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| Astronomy Essentials on Sep 09, 2012

Everything you need to know: zodiacal light or false dawn

It’s an eerie light extending up from the horizon. In autumn, you’re most likely to see it about an hour before true dawn begins.

The zodiacal light – or false dawn – is an eerie light extending up from the horizon. You’re most likely to see it about an hour after sunset in late winter or early spring, or an hour before dawn in the late summer or early autumn.

Maybe you’ve seen the zodiacal light already. Maybe you’ve seen it and not realized it! Maybe it happened while you were driving on a highway or country road – somewhere in the southern U.S. or similar latitudes – at this time of year. Suppose you’re driving east in the hour before dawn. You catch sight of what you think is the light of a nearby town, just over the horizon. You drive on, confident that food or a short rest is not far away. The light stays in view for half an hour or more, until true dawn begins to light the sky. But no town ever appears. This strange light may be the zodiacal light.

Photo credit: Robert Snache

What is the zodiacal light, or false dawn? People used to think this eerie light originated from unknown phenomena in Earth’s upper atmosphere. But today we understand it as sunlight reflecting off dust grains that move in outer space. 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.

Where did the name false dawn come from? The name false dawn originated with the 12th century Persian astronomer, mathematician, and poet Omar Khayyam. In the 200th verse of his poem The Rubaiyat, he wrote:

Zodiacal Light over the Faulkes Telescope in Hawaii. Photo credit: Wikimedia Commons

When false dawn streaks the east with cold, gray line,
Pour in your cups the pure blood of the vine;
The truth, they say, tastes bitter in the mouth,
This is a token that the ‘Truth’ is wine.

It’s beautiful to imagine that people stood where you might stand – outside on autumn morning, gazing eastward in the cool of predawn – and saw what you might also see: the false dawn, or zodiacal light.

At what time of year are you most likely to see it? The false dawn is a seasonal phenomenon. You’re most likely to see it in the east before dawn around the time of the autumnal equinox. But you can see the light in springtime, too, around the time of the spring equinox, in the west in the evening. Either way, it looks like a hazy pyramid of light extending up from the eastern horizon, an hour before true predawn twilight begins. And, by the way, when it’s visible in the west after sunset, no one calls it the false dusk – although that name does seem perfectly appropriate.

The light is most visible before dawn in autumn 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 eastern horizon before dawn.

Likewise, the zodiacal light is easiest to see after sunset in springtime, because then the ecliptic is most perpendicular to the western horizon in the evening.

Under what sky conditions might you see it? You’ll need a dark sky location to see this false dawn, or 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, but if you can see the Milky Way you can also see the zodiacal light.

It helps to have a clear view toward the horizon, of course. Trees or mountains might obscure the view.

In autumn, the zodiacal light can be seen for up to an hour before true dawn begins to break. Or, in spring, it can be seen for up to an hour after twilight ends. Unlike true dawn, 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 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.

Bottom line: Look for the zodiacal light, or false dawn, in the east before sunrise in September and October, around the time of the autumn equinox. Look for it in the west after sunset in March and April, around the time of the spring 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!