The Zodiacal Light
Posted by Brian Ventrudo
“Before the phantom of False morning died,
Methought a Voice within the Tavern cried,
When all the Temple is prepared within,
Why nods the drowsy Worshiper outside?”
- from the Rubaiyat of Omar Khayam
The “false morning” in this 900-year-old passage by the Persian poet and astronomer is not a dreamy literary invention, but a real astronomical phenomenon called the “zodiacal light”. Appearing as a faint, eerie glow before sunrise or after sunset, the zodiacal light has likely been seen by stargazers since since antiquity. And it’s well positioned for viewing over the next month or so by observers in both hemispheres.
The picture above will help you get a better idea what the zodiacal light is all about. It looks like a tilted cone of light rising 20-30 degrees above the horizon over which the sun is about to rise (or has already set). The light always follows the plane of the ecliptic along which are found the constellations of the zodiac. Hence the name.
Zodiacal light is caused by sunlight reflecting off a lens-shaped disk of dust the lies along the plane of the inner solar system, a plane which defines the band of the ecliptic in the sky. Since the zodiacal light is reflected sunlight, it appears whitish-yellow and a detailed examination shows its spectrum is the same as that of the sun. The dust of the inner solar system is slowly spiraling into the sun. But it’s replenished by new dust from comets and asteroid collisions.
The zodiacal light is always there, but it’s easier to see when the ecliptic is nearly vertical to the horizon. For northern observers, that alignment occurs in late-September and October about 2-3 hours before sunrise, and in late February to early March about 2-3 hours after sunset. In the southern hemisphere, the best periods for viewing are August and September just after sunset, and late-March through May a few hours before sunrise.
You need very dark, clear sky to see the zodiacal light. Any light pollution or smoke or haze will obliterate your view. If you have exceptional sky, you might see the light extend in a narrow band all the way along the ecliptic. And in near-perfect sky, you might see an oval patch of faint white light about 15 degrees by 8 degrees directly opposite the sun. This is the Gegenschein (German for “counter shine”). It’s simply sunlight reflected off dust further out in the solar system. The Gegenschein is hard to see, however, and I count myself among the majority of stargazers who’ve never caught a glimpse of it.
So if you’re up before sunrise in the northern hemisphere, like Khayam’s “drowsy worshiper”, try to see the zodiacal light. You southerner’s can see it after sunset right now as you set up your telescopes for a night’s observing. Not many see the zodiacal light, and many who do have no idea what it is. But now, you do.