Sky Watch: Zodiacal light's soft glow

One of my least favorite household chores is dusting.

If I let it go more than a week, I can trace my name on the TV screen. After a month, geologists can easily do sediment dating there!

If this sounds familiar, you've probably also noticed that when the light is just right, the dust is totally invisible. But when the lighting changes -- usually moments before company arrives for dinner -- the room appears aglow with the stuff.

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Since this dusty disk lies mostly in the plane of our solar system -- along the band of constellations we know as the zodiac -- that's where it appears if the lighting is just right. And now is one of those times; during the latter half of September, this softly glowing pyramid of interplanetary dust ascends almost vertically from the eastern horizon at dawn. We call it the "zodiacal light."

To see it, you'll need to rise long before the sun and have a clear view to the east. Then, about 90 minutes or so before sunrise, begin looking for a large faint pyramid with its wide base in the direction of Leo and its tapered end near Gemini. This glow is faint and any moonlight, haze, or light pollution can make your effort futile. Also, the brilliant planet Venus now appears near its center and may make the zodiacal light tough to find.

This mysterious "false dawn" is produced when sunlight is scattered from dust particles, most of which are continually generated by passing comets or by collisions among asteroids. Each is only about four-hundredths of an inch across, and they are separated by an average distance of five miles.

Because it appears brightest in the direction of the sun, we see the zodiacal light best when there's no moonlight and when the plane of our solar system (the ecliptic) forms a steep angle with the horizon. These conditions occur for only a few weeks before morning twilight during September and October. The zodiacal light appears in the evening, too, but that happens in springtime.