<p>Look at the extremes. At any time of the year, it takes only minutes (several minutes) for the Sun to sink well below the horizon at the equator. On the other hand, it can take days (several days) after the September Equinox for the Sun to sink well below the horizon for locations above the Arctic Circle. As an extreme extreme, at the South Pole, it takes multiple days for the Sun to rise from just the top of the Sun showing to the full Sun showing (or to set, six months later). Astronomical twilight lasts for several weeks at the South Pole.</p>

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2021-02-21T09:09:18.917Z
2021-02-21T09:16:00.620Z
2021-02-21T09:16:00.620Z
David Hammen
<p>What's the reason the Astronomical twilight doesn't have constant time although its degrees below the horizon (18 degrees) are constant?</p>

CC BY-SA 4.0
2021-02-21T06:31:00.957Z
I think this has been answered here before but not sure. Searching this site for "astronomical twilight" returns [18 results](https://astronomy.stackexchange.com/search?q=Astronomical+twilight+).
uhoh
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2021-02-22T10:47:48.943Z
Isn't due to the tilt of the terrestrial axe?
Alchimista
<time>
What's the reason the Astronomical twilight doesn't have constant time?
Reckless Glacier
# What's the reason the Astronomical twilight doesn't have constant time?

(Asked by: username_0 on 2021-02-21)

What's the reason the Astronomical twilight doesn't have constant time although its degrees below the horizon (18 degrees) are constant?

**Tags:** time
> **Comments:**
> **username_1**: I think this has been answered here before but not sure. Searching this site for "astronomical twilight" returns [18 results](https://astronomy.stackexchange.com/search?q=Astronomical+twilight+).  
> **username_2**: Isn't due to the tilt of the terrestrial axe?  

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## 1 answers

### Answer by username_3 on 2021-02-21. Score: 3

Look at the extremes. At any time of the year, it takes only minutes (several minutes) for the Sun to sink well below the horizon at the equator. On the other hand, it can take days (several days) after the September Equinox for the Sun to sink well below the horizon for locations above the Arctic Circle. As an extreme extreme, at the South Pole, it takes multiple days for the Sun to rise from just the top of the Sun showing to the full Sun showing (or to set, six months later). Astronomical twilight lasts for several weeks at the South Pole.


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