Morning Mystery Glow Announces Arrival Of Zodiacal Light

The zodiacal light (left) points upward from the eastern horizon and “touches” the Milky Way (right) Sunday morning. The bright object is the planet Venus. Details: 16mm lens at f/2.8, ISO 800 and 2-minute exposure on a tracking mount. Photo: Bob King

When fall comes around I know it’s time to get up at dawn to see the glory of the zodiacal light. It may not be easy to set the alarm for 4:30 a.m. and wander out into 32-degree cold, but you take it a step at a time. Pull on your pants, zip up jacket, pack camera in car and drive to a dark location with an open view to the east.

I’ve never been able to shake the feeling of sneaking around or breaking the rules when I get up in the early morning hours. As a teenager I remember tiptoeing from the bedroom to the front door trying hard to avoid the “creaky spots” in the floor. The last thing you wanted to do was wake up dad.

Illustration showing the tapering wedge of zodiacal light extending upward from the eastern horizon to the Milky Way around 5 a.m. in late September. The Belt stars of Orion are at right. The light is centered on the ecliptic, the path the sun follows through the zodiac during the year. Created with Stellarium

Seen from a dark sky the zodiacal light is large wedge of ghostly light tilting up from the eastern horizon, reaching past the brilliant planet Venus all the way to the band of the winter Milky Way where it touches Orion’s shoulder. The bottom end is brighter and wider than the tapering tip. Many people are surprised when they see the light cone for the first time – it’s much bigger than they think. Brightness-wise the wedge looks like dawn itself at its base, but the top glows only as brightly as the fainter parts of the Milky Way.

The zodiacal light is so called because it’s centered on the zodiac, that ring of 12 constellations defined by the sun’s apparent path through the sky during the year. Right now the sun shines in Virgo; in December it will have moved to Sagittarius and next summer will ascend into Taurus.

Twice a year the zodiacal light makes a good show from mid-northern latitudes when it’s tilted up high at dusk in spring and at dawn in fall. This week it angles up through Leo, Cancer and Gemini.  Just like the sun, the angle of the zodiacal wedge to the horizon varies during the year. When tipped at a low angle, it’s obscured by thick air and haze. When tipped up high, you can’t miss it from a dark sky.

Comets like 168P/Hergenrother, currently visible in the fall evening sky, are the source of most of the material that creates the zodiacal light dust cloud. Credit: Michael Jaeger

The wonder of the zodiacal light is that it’s made of billions of dust particles shed by countless comets orbiting approximately in the plane of the solar system between Jupiter and the sun.

Heat from the sun vaporizes comet ices which are gummed up with dust and small rocks. Some of that liberated dust strikes Earth’s atmosphere, burning up as occasional random meteors we see on any night of the year. Much of it settles into a vast, rarefied cloud in the plane of the solar system where it’s illuminated by the sun like pollen shaken from a pine tree. Dust closer to the sun reflects its light more brightly; dust farther away less so. That’s why the zodiacal light is brighter at its base – which is closer to the rising sun – than at its tip.

Much comet dust slowly spirals into the sun over time. To keep a steady supply available, comets ancient and new have contributed the dribs and drabs that make the zodiacal light an arresting sight. To see it best, find a dark sky location with a great view to the east and start looking about 2 hours before sunrise. For my town, that’s between 5 and 5:30 a.m. 90 minutes before sunrise, you’ll still make out the glowing light but also notice dawn gaining ground.

Sunday morning was special. The cone of comet debris stretched all the way to the blizzard of stars comprising the band of the Milky Way. Comet dust, like all dust, is a gift of the stars. Seeing the eerie light literally reach for the stars hit me like a cosmic version of a baby touching her mother’s face.

Approximate view (not to scale) showing the dust cloud from comets that extends across the inner solar system. Lit by the sun, we see it as the zodiacal light. Illustration: Bob King

Tomorrow through Thursday morning are ideal times to see it for yourself. By Friday the moon will be up at dawn and spoil the view. The next moonless period begins on Oct. 13 and continues through the end of the month. Mark your calendar for an early morning adventure.

2 Responses

  1. MBZ

    “Comet dust, like all dust, is a gift of the stars. Seeing the eerie light literally reach for the stars hit me like a cosmic version of a baby touching her mother’s face.”

    After reading this sentence last night I couldn’t find any words to comment. Still can’t.
    You are a delight to read Astro Bob, even if you do wear goofy shirts 😉

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