This month, if you’re in the northern hemisphere, you might see a large, otherworldly triangle of faint white light appearing above the western horizon about an hour after sunset. Those are not faint city lights. The ghostly phenomenon is called zodiacal light, and it’s the reflection of sunlight off interplanetary debris from comet and asteroid collisions. These cosmic dust particles enter our atmosphere and settle into a diffuse cone of light that is fattest near the sun and tapers as it extends upwards.
This ethereal light was visible as of last week and should continue through February 20th, according to Sky and Telescope, thanks to the moon making itself scarce. Zodiacal light is also known as a false dawn (pictured above). That’s because during the fall, it appears at the eastern horizon about an hour or two before sunrise.
This light-reflecting dust travels around the sun in the same plane as the planets in our solar system: a flat disk known as the ecliptic. The ecliptic forms the center of the zodiac, the band of constellations that the sun, moon, and the planets pass through. That’s also where the zodiacal light gets its name. In ancient times, the 12 constellations correlated with the 12 zodiac signs, but the constellations are drifting apart and their boundaries had to be redefined. The sun’s path, or the ecliptic, now passes through 13 constellations, with Ophiuchus as the newest addition.
Because zodiacal light and the Milky Way are both oriented nearly vertically at times, the two are often confused. You can see the difference below in this NASA APOD. Both bands are standing in the night sky above the Teide Observatory on the island of Tenerife off the coast of Morocco:
To see zodiacal light in the northern hemisphere this month, get away from bright city lights and look west-southwest about an hour to 90 minutes after sunset. First, give your eyes 20 minutes to adjust to the darkness, Space.com explains, then use Venus and Mars (which will be bright and low on the western horizon) to locate the base of the cone of zodiacal light. It will be tilted to the left. The Milky Way will be about 20 degrees to the right.
If you’re in the southern hemisphere, look for zodiacal light in the morning.
Images: Yuri Beletsky/ESO (top), Daniel López/IAC (middle) via NASA Astronomy Photo of the Day
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