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SHORT FEATURES

Western Slope Skies - Tripping the Light Zodiacal

ZodiacalLight7min10Mar2016v2.jpg
Art Trevena
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What’s a sure sign of spring? Daffodils emerging in the garden, cheerful songbirds in the trees? These are familiar, terrestrial indicators.

The celestially-minded look upward for their sign— to a softly luminous beehive appearing above the western horizon, shortly after twilight ends. This faint but lovely sight is known as the zodiacal light.

The phenomenon is so named because the light is aligned with the zodiac, the band of constellations through which the Sun and planets appear to travel.  The line of travel is the ecliptic, indicating the general orbital plane of the Solar System.

A uniform band of dust permeates the Solar System in the ecliptic plane -- the zodiacal cloud. Sunlight reflected off the cloud toward Earth causes the zodiacal light.

Where does the cloud come from? Astronomers originally thought that active comets generated it while releasing dust in their occasional passage through the inner Solar System. However, mathematical modeling now suggests different sources — collisions in the asteroid belt beyond Mars, and fragmentation of dormant comets in Jupiter’s vicinity. Without these events replenishing the cloud, it would have been dissipated by the solar wind long ago.    

The Zodiacal Light appears in both hemispheres, but at opposite times: in the northern hemisphere, evenings in March and April, and mornings during September and October.  It’s just the opposite for the southern hemisphere.   

Giovanni Cassini is usually credited for the first scientific study of the zodiacal light in 1683, but the light has been known since ancient times.

In Islamic oral tradition, Muhammed describes the zodiacal light as the “false dawn” before the sunrise.  He saw it in its northern hemisphere fall apparition.

But don’t wait until fall, or buy a plane ticket to Australia -- catch the evening zodiacal light now, in a dark-sky location near you!

You’ve been listening to Western Slope Skies produced by members of the Black Canyon Astronomical Society.   This piece was written by Michael T. Williams and recorded by Joyce Tanihara.