Earth & Sky

'False dawn' glows on autumn mornings

The zodiacal light
Photo: Dominic Cantin

Saturday, September 22.

At mid-northern latitudes, where summer is giving way to autumn, a seasonal sleight of hand known as the “false dawn” precedes the sunrise.

It’s official name is the zodiacal light, and you might see it 1-and-a-half to 2 hours before sunrise, before true dawn begins to light the sky. The false dawn appears as a pyramid of light jutting upward from the eastern horizon. This year, the faintly milk-colored haze will sweep up to or beyond the brilliant planet Venus, which has recently come back before dawn.

The zodiacal light is caused by the reflection of sunlight from grains of dust moving between the planets in our solar system. If you’re driving down a country road in the predawn hours, you could mistake zodiacal light for the glow of a distant city beyond the horizon.

With the equinox due to arrive tomorrow, this is a great time to start looking for the zodiacal light. From the northern hemisphere, it’ll be visible in dark country skies, in the hour before dawn’s first light. The zodiacal light is always best before dawn in September, October and November from this hemisphere.

South of the equator now, winter is turning into spring, and the zodiacal light is best after sunset.

Written by Bruce McClure