is out of the early evening sky.
Starting a day or two after the February 18, 2011 Full Moon, you'll have a fine two-week window for catching the zodical light. The best time to look is about 90 to 120 minutes after sunset. In the Southern Hemisphere, you can watch for the zodiacal light during this same two-week period, except that it'll appear in the east before dawn.

What Does Zodiacal Light Look Like?

Zodiacal LightZodiacal light appears as a large pyramid of softly-glowing, whitish light, with its base sitting on or near the horizon. Its axis aligns with the ecliptic - Earth's orbital plane projected onto the dome of sky. It is called zodiacal light because it upwardly follows the zodiac - that rather narrow band of stars that goes full circle around the sky. It's about 16o wide, running astride the ecliptic, and it's where you find the sun, moon and planets. You may see this cone-shaped pillar of light climbing halfway up the sky.

Use Jupiter To Locate Evening Zodiacal Light in 2011

Don't know where the constellations of the zodiac cross the sky? No problem. The dazzling planet Jupiter, the most brilliant celestial object to bedeck the moon-free evening sky, serves as your faithful guide "star." It's the first star-like object to pop out at dusk, beaming like a beacon in your western sky. You can't miss seeing this dazzling world at dusk and early evening. (Zodiacal light photo on the right courtesy of Wikimedia Commons)
West After DuskBecause Jupiter shines very close to the ecliptic, the center line of the great zodiacal roadway, watch for the zodiacal light to pass through Jupiter some 90 to 120 minutes after sunset. (Actually, the zodiacal light shines in front of Jupiter, but we'll talk about that later.) Find an unobstructed view of the western sky, away from pesky artificial lights, to witness this otherworldly light adorning the heavens after evening dusk. (Sky chart on left courtesy of Your Sky)

Zodiacal Light: Literally Out Of This World!

Zodiacal light is interplanetary dust reflecting the light of the Sun. A great big disk or cloud of dust fills the inner solar system, and these particles of dust orbit the Sun on or near the same plane that the planets orbit the Sun. That's why the zodiacal light shines along the ecliptic and zodiac. Jupiter, the 5th planet outward from the Sun, resides in the outer solar system, so it's way out there, beyond the sunlit dust particles orbiting the Sun inside of Jupiter's orbit.

Why In Late Winter & Early Spring?

To see the zodiacal light in the evening sky, the ecliptic has to reach its maximum inclination over your horizon in the vicinity of sunset. In the Northern Hemisphere, the ecliptic reaches it maximum inclination whenever the summer solstice point lodges due south in the sky. The summer solstice point, though due south at midday or high noon on the summer solstice, stands due south at sunset on the March equinox.

copyright 2011 by Bruce McClure

January 2011 Feature * March 2011 Feature