It’s fun to be an astronomer. You will feel comfortable under the night sky, and in a few years you will be able to cross the starry vault with the same familiarity as taking a walk in your own neighborhood.
That also applies during the day. I can’t tell you how many times I looked at where the sun was in the sky to orient myself. This literally helped me navigate the real neighborhood.
Astronomy is a great job day and night, but the sky also provides entertainment in between. When the sun becomes low or just sets in the evening and twilight begins, a phenomenon occurs that is worth paying attention to.
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My favorite sky sight from that time was the crepuscular rays, or “twilight rays.” It may be known colloquially as a type of solar ray, a line of light that is sent across the sky, sometimes over great distances. Sunlight is common enough. When the sun is behind the clouds, you can see the rays radiating from the sun. This is a very familiar sight, and when young children draw the sun, they usually depict the sun emitting rays.
But don’t let that familiarity breed contempt. Crepuscular rays are actually much stranger than you think.
Air is so transparent that we cannot see the light passing through it. However, if there are particles suspended in the air (such as dust or fog), the sun’s rays will shine on them and reflect brightly. However, if an object such as a cloud or a mountain is in front of the sun, particles within that line will be in the shadow and appear darker. To us on the ground, it appears as if a long, straight shadow edge is being projected onto the sky itself. If there are multiple objects, different shadows will be cast, creating a series of bright and dark beams that appear to emanate from the sun.
This phenomenon is more common at sunset because the sun’s rays create airborne haze particles 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 saw crepuscular rays was when I was in elementary school. As I was outside cleaning with a chalkboard eraser, I watched the sun’s rays shine through a slowly expanding cloud of chalk dust. When I lived in Colorado, I often saw crepuscular rays receding from the sun as it set behind the Rocky Mountains. Sometimes, especially on foggy or dusty days, rays of light would stretch across the sky from horizon to horizon. And this brings me to a point that I really love about science.
crepuscular rays only look It’s like it’s radiating from the sun. In fact, they are almost exactly parallel to each other.
The reason they appear to diverge is perspective. the that’s right It’s the same effect that makes the edges of railroad tracks or wide highways appear to converge in the distance and meet at the vanishing point of the horizon. It looks like they’re all gathered in the distance, but of course that’s not really the case.
Crepuscular rays do the same thing. They are parallel, but near the point in the sky where the sun sets, they appear closer to each other because they are farther away from you. The higher the sky, the closer they are to you, the more distant they appear and the more they diverge. If they happen to pass overhead, they will actually appear parallel near the zenith, but as they move toward the eastern horizon on the opposite side, they will appear to converge again.
Technically, the rays that are on the opposite side of the sky from the sun are called anti-crepuscular rays, and if they meet again at the horizon, they converge at the anti-solar point, i.e. a point on the horizon directly opposite the position of the sun. I say. Terminology is fun!
Crepuscular rays seen from the International Space Station in low Earth orbit.
NASA/piemags/Alamy Stock Photo
When I explain this parallelism to people, some still don’t believe me. But the evidence can be seen from space. Photos of long crepuscular rays taken from the International Space Station at local dusk clearly show that the rays are parallel. But anyone watching them from the ground will see them moving away from the sun in the west and converging in the east. It literally helps you get a new perspective on some sights.
But the rays aren’t the only dusk phenomenon you’ll want to keep an eye out for.
If you look east with the sun setting in the west, you may see a pinkish or reddish band on the horizon. The more you look, the higher the sky becomes, and the colors deepen. Eventually, a long wedge of darkness appears on the eastern horizon, highest directly opposite the sun and tapering to narrow angles on each side.
This is the Belt of Venus – and it actually is Earth’s shadow above Earth’s own atmosphere! As you head east, the sun will set earlier, further below the horizon. As dusk progresses and the sun lowers below the horizon, the air above the surface to the east darkens, forming a dark band of shadow that contrasts with the still bright air aloft. The reddish color on it appears because red is not scattered by molecules in the air, and that air receives only the reddest light from the setting sun.
Note that Venus bands can also occur at sunrise. But seeing Earth’s shadow helps when there are more particles in the air, which is more common at sunset. Additionally, more people are generally awake at dusk than at dawn, so we see more of the Earth’s shadow after sunset.
Please note that the Venus belt must always appear Opposition Ironically the sun. Venus orbits closer to its star than Earth, so you won’t see Venus from further than about 45 degrees from the Sun. Therefore, Venus will never appear in its own belt. However, the term actually derives from the girdle or band typically associated with the Roman goddess Venus’ clothing style, rather than the planet itself.
One of the joys of being an astronomer is paying attention to the sky. Because the sky contains countless things to enjoy while you wait for darkness to set in. It’s also fun to know what you’re looking at because that knowledge adds spice to the event. There are beautiful things no matter when you look at them, understanding Gives depth.
