The zodiacal light – or false dusk – is an eerie light extending up from the western horizon in springtime. You might also see it in the east before dawn begins to light the sky, in autumn, in which case it’s called the false dawn.
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 the zodiacal light. Suppose you’re driving toward the west in springtime the hour after dusk. You catch sight of what you think is the lingering evening twilight, or the light of a nearby town, just over the horizon. Instead, what you’re seeing is the zodiacal light. It looks like a hazy pyramid of light extending up from the western horizon, after evening twilight ends.
Springtime? Autumn? Is there a difference between Earth’s two hemispheres. Yes and no. For both hemisphere, springtime is the best time to see the zodiacal light in the evening. Autumn is the best time to see it before dawn. But, of course, spring and autumn fall in different months for Earth’s Northern and Southern Hemisphere.
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 you 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 after dusk in spring because, as seen from the northern hemisphere, the ecliptic – or path of the sun and moon – stands nearly straight up in autumn with respect to the western horizon after dusk. Likewise, the zodiacal light is easiest to see before dawn in autumn, because then the ecliptic is most perpendicular to the eastern horizon in the morning.
In spring, the zodiacal light can be seen for up to an hour after dusk ends. Or, in autumn, it can be seen for up to an hour before dawn. Unlike true 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.
Look for the zodiacal light in the west after sunset in late winter and early spring, around the time of the spring equinox. Look for it in the east before sunrise in late summer and early autumn, around the time of the autumn equinox. 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 is a hazy pyramid of light extending up from the western horizon, beginning about an hour after sunset, in springtime. It extends up from the eastern horizon, a couple of hours before dawn. It’s sunlight reflecting on dust grains that move in the plane of the solar system.