Being an astronomer is fun. You get to feel comfortable under the night sky, and after a few years, you’re able to make your way across the starry vault with a familiarity similar to walking around your own neighborhood.
That’s true during the day, too; I can’t tell you how many times I’ve oriented myself by noting where the sun is in the sky, which has literally helped me navigate my actual neighborhood.
Astronomy is a lovely undertaking by day or by night, but the sky has delights to offer in the between times as well. When the sun is low or has just barely set for the evening and twilight takes hold, there are phenomena well worth keeping a watch out for.
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Among my favorite sky sights at that time are crepuscular rays, or “twilight rays.” You may know them more colloquially as a type of sunbeam, a line of light sent across the sky, sometimes for quite a long distance. Sunbeams are common enough; you can see beams of light radiating away from the sun when it’s behind a cloud—a sight so familiar that when little kids draw the sun, they usually depict it emanating rays of light.
But don’t let that familiarity breed contempt. Crepuscular rays are actually a lot weirder than you might think.
Air is quite transparent, so you can’t see the light passing through it. But if there are particles suspended in the air—dust or haze, say—then the sun’s light illuminates them, and they reflect it brightly. If some object such as a cloud or a mountain is in front of the sun, however, then any particle in that line is in shadow and appears darker. To us on the ground, we see a long, straight shadow edge that appears cast on the sky itself. If there are multiple objects, then there are many different shadows cast, creating a series of bright and dark beams that appear to radiate from the sun.
This effect is more common at sunset because the sun’s light creates haze particles suspended in the air that accumulate during the day. Also, as twilight begins and the sky darkens, the contrast between light and shadow is easier to see.
The first time I remember seeing crepuscular rays was in elementary school, when I was cleaning blackboard erasers by banging them together outside, watching the sunbeams shine through the slowly expanding clouds of suspended chalk dust. When I lived in Colorado, I saw crepuscular rays quite often, with the beams fanning away from the sun as it set behind the Rocky Mountains. Sometimes, on particularly hazy or dusty days, the rays stretched all the way across the sky, from horizon to horizon. And this brings up a point that I love with all my science heart:
Crepuscular rays only look like they’re radiating away from the sun. They’re actually almost exactly parallel to one another!
The reason they appear to diverge is perspective. It’s exactly the same effect that makes it look like railroad tracks or the edges of a wide highway appear to converge in the distance, meeting at the horizon—at the vanishing point. They only look like they converge in the distance, but of course they don’t, really.
Crepuscular rays are doing the same thing. They’re parallel, but near the point in the sky where the sun sets, they’re far away from you, so they appear close together. As they get higher in the sky, they’re closer to you, so they appear farther apart, diverging. If they happen to pass directly overhead, they do in fact look parallel near the zenith, but if they continue on to the opposite, eastern horizon, they appear to converge again.
Technically, rays on the opposite side of the sky as the sun are called anticrepuscular rays, and if they meet again at the horizon, we say they converge on the antisolar point, the spot on the horizon directly opposite the sun’s position. Jargon is fun!
Crepuscular rays as seen from the International Space Station in low-Earth orbit.
NASA/piemags/Alamy Stock Photo
Even after I explain this parallelism to people, some still don’t believe me. But the proof can be seen from space! Photographs of long crepuscular rays taken from the International Space Station at local twilight clearly show that the rays are parallel. Anyone from the ground looking at them, though, would see them diverge away from the sun in the west and converge to the east. It literally helps to get a new perspective on some sights.
But rays are not the only twilight phenomenon to keep your eyes open for.
If you look east, with your back to the sun as it sets in the west, you can sometimes see a pinkish or reddish band on the horizon. It gets higher in the sky the longer you watch, and the colors deepen. Eventually a long wedge of darkness appears on the eastern horizon, highest in the direction directly opposite the sun and tapering to a thin angle on either side.
This is the Belt of Venus—and it’s actually the shadow of Earth on its own atmosphere! Toward your east, the sun sets earlier, so it’s farther below the horizon. As twilight progresses and the sun gets lower below the horizon, the air above Earth’s surface to the east darkens, contrasting with air higher up that’s still illuminated, creating the dark, shadowy band. The reddish colors above it appear because red is not scattered as well by airborne molecules and that air is only getting the reddest light from the setting sun.
I’ll note the Belt of Venus can occur at sunrise, too, but it helps to have more particulates in the air to see Earth’s shadow, and this is more common at sunset. Also, in general, more people are awake at dusk than dawn, so it’s more common to see Earth’s shadow after sunset.
Note that the Belt of Venus always has to appear opposite the sun, which is ironic; the planet Venus orbits closer to our star than Earth does, so we never see it more than around 45 degrees from the sun. So Venus can never appear in its own belt! The term, though, actually comes from the girdle or band usually associated with the sartorial stylings of the Roman goddess Venus and not the planet itself.
One of the joys of being an astronomer is paying attention to the sky because it contains innumerable delights to keep you entertained as you wait for it to darken. It’s also a joy to know what you’re seeing because that knowledge adds spice to the event. There is always beauty to see, but understanding gives it depth.
