Astronomy Essentials

Zodiacal light: All you need to know

Person standing watching hazy triangular area of light from horizon to near zenith at upper right corner.
Zodiacal light before dawn. Image via Jeff Dai. If you’re in the Southern Hemisphere now, where it’s spring, the light appears best in the west in the evening and slants the other way. Photos of the evening zodiacal light here.

Zodiacal light is false dawn (or dusk)

The zodiacal light is a cone of eerie light above the sunrise or sunset point on the horizon. You’ll see it 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. Unlike true dawn or dusk, there’s no rosy color to the zodiacal light. The reddish skies at dawn and dusk are caused by Earth’s atmosphere. But the zodiacal light originates far outside our atmosphere.

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The darker your sky, the better your chances of seeing the zodiacal light. Your best bet is to pick a night when the moon is out of the sky, although it’s possible, and very lovely, to see a slim crescent moon in the midst of this strange milky pyramid of light.

Zodiacal light or false dawn. Faint cone of light pointing to upper right in starry sky.
View at EarthSky Community Photos. | David Hoskin caught the “false dawn,” or zodiacal light, in the morning of September 8, 2021, at Chebucto Head, Nova Scotia. David wrote: “At 3 a.m. I decided to drive out to Chebucto Head, hoping that it would be dark enough there to image the zodiacal light. Given the presence of a nearby lighthouse, I am satisfied with the result.”

Most visible around the equinoxes

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. Suppose you’re driving toward the east – in the dark hour before dawn – in late summer or early autumn. You might see 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.

Zodiacal light before dawn in star field, slanting upward toward the right.
Zodiacal light on the morning of August 31, 2017, with Venus in its midst, captured at Mono Lake in California. The photographer, Eric Barnett, wrote: “I woke from sleeping in the car thinking sunrise was coming. My photographer friend, Paul Rutigliano, said it was the zodiacal light. I jumped up, got my camera into position and captured about a dozen or so shots.” Thank you, Eric! Photos of the evening zodiacal light here.

The zodiacal light is most visible around the equinoxes. That’s because the light follows the ecliptic, or pathway of the sun, moon and planets. And the ecliptic makes its most extreme angles with the horizon around the equinoxes. It makes its steepest angle on spring evenings (autumn mornings). It makes its narrowest angle on autumn evenings (spring mornings).

For the same reason, because it follows the ecliptic, the zodiacal light is often visible from Earth’s tropical latitudes. 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.

But the zodiacal light can be extremely bright and easy to see from latitudes close to the tropics. It’s very easy to see in dark skies in the southern U.S., for example. 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.

Very bright long exposure of zodiacal light with observatory to one side and slightly smudged stars in the sky.
Zodiacal Light over the Faulkes Telescope on the volcano Haleakala on Maui, Hawai’i. The zodiacal light is often visible at tropical latitudes. 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. Image via Rob Ratkowski.

Spring? Autumn? When should I look?

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.

Diagram of slant of ecliptic at different times of day and year.
The inclination of the ecliptic to the horizon changes over the course of the year. Its slant determines how high or low planets or the moon – or the zodiacal light – appear in a morning or evening twilight sky. Image via Dominic Ford/ In-The-Sky.org.

What is zodiacal light?

People used to think zodiacal light originated in Earth’s upper atmosphere. They thought it was a meteorological phenomenon (related to weather). But today we understand the zodiacal light as sunlight reflecting off dust grains that orbit 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.

A faint band of light, with a concentration of light toward the center of the image.
See the faint concentration of light toward the center of this image? That’s the elusive gegenschein – aka the counterglow – a diffuse spot visible in the darkest of skies, centered at the point directly opposite the sun. Sunlight reflecting on interplanetary dust causes it. Image via Project Nightflight.

Zodiacal light and gegenschein

The grains of dust that create the zodiacal light are like tiny worlds. They range from meter-sized to micron-sized. They’re densest around the immediate vicinity of the sun and extend 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. If we could see them, they’d mark 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, an illuminated spot, possible to glimpse in a dark night sky, centered at the point directly opposite the sun (the antisolar point).

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.

It’s time to watch for this elusive light! We hope you see it, and enjoy it. If you do see it, let us know! And if you catch a photo, submit it here.

Starry sky with wide, fuzzy triangle of light sticking up from the horizon.
View larger. | Lubomir Lenko wrote from Brehov, Slovakia, on August 18, 2018: “The rise of Orion is back with the fine shine of zodiacal light.” Thank you, Lubomir! Orion is in the lower right. See its Belt, the 3 stars in a short, straight row? The zodiacal light nearly fills the frame in this photo. Can you see that the light is pyramid-shaped?

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. You need a dark sky to see it. Northern Hemisphere dwellers look east before dawn from about late August through early November. Southern Hemisphere dwellers look in the evening in those same months. Or, from the Northern Hemisphere, look for the evening zodiacal light from late February through early May. From the Southern Hemisphere, look for the light in the morning in those same months.

Photos of the evening zodiacal light here

Posted 
September 6, 2021
 in 
Astronomy Essentials

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