Starship Asterisk*

Crepuscular Moon Rays over Denmark

Explanation: This moon made quite an entrance. Typically, a moonrise is quiet and serene. Taking a few minutes to fully peek above the horizon, Earth's largest orbital companion can remain relatively obscure until it rises high in the nighttime sky. About a week ago, however, and despite being only half lit by the Sun, this rising moon put on a show -- at least from this location. The reason was that, as seen from Limfjord in Nykøbing Mors, Denmark, the moon rose below scattered clouds near the horizon. The result, captured here in a single exposure, was that moonlight poured through gaps in the clouds to created what are called crepuscular rays. These rays can fan out dramatically across the sky when starting near the horizon, and can even appear to converge on the other side of the sky. Well behind our Moon, stars from our Milky Way galaxy dot the background, and our galaxy's largest orbital companion -- the Andromeda galaxy -- can be found on the upper left.

<< Previous APOD

This Day in APOD

Next APOD >>

That's a very beautiful picture indeed! It looks almost like a painting. Do note Andromeda at upper left, and note how bright its center is, and how faint the disk is. "Normal" pictures of Andromeda don't show us this great contrast in brightness.

Not only is the composition lovely here, but the colors are strikingly beautiful, too. You can find the stars in the Great Square of Pegasus (which looks a little squashed) and note their colors.

There are three orange stars at upper right, and the brightest and leftmost of these is Scheat, Beta Pegasi, a reddish M-type giant. To the lower right of Scheat is a bright white star, which is Markab, Alpha Pegasi, an A-type star. At upper center, in a bluish gap between two golden-colored crepuscular rays, you find a blue-white star (with a lot of small stars to the right of it), which is Alpha Andromedae, Alpheratz, of spectral type B9. And halfway between Alpheratz and Venus near the horizon is a bluish star, Algenib, Gamma Pegasi, an intrinsically bright and blue B2-type star.

I would annotate the image and show you, except my computer is groaning under the weight of all the images I have uploaded to it! So I'll help you by adding two annotated pictures of the Great Square of Pegasus. I had to upload one of them, but at least I didn't have to add annotation:


I've got to show you a picture of anticrepuscular rays, too.

Wikipedia wrote:

Anticrepuscular rays, or antisolar rays, are meteorological optical phenomena similar to crepuscular rays, but appear opposite the Sun in the sky. Anticrepuscular rays are essentially parallel, but appear to converge toward the antisolar point, the vanishing point, due to a visual illusion from linear perspective.

Anticrepuscular rays are most frequently visible around sunrise or sunset. This is because the atmospheric light scattering that makes them visible (backscattering) is larger for low angles to the horizon than most other angles. Anticrepuscular rays are dimmer than crepuscular rays because backscattering is less than forward scattering.

Anticrepuscular rays can be continuous with crepuscular rays, curving across the whole sky in great circles.

Did you get that? I can't say I did, but... we can enjoy the picture anyway, can't we?

Ann

Ann wrote: ↑Wed Jul 27, 2022 5:12 am

Wikipedia wrote:

Anticrepuscular rays, or antisolar rays, are meteorological optical phenomena similar to crepuscular rays, but appear opposite the Sun in the sky. Anticrepuscular rays are essentially parallel, but appear to converge toward the antisolar point, the vanishing point, due to a visual illusion from linear perspective.

Anticrepuscular rays are most frequently visible around sunrise or sunset. This is because the atmospheric light scattering that makes them visible (backscattering) is larger for low angles to the horizon than most other angles. Anticrepuscular rays are dimmer than crepuscular rays because backscattering is less than forward scattering.

Anticrepuscular rays can be continuous with crepuscular rays, curving across the whole sky in great circles.

Did you get that? I can't say I did, but... we can enjoy the picture anyway, can't we? :wink:

What part didn't you get?

I agree that it is very beautiful; and most APOD's are! Who are you to talk?

Chris Peterson wrote: ↑Wed Jul 27, 2022 1:06 pm

Ann wrote: ↑Wed Jul 27, 2022 5:12 am

Wikipedia wrote:

Anticrepuscular rays, or antisolar rays, are meteorological optical phenomena similar to crepuscular rays, but appear opposite the Sun in the sky. Anticrepuscular rays are essentially parallel, but appear to converge toward the antisolar point, the vanishing point, due to a visual illusion from linear perspective.

Anticrepuscular rays are most frequently visible around sunrise or sunset. This is because the atmospheric light scattering that makes them visible (backscattering) is larger for low angles to the horizon than most other angles. Anticrepuscular rays are dimmer than crepuscular rays because backscattering is less than forward scattering.

Anticrepuscular rays can be continuous with crepuscular rays, curving across the whole sky in great circles.

Did you get that? I can't say I did, but... we can enjoy the picture anyway, can't we?

What part didn't you get?

Why the rays are dark?

Ann

Ann wrote: ↑Wed Jul 27, 2022 5:12 am

Anticrepuscular rays toward the eastern horizon, as seen from Colorado at dusk. Image: Seppukudetat at English Wikipedia.

---

Wikipedia wrote:
Anticrepuscular rays, or antisolar rays, are meteorological optical phenomena similar to crepuscular rays, but appear opposite the Sun in the sky. Anticrepuscular rays are essentially parallel, but appear to converge toward the antisolar point, the vanishing point, due to a visual illusion from linear perspective.
Anticrepuscular rays are most frequently visible around sunrise or sunset. This is because the atmospheric light scattering that makes them visible (backscattering) is larger for low angles to the horizon than most other angles. Anticrepuscular rays are dimmer than crepuscular rays because backscattering is less than forward scattering.
Anticrepuscular rays can be continuous with crepuscular rays, curving across the whole sky in great circles.

Did you get that? I can't say I did, but... we can enjoy the picture anyway, can't we?
Ann

I doubt that those rays were anticrepuscular. Why that part of the low sky was so red if it was opposite to Sun's side?
Another wiki's example has rays incident at the centre of a double rainbow. That seems reasonable.

Anticrepuscular rays and double rainbow opposite the setting sun on the northern shore of Lake Ontario. Taken at Meadow Wood Park in Mississauga.

Ann wrote: ↑Wed Jul 27, 2022 1:42 pm Why the rays are dark?
Ann

When clouds are 50 % holes, the rays occupy 50 % of space and the shadows own the rest, don't they?

Ann wrote: ↑Wed Jul 27, 2022 1:42 pm

Chris Peterson wrote: ↑Wed Jul 27, 2022 1:06 pm

Ann wrote: ↑Wed Jul 27, 2022 5:12 am

Did you get that? I can't say I did, but... we can enjoy the picture anyway, can't we? :wink:

What part didn't you get?

Why the rays are dark? :wink:

Ann

Are they? Which are the rays... the shadows or the lit areas between the shadows? Or both?

Is the Andromeda Galaxy really an "orbital companion" of ours? Has its orbit been determined?

John

j_isles wrote: ↑Wed Jul 27, 2022 2:46 pm Is the Andromeda Galaxy really an "orbital companion" of ours? Has its orbit been determined?

Its orbit is not accurately known. But the two galaxies are gravitationally bound, which means they are in orbit around each other.

This article bears on my question:
https://www.sciencealert.com/we-now-kno ... -andromeda

jisles wrote: ↑Wed Jul 27, 2022 2:49 pm This article bears on my question:
https://www.sciencealert.com/we-now-kno ... -andromeda

Right. As noted, the orbit is not well characterized. But all the galaxies in the Local Group are in orbit around each other, because none is moving at escape velocity with respect to the cluster.

I wonder if this fanning out is crepuscular rays too

VictorBorun wrote: ↑Wed Jul 27, 2022 3:29 pm I wonder if this fanning out is crepuscular rays too
Southern Ring Nebula rays.jpg

We do see interstellar dust clouds cast shadows. Not sure we can call them "crepuscular", though, in an environment without night or day.

Chris Peterson wrote: ↑Wed Jul 27, 2022 3:46 pm

VictorBorun wrote: ↑Wed Jul 27, 2022 3:29 pm I wonder if this fanning out is crepuscular rays too
Southern Ring Nebula rays.jpg

We do see interstellar dust clouds cast shadows. Not sure we can call them "crepuscular", though, in an environment without night or day.

well I did not mean crepuscular like low-sun (or low-moon), but the visual fanning out with so strong perspective that the lines are in fact as well as parallel.
Are Southern Ring Nebula's rays like that, going our way many times more than crossing our field of view?

I thought it was the Sun. (Poor me!)