Zapped By Zodiacal Light

I took this photo of the zodiacal light at the start of morning twilight on Sept.13. Notice how the light cone is wider and brighter near the horizon and tapers and fades as you look upward. At right, the cone meets the band of the Milky Way. City glow from Two Harbors, Minn. creates the brighter, horizontal patch just above the horizon at lower left. Details: ISO 1600, 30 seconds, 16mm lens.  Bob King

Every fall, a big, faint, cone-shaped glow towers in the eastern sky at the start of dawn called the zodiacal light. It resembles the Milky Way’s smoky appearance but has a smoother texture, and instead of being comprised of stars, originates from comet and asteroid dust. Dust and fine rocky grit sloughed by comets as they approach the sun lingers in the plane of the solar system. A smaller amount of dust from asteroid collisions is also part of the mix.

Sunbeams form when dust in the air scatters sunlight; interplanetary dust likewise scatters sunlight to shape the zodiacal light. Reader Tom Polakis suggests imagining the sun shining through a cloud of dust suspended over a dirt road after a car has passed. Sunlight scattered by the dust creates a glowing cloud, brightest near the sun and fading as you look off to the side.

In this labeled view, you can see that the zodiacal light cone is centered on the ecliptic, the plane of the planets and solar system. When you look for the light, find a place with a great view to the east. You need a chunk of sky to see it best because it’s so BIG. The base of the cone is about three fists wide, while its height is more than six fists! In fall, the zodiacal light extends all the way up to the winter Milky Way. Stellarium with additions by the author

Dust closer to the sun, the source of light, glows more brightly, the reason the zodiacal light is brightest and broadest low in the east where the sun comes up. The farther from the sun you gaze, the narrower and fainter the ghostly cone. I recently made a special trip to a field with a clear view of the eastern sky to take in the sight — an impressive one!

You know how you feel when you look up to see the Milky Way arching from one end of the sky to the other. The zodiacal light is similarly large. From a dark, moonless sky, I think you’ll be surprised by its magnificence. From mid-September to mid-November it stands high in the east from about a half-hour before to the start of dawn. Since dawn begins about an hour and 40 minutes before local sunrise, all you need do is click the link to find the sunrise time for your location and back the time up 2 hours. For me, the sun comes up at 6:50 a.m., so views are perfect around 5 right now.

Wow — what a photo! Speaking of comet dust, 21P/Giacobini-Zinner, seen here passing over the star clusters M35 and NGC 2158 (lower right) in Gemini this morning (Sept. 16), makes regular contributions to the zodiacal light during its regular spins around the sun. Rolando Ligustri

Find a place with a great exposure to the east. You entire sky doesn’t have to be light pollution free. As long as the eastern half is dark, moonless and rich with stars, you have a good shot at finding this monster interplanetary glow. Think big. The finger-like light cone starts at the horizon and slants up and to the right all the way to Orion.

The reason the zodiacal is more prominent in the fall morning sky (and spring evening sky) has to do with the angle the ecliptic makes to the horizon. In fall, it tilts at a steep angle at dawn, so the light cone makes it above the horizon haze into good visibility.

We have from tomorrow morning (Sept. 17) through about Sept. 23, when the moon returns to the morning sky, for excellent views. After that, the next viewing window opens up from Oct. 7-22. Generations of comets have provided the fodder for the phenomenon. So long as comets come and go,