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Zodiacal Light, the False Dawn of Autumn

The famous proverb claiming "the darkest hour is just before dawn" rings out often in word and song. But with the equinox falling on September 23 - and autumn returning to the Northern Hemisphere - the seasonal sleigh of hand known as the "false dawn" now lightens the "darkest hour" before dawn. This false dawn is caused by zodiacal light.
To see the soft luminescence of zodiacal light, you need a moonless, clear and (otherwise) dark sky, and dark-adapted eyes. Roughly 1.5 to 2 hours before sunrise, the false dawn appears as a feeble, cone-shaped pillar of light jutting upward from the eastern horizon. In the autumn of 2007, look for the faint white-hazed light to sweep up to or beyond Venus - easily the most brilliant celestial object in the early morning sky. For some idea of what zodiacal light looks like, I enclose this zodiacal light gallery.
Zodiacal light glimmers along the zodiac: the pathway of the Sun, Moon and planets as mapped by the backdrop stars. When the zodiac arcs high up above the horizon before dawn (or after dusk), you are much more likely to see sunlight reflecting off the interplanetary dust particles that orbit the the Sun. This dust is probably the debris left behind by comets and the pulverized remains of asteroids.

Why on Autumn Mornings?

Zodiacal light is commonplace at the tropics, because the zodiac stands almost straight up from the horizon all year round. However, at northern (or southern) temperate latitudes, the angle of the zodiac to the horizon is at its steepest before dawn in autumn. For this reason, the prime time for catching zodiacal light before dawn at northern temperate latitudes is in later September, October and early November.
From the Northern Hemisphere, the zodiac is highest in the sky when the constellation Orion the Mighty Hunter shines due south in your sky. When Orion stands south, the zodiac arcs high above Orion, extending east to west. So whenever you see Orion (or more specifically: Orion's bright star Betelgeuse) due south just before dawn, look for the zodiacal light.
(To know Betelgeuse's transit time - when this star is due south - click here. Try this almanac or the US Naval Observatory almanac to find out when twilight starts to color your sky.)

Orion before Dawn or after Dusk!

At northern (or southern) temperate latitudes, the best time to see the zodiacal light in the EVENING is in late winter and early spring. From the Northern Hemisphere, the zodiac climbs highest after dusk when - once again - Orion is due south after dusk. From northern temperate latitudes, look for the zodiacal light after the twilight dusk, starting in early February.

copyright 2007 by Bruce McClure

August 2007 Feature * October 2007 Feature