Zodiacal light: All you need to know
Zodiacal light before dawn or after dusk
The zodiacal light is a cone of eerie light above the sunrise or sunset point on the horizon, before morning dawn breaks or after evening twilight ends. No matter where you are on Earth, your best chance to see it in the east before dawn is late summer or early autumn (false dawn). Your best chance to see it in the west at dusk is late winter or early spring (false dusk).
The light looks like a hazy pyramid. It appears in the sky just before true dawn lights the sky. It’s comparable in brightness to the Milky Way, but even milkier in appearance.
Maybe you’ve seen the zodiacal light in the sky already and not realized it. Maybe you glimpsed it while driving on a highway or country road. This strange light is a seasonal phenomenon. Springtime and autumn are best for seeing it, no matter where you live on Earth.
How can I see the zodiacal light?
Suppose you’re driving toward the east – in the dark hour before dawn – in late summer or early autumn. You might sight what you think is the light of a nearby town, just over the horizon. But it may not be a town, but the zodiacal light. The light extends up from the eastern horizon, shortly before morning twilight begins. The zodiacal light can be extremely bright and easy to see from latitudes like those in the southern U.S.
We also sometimes hear from skywatchers in the northern U.S. or Canada who’ve captured images of the zodiacal light.
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.
Most visible around the equinoxes
The zodiacal light is often visible from the tropics. That’s because the ecliptic – pathway of the sun and moon – hits the horizon at a steep angle from this part of the world all year long.
Outside the tropics, the zodiacal light is most likely to be visible before dawn in late summer/early autumn. That’s because the ecliptic – or path of the sun and moon – intersects the horizon most steeply for the year at sunrise on the autumn equinox.
On the other hand, the zodiacal light is most prevalent after dusk in late winter/early spring. That’s because then the ecliptic is most perpendicular to the western horizon at sunset on the spring equinox. This holds true for either the Northern or Southern Hemisphere.
Ecliptic steepest at sunrise on autumn equinox, at sunset on spring equinox
In late summer/early autumn, the zodiacal light can be seen in the hour before true dawn begins. Or, in late winter/early 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 below.
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! If you catch a photo, submit it here.
Springtime? Autumn? When should I look?
Springtime? Autumn? When should I look? Is there a Northern/Southern Hemisphere difference between the best time of year to view the zodiacal light? 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.
No matter where you live on Earth, look for the zodiacal light in the east before dawn around the time of your autumn equinox. Look for it in the west after sunset around the time of your spring equinox.
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?
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.
Zodiacal light is a solar system phenomenon
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.
Bottom line: The zodiacal light – aka false dawn or dusk – is a hazy pyramid of light, really sunlight reflecting off dust grains in the plane of our solar system. Northern Hemisphere dwellers, look east before dawn. Southern Hemisphere … look west when all traces of evening twilight are gone.