<p>The different <a href="https://en.wikipedia.org/wiki/Twilight" rel="nofollow noreferrer">twilights</a> are distinguished - as you write - by how many degrees the Sun is below the horizon:</p>
<ul>
<li>day: sun &gt; 0° (or actually &gt; -0.7° due to <a href="https://en.wikipedia.org/wiki/Refraction" rel="nofollow noreferrer">refraction</a>)</li>
<li>civil twilight: 0 ... -6°</li>
<li>nautical twilight: -6° .. -12°</li>
<li>astronomical twilight: -12° .. -18°</li>
<li>astronomical night: &lt; -18°</li>
</ul>
<p>The reason for the distinction is the different brightness of the sky itself due to the Sun's light which gets scattered within the atmosphere. As such: yes, also during astronomical twilight you can make out that the sky is brighter in the West (in the evening hours) or in the East (in the morning hours) - but of course not as bright as during nautical twilight as the progression from day to night is of course a continuous one. Within urban areas one will not detect a difference in the sky between astronomical night and twilight as the artifial sources already add so much light pollution and thus brightness to the background of the sky that the really dark night is not observable, or the distinction between astronomical twilight and night becomes nearly an academic exercise.</p>
<p>With a naked eye the difference is often hard up to impossible to distinguish (depending on general light conditions, but also as you gradually get accustomed to the darkness and see increasingly more. However on clear nights, and if you compare carefully, you can see small differences in sky brightness when you compare the Eastern and Western directions even during astronomical twilight, but easily visible on full-sky images). Yet as a person taking photos of <a href="https://en.wikipedia.org/wiki/Deep-sky_object" rel="nofollow noreferrer">deep sky objects</a> one would preferentially want to take photos during the time of &quot;real&quot; astronomical night in order to get the highest signal-to-noise ratio and have an as-dark-as-possible background of the sky itself.</p>
<p>There is a recent article in Nature by <a href="https://www.nature.com/articles/s41598-024-76550-3" rel="nofollow noreferrer">Faid et al</a> which discusses the sky brightness in relation to location, zenith angle, air polution and some other factors. They present this figure which gives the expected change in sky brightness depending on your location. You need good observing conditions to really be able to appreciate the effect of astronomical twilight:
<a href="https://i.sstatic.net/WiE1d8cw.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/WiE1d8cw.png" alt="sky brightness for different locations" /></a>
This is possibly better illustrated by this figure taken from <a href="https://www.nature.com/articles/s41550-022-01756-2" rel="nofollow noreferrer">John Barentine (2022)</a> in Nature Astronomy (sadly not open access) as it uses lux as brightness measure which is a unit geared to mimic the sensitivity of the human eye. CT, NT and AT indicate civil, nautical and astronomical twilight respectively. It's apparent that the difference between nautical and astronomical twilight is not as big as the one between civil and nautical twilight: <a href="https://i.sstatic.net/AJbhRfG8.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/AJbhRfG8.png" alt="sky brightness as function of solar altitude" /></a></p>

CC BY-SA 4.0
2024-12-29T04:13:36.787Z
a good source for information can be found here https://www.timeanddate.com/
trond hansen
CC BY-SA 4.0
2024-12-29T09:32:31.883Z
Thank you. The last image, is this civil twilight?
Little Mind
CC BY-SA 4.0
2024-12-29T10:32:58.483Z
@LittleMind hard to tell as you can vary both sensitivity and exposure time to get similar results. The paper does not give these details and included it with the explanation "Opposite the setting Sun in the sky, the anti-twilight arch rises (Fig. 1b). Its warm tones represent the enhancement of backscattered sunset light deblued by its passage through the atmosphere at near-grazing angles. It reaches a maximum width of between 3 and 6° above the anti-solar point; below the arch, a deep blue tinged with purple indicates the projection of the Earth’s shadow onto the twilight sky." (civi lTL)
planetmaker
CC BY-SA 4.0
2024-12-29T01:19:23.523Z
2024-12-29T08:00:33.383Z
2024-12-29T08:00:33.383Z
planetmaker
<p>I don't quite get the difference between astronomical twilight and a deeper night. Astronomical twilight is when the Sun is between 12 and 18 degrees below the horizon. Is there any difference (discernible to the human eye) between what the sky looks like during astronomical twilight versus the night? I guess the night sky never gets absolutely black due to the atmosphere.</p>
<p>When looking at when there's astronomical twilight at my location according to <a href="https://timeanddate.com" rel="nofollow noreferrer">timeanddate.com</a> it seems indiscernible from a deeper night to me. Civil twilight looks clearly like a twilight to me (you can still see well without street lights, after all) while nautical twilight almost looks like night except there's a brighter sky around the location the Sun has set at the horizon. Is such a brighter sky visible near the horizon during astronomical twilight too?</p>

<solar-terrestrial-physics><astronomy><atmospheric-optics>
Is there any difference between astronomical twilight and night, other than the Sun's angle?
Little Mind
# Is there any difference between astronomical twilight and night, other than the Sun's angle?

(Asked by: username_0 on 2024-12-28)

I don't quite get the difference between astronomical twilight and a deeper night. Astronomical twilight is when the Sun is between 12 and 18 degrees below the horizon. Is there any difference (discernible to the human eye) between what the sky looks like during astronomical twilight versus the night? I guess the night sky never gets absolutely black due to the atmosphere.

When looking at when there's astronomical twilight at my location according to [timeanddate.com](https://timeanddate.com) it seems indiscernible from a deeper night to me. Civil twilight looks clearly like a twilight to me (you can still see well without street lights, after all) while nautical twilight almost looks like night except there's a brighter sky around the location the Sun has set at the horizon. Is such a brighter sky visible near the horizon during astronomical twilight too?

**Tags:** solar-terrestrial-physics, astronomy, atmospheric-optics


---

## 1 answers

### Answer by username_1 on 2024-12-29. Score: 2 | Accepted answer

The different [twilights](https://en.wikipedia.org/wiki/Twilight) are distinguished - as you write - by how many degrees the Sun is below the horizon:

* day: sun > 0° (or actually > -0.7° due to [refraction](https://en.wikipedia.org/wiki/Refraction))
* civil twilight: 0 ... -6°
* nautical twilight: -6° .. -12°
* astronomical twilight: -12° .. -18°
* astronomical night: < -18°

The reason for the distinction is the different brightness of the sky itself due to the Sun's light which gets scattered within the atmosphere. As such: yes, also during astronomical twilight you can make out that the sky is brighter in the West (in the evening hours) or in the East (in the morning hours) - but of course not as bright as during nautical twilight as the progression from day to night is of course a continuous one. Within urban areas one will not detect a difference in the sky between astronomical night and twilight as the artifial sources already add so much light pollution and thus brightness to the background of the sky that the really dark night is not observable, or the distinction between astronomical twilight and night becomes nearly an academic exercise.

With a naked eye the difference is often hard up to impossible to distinguish (depending on general light conditions, but also as you gradually get accustomed to the darkness and see increasingly more. However on clear nights, and if you compare carefully, you can see small differences in sky brightness when you compare the Eastern and Western directions even during astronomical twilight, but easily visible on full-sky images). Yet as a person taking photos of [deep sky objects](https://en.wikipedia.org/wiki/Deep-sky_object) one would preferentially want to take photos during the time of "real" astronomical night in order to get the highest signal-to-noise ratio and have an as-dark-as-possible background of the sky itself.

There is a recent article in Nature by [Faid et al](https://www.nature.com/articles/s41598-024-76550-3) which discusses the sky brightness in relation to location, zenith angle, air polution and some other factors. They present this figure which gives the expected change in sky brightness depending on your location. You need good observing conditions to really be able to appreciate the effect of astronomical twilight:
[![sky brightness for different locations](https://i.sstatic.net/WiE1d8cw.png)](https://i.sstatic.net/WiE1d8cw.png)
This is possibly better illustrated by this figure taken from [John Barentine (2022)](https://www.nature.com/articles/s41550-022-01756-2) in Nature Astronomy (sadly not open access) as it uses lux as brightness measure which is a unit geared to mimic the sensitivity of the human eye. CT, NT and AT indicate civil, nautical and astronomical twilight respectively. It's apparent that the difference between nautical and astronomical twilight is not as big as the one between civil and nautical twilight: [![sky brightness as function of solar altitude](https://i.sstatic.net/AJbhRfG8.png)](https://i.sstatic.net/AJbhRfG8.png)

> **Comments:**
> **username_2**: a good source for information can be found here https://www.timeanddate.com/  
> **username_0**: Thank you. The last image, is this civil twilight?  
> **username_1**: @username_0 hard to tell as you can vary both sensitivity and exposure time to get similar results. The paper does not give these details and included it with the explanation "Opposite the setting Sun in the sky, the anti-twilight arch rises (Fig. 1b). Its warm tones represent the enhancement of backscattered sunset light deblued by its passage through the atmosphere at near-grazing angles. It reaches a maximum width of between 3 and 6° above the anti-solar point; below the arch, a deep blue tinged with purple indicates the projection of the Earth’s shadow onto the twilight sky." (civi lTL)  

---

