Our Dusty Solar System
Last updated 4/28/2023 at 10:53am
One of my least favorite household chores is dusting.
Here in the desert, if I let it go more than a week, I can trace my name on the coffee table. After a month, geologists can easily do sediment dating there!
If this sounds familiar, you've probably also noticed that airborne dust is mostly invisible, but when the light is just right – usually moments before guests arrive for dinner – the room appears aglow with the stuff.
Well, the same is true in our part of the Solar system. Whoever's in charge is apparently no neater than I, because the entire inner Solar system is littered with a thick, dusty disk. And, while this interplanetary cloud is composed of different material than that blanketing the coffee table, the principles for seeing it are rather similar.
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 springtime is one of those times; during March and early April this softly glowing pyramid of interplanetary dust ascends almost vertically from the western horizon at dusk. We call it the "zodiacal light."
Stargazers who have never seen this phenomenon tend to expect a much smaller or brighter glow. 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.
To see it you'll need to head outdoors after sunset to a place where you'll have a clear view to the west and no moonlight or glow of city lights in that direction. 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 and reaching its top near the tiny Pleiades star cluster high in the west. Remember, any light pollution, haze or moonlight anywhere in the western sky and you'll have quite a challenge spotting it.
This phenomenon was described nearly a millennium ago by the Persian poet Omar Khayyam in his famous book "The Rubaiyat." Today we know that it 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 microscopic – only about four-hundredths of an inch across – and these 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. In the Northern Hemisphere, the best conditions occur after dusk in the springtime, and before dawn during autumn.
Most folks don't even know that this phenomenon exists, let alone that they can see it in a dark nighttime sky. Later this week, take a drive to the country or mountains, far from the lights of any cities, and search the western sky for this elusive glow.