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| Astronomy Essentials on Oct 20, 2014

Glowing pyramid of light in east might be false dawn

Autumn is the best time of the year to see the false dawn, also known as zodiacal light.

Autumn is the best time of year to see the false dawn, also known as the zodiacal light. For us in the Northern Hemisphere, this time of year is the best time of year to see it. If you’re in the Southern Hemisphere, click here.

This light can be noticeable and easy to see from latitudes like those in the southern U.S. I’ve seen it many times from the latitude of southern Texas, sometimes while driving a lonely highway far from city lights, in the hour or so before true dawn begins to light the sky. In that case, the zodiacal light can resemble the lights of a city or town just over the horizon. Meanwhile, skywatchers in the northern U.S. or Canada sometimes say, wistfully, that they’ve never seen it.

Also before dawn, the Orionid meteor shower

You need a dark sky location to see the zodiacal light, someplace where city lights aren’t obscuring the natural lights in the sky. And you have to be up before dawn. If you don’t know when morning twilight begins in your sky, an astronomical almanac can tell you.

Looking for a sky almanac? EarthSky recommends …

Milky Way on left in this photo. Zodiacal light on right. This photo is from EarthSky Facebook friend Sean Parker Photography. He captured it in October 2012 at Kitt Peak National Observatory in Arizona. Click here to expand this image.

What does it look like? The zodiacal light is a pyramid-shaped glow in the east before dawn. It’s even “milkier” in appearance than the starlit trail of the summer Milky Way. It’s most visible before dawn at this time of year because (as seen from the northern hemisphere) the ecliptic – or path of the sun, moon and planets – stands nearly straight up with respect to the eastern horizon before dawn now. At temperate latitudes in the southern hemisphere, where it is now spring, the zodiacal light is seen after dusk, rather than before dawn.

The zodiacal light can be seen for up to an hour before true dawn begins to break. Unlike true dawn, though, there’s no rosy color to the zodiacal light. The reddish skies at dawn and dusk are caused by Earth’s atmosphere, and the zodiacal light originates far outside our atmosphere.

Zodiacal Light

Zodiacal light at Paranal. Credit: ESO/Y. Beletsky

When you see the zodiacal light, whether it’s before dawn or after dusk, you are looking far beyond our atmosphere – edgewise into the space of our own solar system. The zodiacal light is actually sunlight reflecting off dust particles that move in the same plane as Earth and the other planets orbiting our sun.

If you’re in the Southern Hemisphere, you can see the zodiacal light, too. At this time of year, instead of finding it in the east before dawn, you should look west after sunset. Start looking about an hour after the sun goes down, when all traces of regular twilight have left the sky.

Bottom line: The zodiacal light, or false dawn, is best seen before true dawn breaks during the autumn months from either the Northern or Southern Hemisphere. September-October are good times to look for the Northern Hemisphere. March-April are good times to look for the Southern Hemisphere. Want to see the zodiacal light in the evening? In that case, spring is best. The light appears as a hazy pyramid in the location of the coming sunrise (or sunset). You need a dark sky to see it.