Stargazers | The zodiacal light: Seeing our dusty solar system

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Stargazers who spend time in dark, rural locations are fortunate to see amazing celestial phenomena invisible — or even unknown — to those who live near the bright lights of a city.

The sheer number of stars visible under the clear, dark skies of the mountains or deserts is easily in the thousands. Then there's the Milky Way, the cloudy band of light that represents the central plane of our galaxy — our home star city. And the many distant star clusters, nebulae and galaxies known to astronomers as "deep sky objects," appear as smudges of light to the unaided eye when we're far from the devastating effects of light pollution. All of these are visible on most nights of the year.

But there's another remarkable phenomenon — one that we can witness best during the late glimmers of dusk during early spring. Astronomers know it as the zodiacal light.

To see it, you'll need to head outdoors after sunset in a place where you'll have a clear view to the west without a glow of city lights. During twilight, let your eyes adjust to the darkness and then, about 90 minutes or so after sunset, begin looking for a tall, softly glowing pyramid rising out of the western horizon.

Stargazers who have never seen this phenomenon tend to expect a much smaller or brighter glow. Often it appears to ascend to one-third or even halfway up in the western sky. Its base typically appears to be some 15 degrees wide, and the cone tapers to only about five degrees wide at the top. Remember, any light pollution, haze or moonlight anywhere in the western sky and you'll have quite a challenge spotting it.

You can be sure you've found it if you can trace its very faint cone-shaped glow along constellations of the zodiac (from which it derives its name); this week it appears to ascend out of the constellation Pisces near the horizon, and it continues through Aries toward the Pleiades star cluster in Taurus.

The zodiacal light is produced when sunlight is scattered from dust particles spread along the plane of our solar system. Most of these are continually generated by passing comets or by collisions among asteroids; each is only about four-hundredths of an inch across and the particles are separated by an average distance of five miles.

Because it generally appears brightest in the direction of the sun, which illuminates it from behind, we can see the zodiacal light best after the end of evening twilight or before the onset of morning twilight, and then only at times when the plane of our solar system (the ecliptic) forms a steep angle with the horizon. These conditions — along with an absence of moonlight — occur for only a few weeks after evening twilight in the springtime, and before morning twilight in the autumn.

Most people don't even know this phenomenon exists, let alone that they can see it in a dark night-time sky. So take a drive to the country or mountains this week, far from the lights of any cities, and search the skies for this elusive glow.

Visit Dennis Mammana at www.dennismammana.com.


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