Recently, I made a special trip to a dark, rural area about 25 miles north of my home to see the return of the zodiacal light. Every March and April, this cone of glowing cosmic dust points up from the western horizon at nightfall like a ghostly index finger. Unless your sky is exceptionally dark, it won't be obvious at first glance. To spot it, sweep your gaze across the western sky in the direction of Venus, looking for a large, faint, softly glowing band resembling the Milky Way.
The zodiacal light glows brightest toward the horizon and fades and narrows along its length. I could trace its extent all the way from Jupiter, very low in the west, up past the Pleiades star cluster where it intersected with the dim band of the Milky Way in Taurus and Perseus. Near the horizon the glow spans about two fists (20°) and reaches up half the sky to 45-degree altitude or more.
Centrally located within the finger, Venus gleamed like a shiny pendant. Nature can be as quiet as it is raucous. I strained to hear any sound at all while watching the zodiacal light slowly emerge from the enveloping night but was met with one of my favorite things — silence.
The zodiacal light gets its name because it stretches along the ecliptic — the imaginary circle in the sky traveled by the sun, moon and planets — which passes through the twelve constellations of the zodiac. That's why Venus (also Jupiter, but difficult to see in the photo) shine from inside the fuzzy finger. The ecliptic defines the plane of the solar system. Earth and all the planets appear to travel along it because they all orbit within that same plane. Viewed from Venus, Earth would shine like a bright, blue star inside the zodiacal light.
In spring the ecliptic makes a steep angle to the western horizon at dusk, which tilts the faint light upward into good view. Other times of year, it rests at a lower angle at nightfall and gets lost in the horizon glow. The ecliptic also tips upward at dawn in October and November, making those months the best time to see it in the morning sky.
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The cause of the light is dust. Dust from comets, asteroids (including asteroid collisions) and even ejected by Mars settles in the plane of the solar system and glows faintly by reflected sunlight.
Technically, the zodiacal light arises from forward scattering. If you've ever driven down a dirt road with a low sun in your eyes, and the car in front of you is kicking up dust, you've experienced forward scattering. Dust scatters sunlight into your eyes making it nearly impossible to see what's ahead. The same principle applies to zodiacal dust and comet tails.
If we could remove the Earth from our view and hover weightless in space, we'd see the zodiacal light extending on either side of the sun as a disk. Physically, some of the dust is found inside Earth's orbit, but the majority lies between Earth and just beyond Mars. Vast in dimension, this enormous dusty doughnut is composed of almost nothing. Yet its presence adds a touch of magic to the night.
The best time to see it is from now through March 22 and again from April 7-20 when the moon is absent from the sky. Look west at the end of evening twilight or about 90 minutes to 2 hours after sunset. You can get your local sunset time at timeanddate.com . The light remains visible for about an hour after nightfall, but catch it early when it towers highest.
For about the next week northern hemisphere observers can also spot China's Tiangong space station. When it passes high overhead it can briefly shine as brightly as Jupiter. During most appearances it's about as bright as Saturn and quite easy to see. Go to Heavens Above , add your city (click the link under Configuration heading on the left side of the page) and then click the blue Tiangong link for a list of times. A negative number under the magnitude heading indicates a bright pass. Click on a date for a map that will show satellite's path through the constellations.
On the way home I stopped to see if the aurora was active. My suspicions were confirmed! A classic, pale green bow arched across the bottom of the northern sky. Photos showed mottling in the bow from faint rays. Although no aurora was forecast for that night (March 13), there's a minor G1 storm predicted for Tuesday night, March 14 into the early morning hours. Keep your eyes on the northern sky in case the green fuzz returns!
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