Stand In Awe Before Dawn’s Early Light

A great cone of softly luminous dust, the zodiacal light, reaches up from the eastern horizon at the start of dawn earlier this week. The dust originates from comets and colliding asteroids. Credit: Bob King
A great cone of softly luminous dust, the zodiacal light, reaches up from the eastern horizon at the start of dawn earlier this week. The dust originates from comets and colliding asteroids. The Beehive Cluster is just above center, and the band of the winter Milky Way is off to the upper right. Credit: Bob King

What sort of things make for jaw-dropping sky sights? You might answer a total eclipse of the sun or a raging, all-sky aurora. The other morning just before the start of dawn, I stood in awe before a phenomenon consisting of practically nothing at all. A great cone of luminous haze called the zodiacal light stood up from the eastern horizon pointing to Orion and the Milky Way.

Looking something like a beam cast by a spotlight in fog, it’s made of dust lost by comets and shed by colliding asteroids. The particles settle into a flattened disk in the plane of the solar system, extending from the vicinity of the sun all the way out to the orbit of Jupiter. While vast, the separation between motes is on the order of several miles, so the zodiacal cloud (as it’s called), is spectacularly dilute. But thanks to the ability of fine dust to scatter sunlight, it makes an impressive sight on moonless fall mornings.

Every October and November, the ecliptic, the path taken by the sun, moon and planets in Earth’s sky, tilts up at a steep angle from the eastern horizon before dawn. Recall that the ecliptic passes through the familiar constellations of the zodiac. It also defines the plane of the solar system, and since comet and asteroid dust are concentrated in that same plane, the zodiacal light naturally follows the ecliptic.

This view shows the sky facing east about 1 hour 40 minutes before sunrise. The tilted cone or wedge is the zodiacal light, which follows along the ecliptic from Leo up into Gemini. Map: Bob King, Source: Stellarium
This view shows the sky facing east about 1 hour 45 minutes before sunrise. The tilted cone or wedge is the zodiacal light, which follows along the ecliptic from Leo up into Gemini. Map: Bob King, Source: Stellarium

In fall, the light cone extends up from the eastern horizon through Leo the Lion, Cancer the Crab and into Gemini the Twins before appearing to “dead-end” in the Milky Way. But if your sky is free of light pollution, you can continue to follow the zodiacal light all around the sky to the western horizon as the zodiacal band. At other times of year, the angle the ecliptic makes to the eastern horizon at dawn is shallow, and the light cone is lost from view in low-altitude haze, but during fall mornings (and again on spring evenings in the western sky), it stands nearly straight up, towering in a dark sky.

The zodiacal light comes into good view in the eastern sky starting about 2-2 ½ hours before sunrise, but if you wait until about 1 hour 45 minutes before the sun comes up — just before the first hint of dawn in the east — it will appear highest and brightest. Sunlight scattering off the dust gives it a glowing appearance, and the closer the dust is in the direction of the sun, the brighter it appears. That’s why the cone is brightest nearer the horizon and fainter at its tip.

Comet C/2013 US10 Catalina shows two spectacular tails and a bright aqua coma this morning (Dec. 6) . The dust tail, made of dust boiled off the comet during it recent close approach to the sun, points to the southeast. The gas tail is made of gases such as carbon monoxide that fluoresce in ultraviolet light coming from the sun. Credit: Michael Jaeger
Sunlight boils off dust from comets, such as C/2013 US10 Catalina pictured here, that contributes to the zodiacal light. Much of the dust eventually falls into the sun and gets replenished by new comets arriving in the inner solar system from the distant comet reservoir known as the Oort Cloud. Credit: Michael Jaeger

The bottom half is similar to the summer Milky Way in brightness; the top more like the fainter portions of the winter Milky Way. Cast your gaze wide. This huge wedge spans 2-3 fists wide at its base and 6 fists tall! Just as the dust begins to fade in the bluing dawn, you’ll see the planet Mercury pop over the eastern horizon. The planet will be in view for about another week.

There are two windows this season to get your best look at the zodiacal light: now through October 14, when the moon returns to the morning sky, and from October 30 through Nov. 12, when the moon’s out of the picture again. Find a place with a dark, light-pollution free view to the east for a jaw-dropping sight.