The zodiacal light – or false dawn – is an eerie light extending up from the eastern horizon, before sunrise, in autumn. You might also see it in the west after sunset, in springtime, in which case it’s called the false dusk.
Maybe you’ve seen the zodiacal light in the sky and not realized it. Maybe you glimpsed it while driving on a highway or country road at this time of year. This strange light is a seasonal phenomenon. Springtime and autumn are best for seeing it, no matter where you live on Earth. Suppose you’re driving toward the east – in the hour before dawn – in autumn. You catch sight of what you think is the light of a nearby town, just over the horizon. But it might not be a town. It might be the zodiacal light. The light looks like a hazy pyramid of light extending up from the eastern horizon, shortly before morning twilight begins. Follow the links below to learn more about the zodiacal light.
Springtime? Autumn? Is there a difference between Earth’s two hemispheres? Yes and no. For both hemispheres, springtime is the best time to see the zodiacal light in the evening. Autumn is the best time to see it before dawn. Look for the zodiacal light in the east around the time of the autumn equinox. Look for it in the west after sunset around the time of the spring equinox.
But, of course, spring and autumn fall in different months for Earth’s Northern and Southern Hemispheres. So if you’re in the Northern Hemisphere look for the zodiacal light before dawn from about late August through early November. In those same months, if you’re in the Southern Hemisphere, look for the light in the evening.
Likewise, if you’re in the Northern Hemisphere, look for the evening zodiacal light from late February through early May. During those months, from the Southern Hemisphere, look for the light in the morning.
What is zodiacal light? People used to think zodiacal light originated somehow from phenomena in Earth’s upper atmosphere, but today we understand it as sunlight reflecting off dust grains that circle the sun in the inner solar system. These grains are thought to be left over from the process that created our Earth and the other planets of our solar system 4.5 billion years ago.
These dust grains in space spread out from the sun in the same flat disc of space inhabited by Mercury, Venus, Earth, Mars and the other planets in our sun’s family. This flat space around the sun – the plane of our solar system – translates on our sky to a narrow pathway called the ecliptic. This is the same pathway traveled by the sun and moon as they journey across our sky.
The pathway of the sun and moon was called the Zodiac or Pathway of Animals by our ancestors in honor of the constellations seen beyond it. The word zodiacal stems from the word Zodiac.
In other words, the zodiacal light is a solar system phenomenon. The grains of dust that create it are like tiny worlds – ranging from meter-sized to micron-sized – densest around the immediate vicinity of the sun and extending outward beyond the orbit of Mars. Sunlight shines on these grains of dust to create the light we see. Since they lie in the flat sheet of space around the sun, we could, in theory, see them as a band of dust across our entire sky, marking the same path that the sun follows during the day. And indeed there are sky phenomena associated with this band of dust, such as the gegenschein. But seeing such elusive sky phenomena as the gegenschein is difficult. Most of us see only the more obvious part of this dust band – the zodiacal light – in either spring or fall.
How I can see the zodiacal light? The zodiacal light can be extremely bright and easy to see from latitudes like those in the southern U.S., sometimes leading to the above situation where drivers mistake the lights for a town just over the horizon.
Meanwhile, skywatchers in the northern U.S. or Canada sometimes say, wistfully, that they’ve never seen it.
You’ll need a dark sky location to see the zodiacal light, someplace where city lights aren’t obscuring the natural lights in the sky. The zodiacal light is even milkier in appearance than the summer Milky Way. It’s most visible before dawn in autumn because, that’s when the ecliptic – or path of the sun and moon – stands nearly straight up with respect to the eastern horizon before dawn. Likewise, the zodiacal light is easiest to see when true night falls in spring, because then the ecliptic is most perpendicular to the western horizon in the evening.
In autumn, the zodiacal light can be seen in the hour before true dawn begins. Or, in spring, it can be seen for up to an hour after all traces of evening twilight leave the sky. Unlike true dawn or dusk, though, there’s no rosy color to the zodiacal light. The reddish skies at dawn and dusk are caused by Earth’s atmosphere, while the zodiacal light originates far outside our atmosphere, as explained above.
The darker your sky, the better your chances of seeing it. Your best bet is to pick a night when the moon is out of the sky, although it’s definitely possible, and very lovely, to see a slim crescent moon in the midst of this strange milky pyramid of light.
If you see it, let us know!
Bottom line: The zodiacal light – sometimes called the false dawn – is a hazy pyramid of light extending up from the eastern horizon, beginning about an hour before true dawn begins to break, in autumn. It extends up from the western horizon, a couple of hours after sunset. The light is caused by sunlight reflecting on dust grains that move in the plane of the solar system.