<p>What is happening here? Photoshop!</p>

<blockquote>
  <p>The number of visible satellites plummets towards the middle of the night when more satellites fall into the shadow of the Earth, <strong>represented by the dark area on the left of the image</strong>. (emphasis added)</p>
</blockquote>

CC BY-SA 4.0
2020-03-18T00:13:08.337Z
yep, I can't imagine that this could possibly not be the answer. Thanks!
uhoh
CC BY-SA 4.0
2020-03-14T17:37:23.440Z
2020-03-14T17:37:23.440Z
JohnHoltz
<p><a href="https://astronomy.stackexchange.com/questions/34219/did-nobody-in-the-astronomy-community-think-12-000-new-satellites-in-leo-might-b?noredirect=1#comment68924_34219">This comment</a> to <em>Did nobody in the Astronomy community think 12,000 new satellites in LEO might be a problem?</em> links to Phys.org's <a href="https://phys.org/news/2020-03-eso-impact-satellite-constellations-astronomical.html" rel="nofollow noreferrer">New ESO study evaluates impact of satellite constellations on astronomical observations</a> which includes ESO's <a href="https://www.eso.org/public/switzerland-it/images/eso2004a/" rel="nofollow noreferrer">Areas of the sky most affected by satellite constellations</a> shown below.</p>
<p>It's a fisheye lens view (FOV wider than 180 degrees)looking straight up.</p>
<p>There is an large arc that begins at the bottom of the image (where the elevations of 20 and 30 degrees are deonted) and extends upward and toward the left. It looks like it could be a circle about the anti-lunar point since the moon is also seen on the right side slightly above center.</p>
<p><strong>Question:</strong> What is happening here? Is this an atmospheric effect of scattered moonlight? If so, what causes such a sharp transition in the amount of scattered light? Does this have a name?</p>
<p><a href="https://i.sstatic.net/aZNZU.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/aZNZU.jpg" alt="the night sky at ESO's Paranal Observatory around twilight, about 90 minutes before sunrise Crediti: ESO/Y. Beletsky/L. Calçada" /></a></p>
<blockquote>
<p>This annotated image shows the night sky at ESO's Paranal Observatory around twilight, about 90 minutes before sunrise. The blue lines mark degrees of elevation above the horizon.</p>
<p>A new ESO study looking into the impact of satellite constellations on astronomical observations shows that up to about 100 satellites could be bright enough to be visible with the naked eye during twilight hours (magnitude 5–6 or brighter). The vast majority of these, their locations marked with small green circles in the image, would be low in the sky, below about 30 degrees elevation, and/or would be rather faint. Only a few satellites, their locations marked in red, would be above 30 degrees of the horizon — the part of the sky where most astronomical observations take place — and be relatively bright (magnitude of about 3–4). For comparison, Polaris, the North Star, has a magnitude of 2, which is 2.5 times brighter than an object of magnitude 3.</p>
<p>The number of visible satellites plummets towards the middle of the night when more satellites fall into the shadow of the Earth, represented by the dark area on the left of the image. Satellites within the Earth's shadow are invisible.</p>
<p>Crediti: ESO/Y. Beletsky/L. Calçada</p>
</blockquote>

CC BY-SA 4.0
2020-03-14T06:00:46.340Z
companion question in Astronomy: [Which kinds of astronomical observations most need to avoid the Moon being up?](https://astronomy.stackexchange.com/q/35498/7982)
uhoh
CC BY-SA 4.0
2020-03-14T09:57:50.960Z
I would assume it is an effect of the camera system. For example some shadowing effect of the moonlight since the moon is directly opposed to the arc
Christoph
CC BY-SA 4.0
2020-03-14T15:04:25.043Z
Could it be that we don't see the Zodiacal light from the Earth's shadow?
Keith McClary
CC BY-SA 4.0
2020-03-14T16:15:21.613Z
@KeithMcClary that's a really cool thought, but I think that the Zodiacal light comes from particles in orbit around the Sun mostly quite far from Earth. They orbit in the ecliptic like everything else, thus the name, and so the light is fairly well constrained to a band near the ecliptic and not too far from the Sun, whereas this seems to be everywhere.  See [1](https://astronomy.stackexchange.com/q/28199/7982), [2](https://astronomy.stackexchange.com/q/28736/7982), and of course [3](https://astronomy.stackexchange.com/a/29865/7982) :-)
uhoh
CC BY-SA 4.0
2020-03-14T16:31:39.367Z
It says on the ESO page: "the shadow of the Earth, represented by the dark area on the left of the image".
Keith McClary
CC BY-SA 4.0
2020-03-14T16:35:39.777Z
@KeithMcClary okay so its not a shadow of the Zodiacal light proper, but it could be artificially shadowed to illustrate where artificial satellites in LEO will go into Earth's shadow. Since they've gone and added those glowing blue rings and annotations for elevation and marked the satellites, maybe this is artificial too. I'll be surprised if it's a real shadow, but if it is it will be quite a pleasant one!
uhoh
CC BY-SA 4.0
2020-03-14T17:10:49.260Z
This is from the ["Fulldome"](https://www.eso.org/public/images/archive/category/fulldome/) (First of 4 pages.)  I don't see a shadow in any other images.
Keith McClary
<atmosphere><astronomy><identification-request><atmospheric-optics>
What causes this arc in the night sky where the background is brighter on one side than the other?
uhoh
# What causes this arc in the night sky where the background is brighter on one side than the other?

(Asked by: username_0 on 2020-03-14)

[This comment](https://astronomy.stackexchange.com/questions/34219/did-nobody-in-the-astronomy-community-think-12-000-new-satellites-in-leo-might-b?noredirect=1#comment68924_34219) to *Did nobody in the Astronomy community think 12,000 new satellites in LEO might be a problem?* links to Phys.org's [New ESO study evaluates impact of satellite constellations on astronomical observations](https://phys.org/news/2020-03-eso-impact-satellite-constellations-astronomical.html) which includes ESO's [Areas of the sky most affected by satellite constellations](https://www.eso.org/public/switzerland-it/images/eso2004a/) shown below.

It's a fisheye lens view (FOV wider than 180 degrees)looking straight up.

There is an large arc that begins at the bottom of the image (where the elevations of 20 and 30 degrees are deonted) and extends upward and toward the left. It looks like it could be a circle about the anti-lunar point since the moon is also seen on the right side slightly above center.

**Question:** What is happening here? Is this an atmospheric effect of scattered moonlight? If so, what causes such a sharp transition in the amount of scattered light? Does this have a name?

[![the night sky at ESO's Paranal Observatory around twilight, about 90 minutes before sunrise Crediti: ESO/Y. Beletsky/L. Calçada](https://i.sstatic.net/aZNZU.jpg)](https://i.sstatic.net/aZNZU.jpg)

> This annotated image shows the night sky at ESO's Paranal Observatory around twilight, about 90 minutes before sunrise. The blue lines mark degrees of elevation above the horizon.
>
> A new ESO study looking into the impact of satellite constellations on astronomical observations shows that up to about 100 satellites could be bright enough to be visible with the naked eye during twilight hours (magnitude 5–6 or brighter). The vast majority of these, their locations marked with small green circles in the image, would be low in the sky, below about 30 degrees elevation, and/or would be rather faint. Only a few satellites, their locations marked in red, would be above 30 degrees of the horizon — the part of the sky where most astronomical observations take place — and be relatively bright (magnitude of about 3–4). For comparison, Polaris, the North Star, has a magnitude of 2, which is 2.5 times brighter than an object of magnitude 3.
>
> The number of visible satellites plummets towards the middle of the night when more satellites fall into the shadow of the Earth, represented by the dark area on the left of the image. Satellites within the Earth's shadow are invisible.
>
> Crediti: ESO/Y. Beletsky/L. Calçada

**Tags:** atmosphere, astronomy, identification-request, atmospheric-optics

> **Comments:**
> **username_0**: companion question in Astronomy: [Which kinds of astronomical observations most need to avoid the Moon being up?](https://astronomy.stackexchange.com/q/35498/7982)  
> **username_1**: I would assume it is an effect of the camera system. For example some shadowing effect of the moonlight since the moon is directly opposed to the arc  
> **username_2**: Could it be that we don't see the Zodiacal light from the Earth's shadow?  
> **username_0**: @username_2 that's a really cool thought, but I think that the Zodiacal light comes from particles in orbit around the Sun mostly quite far from Earth. They orbit in the ecliptic like everything else, thus the name, and so the light is fairly well constrained to a band near the ecliptic and not too far from the Sun, whereas this seems to be everywhere.  See [1](https://astronomy.stackexchange.com/q/28199/7982), [2](https://astronomy.stackexchange.com/q/28736/7982), and of course [3](https://astronomy.stackexchange.com/a/29865/7982) :-)  
> **username_2**: It says on the ESO page: "the shadow of the Earth, represented by the dark area on the left of the image".  
> **username_0**: @username_2 okay so its not a shadow of the Zodiacal light proper, but it could be artificially shadowed to illustrate where artificial satellites in LEO will go into Earth's shadow. Since they've gone and added those glowing blue rings and annotations for elevation and marked the satellites, maybe this is artificial too. I'll be surprised if it's a real shadow, but if it is it will be quite a pleasant one!  
> **username_2**: This is from the ["Fulldome"](https://www.eso.org/public/images/archive/category/fulldome/) (First of 4 pages.)  I don't see a shadow in any other images.  

---

## 1 answers

### Answer by username_3 on 2020-03-14. Score: 4 | Accepted answer

What is happening here? Photoshop!

> The number of visible satellites plummets towards the middle of the night when more satellites fall into the shadow of the Earth, **represented by the dark area on the left of the image**. (emphasis added)

> **Comments:**
> **username_0**: yep, I can't imagine that this could possibly not be the answer. Thanks!  

---

