Being an astronomer is fun. You’ll feel comfortable under the night sky, and after a few years, you’ll be able to walk through the starry vault with a familiarity similar to walking around your neighborhood.
That is also during the day; 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 real neighborhood.
Astronomy is beautiful day or night, but the sky has its delights in between too. When the sun is low or when it has just set in the evening and the sunset takes placethere are phenomena that must be taken care of.
About supporting science journalism
If you like this article, please consider supporting our award-winning journalism subscribe. By purchasing a subscription, you’re helping to ensure a future of impactful stories about the discoveries and ideas that shape our world.
Among my favorite celestial sightings at that time are the crepuscular rays, or “dusk rays.” You may know them more commonly as a type of sunbeam, a line of light sent across the sky, sometimes for quite a long distance. Sun rays are quite common; when behind a cloud you can see the rays of light moving away from the sun; a view so familiar that when young children draw the sun, they usually picture it as radiating rays of light.
But don’t let that familiarity breed contempt. Crepuscular rays are much rarer than you might think.
Air is quite transparent, so you can’t see light passing through it. But if there are particles suspended in the air – say dust or fog – then the light of the sun illuminates themand they reflect brightly. However, if an object like a cloud or a mountain is in front of the sun, any particle along that line is shadowed and appears darker. For us on earth, we see a long, straight edge of shadow cast in the sky itself. If there are multiple objects, then there are many different shadows, creating a series of bright and dark rays that appear to radiate from the sun.
This effect is more common at sunset, because sunlight creates suspended particles of fog that accumulate during the day. Also, as dusk begins and the sky darkens, the contrast between light and shadow becomes easier to see.
The first time I remember seeing the crepuscular rays was in elementary school, when I was erasing the blackboard erasers by tapping them together outside, watching the rays of the sun shine as the clouds of suspended chalk dust slowly spread. When I lived in Colorado, I often saw the crepuscular rays, the rays receding from the sun as it set behind the Rocky Mountains. Sometimes, on particularly cloudy or dusty days, the rays spread across the sky, from horizon to horizon. And this brings up a point I love with a science heart:
Crepuscular rays only look at like they are moving away from the sun. In fact, they are almost completely parallel to each other!
The reason they seem to leave is perspective. is exactly the same effect that railway tracks or the edges of a wide highway seem to merge in the distance, meeting on the horizon. vanishing point. From a distance they seem to match, but of course they don’t, in fact.
The crepuscular rays are doing the same thing. They are parallel, but near the point in the sky where the sun sets, they are far from you, so they appear close together. As they move higher in the sky, they are closer to you, so they appear further away, further apart. If they pass directly overhead, they actually look parallel near the zenith, but if they continue toward the opposite eastern horizon, they appear to merge again.
Technically, the rays that are on the opposite side of the sky as the sun are called anti-crepuscular rays, and if they meet again at the horizon, we say that they converge at the anti-sun point, the place that is opposite the sun’s position on the horizon. Jargon is fun!
Crepuscular rays as seen from the International Space Station in low Earth orbit.
NASA/piemags/Alamy Stock Photo
Even after explaining this parallel to people, some still don’t believe me. But the proof can be seen from space! Photographs of long crepuscular rays from the International Space Station at sunset clearly show that the rays are parallel.. Anyone on Earth looking at them, however, would see them moving away from the sun in the west and converging towards the east. It literally helps to gain a new perspective on some points of view.
But the rays aren’t the only sunset 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 pink or reddish band on the horizon. The longer you look at it, the higher the sky, and the deeper the colors. Finally, a long wedge of darkness appears on the eastern horizon, highest in the direction opposite the sun and tapering to a thin angle on either side.
This is the Belt of Venus, and it really is Earth’s shadow in its atmosphere! To your east, the sun sets earlier, so it is farther from the horizon. As dusk progresses and the sun descends below the horizon, the air at Earth’s surface to the east darkens, contrasting with the still-illuminated air above, creating a dark, shadowy band. The reddish colors above appear because it is red Air molecules do not disperse and that air receives only the reddest light from the setting sun.
I will note that the Belt of Venus can also occur at sunrise, but seeing the Earth’s shadow is helped by the fact that there are more particles in the air, which is more common at sunset. Also, generally more people are awake at dusk than at dawn, so it’s more common to see the Earth’s shadow at sunset.
Note that the Belt of Venus must always appear the opposite the sun, which is ironic; The planet Venus orbits closer to our star than Earth, so we never see it more than about 45 degrees from the sun. So Venus can never appear in her belt! The term, however, is really coming A belt or band associated with the clothing styles of the Roman goddess Venus and not the planet itself.
One of the joys of being an astronomer is looking at the sky, because it has countless delights to keep you entertained until dark. 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, though understanding it gives it depth.
