<p>The length of Astronomical twilight isn't constant, but the main reason is the tilt of the Earth, and not the orbital speed of the Earth.</p>
<p>The change from day to night is a result of the Earth spinning.  In summer the sun is high in the Northern Sky, as a result its angle relative to the horizon when it sets is shallow and twilight will take longer.  Back when I lived in York, twilight was permanent in June.  The sky was still slightly pale at midnight.</p>
<p>This image show the day of the year from left to right and time from midnight to midnight vertically. In the centre you can see that in York truely dark skies are never achieved at Midsummer.</p>
<p><a href="https://i.sstatic.net/Kv9B7.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Kv9B7.png" alt="enter image description here" /></a></p>
<p>The best way to find the length of twilight at anytime and date is to use a service like <a href="https://www.timeanddate.com/sun/uk/york" rel="nofollow noreferrer">time and date|</a>. The calculations are not impossible, but not trivial either.  Alternatively astronomical software such as <a href="https://rhodesmill.org/skyfield/" rel="nofollow noreferrer">skyfield</a>  can calculate twilight times too.</p>

CC BY-SA 4.0
2021-06-02T21:26:04.117Z
Thank you very much for the answer. Basically, I want to find minimal and maximal time for this period (18-0 degrees).
Reckless Glacier
CC BY-SA 4.0
2021-06-02T21:27:41.510Z
Well, it can last weeks in the arctic.
James K
CC BY-SA 4.0
2021-06-02T22:37:35.650Z
That's right... If I want to find the minimal or maximal time for this period in a specific country, can I use Time-and-date website and find these two specific times without checking 365 days a year seperately?
Reckless Glacier
CC BY-SA 4.0
2021-06-02T22:39:04.633Z
Should be possible
James K
CC BY-SA 4.0
2021-06-02T22:50:13.823Z
Great. Taking an example of London, using this site, can I say that in Oct this period is shortest (1:17 h) and in Jul this period is the longest (and takes 4:33 hours)?  
https://www.timeanddate.com/sun/uk/london  Just to be sure I understood the system.
Reckless Glacier
CC BY-SA 4.0
2021-06-03T04:19:14.807Z
That looks correct.
James K
CC BY-SA 4.0
2021-06-03T11:21:40.590Z
But it seems that the times I mentioned refer only to the interval between 18-12 degrees, rather than until the sunrise (which is almost 0 degrees). Do I not have to add the time for nautical twilight and civil twilight, in order to get the interval time between 18-0 degrees?
Reckless Glacier
CC BY-SA 4.0
2021-06-03T12:20:49.950Z
I don't know what you want!  The time and date site will give you the times when the sun is 0, 6, 12 and 18 degrees below the horizon for every day of the year at any given location.  From that anyone can calculate the length between any two times. It is just a difference of times.  If you want to find the length of time from sunset to  the end of astronomical twilight you can work it out.  If you want to find the time from the start to the end of astronimical twilight, you can do that too.
James K
CC BY-SA 4.0
2021-06-02T20:14:35.680Z
2021-06-02T20:14:35.680Z
James K
<p>I've been told by an astronomer in past, that the appearance of the twilight isn't constant in the interval before the sunrise. That is to say that not every day the earth is going at the same speed, so for one day it takes 70 minutes for the earth to move from 18 degrees under the horizon (=astronomical twilight) to 0 degrees (horizon), and on other days it may be even 85 minutes. It depends on the speed of the earth circles the Sun. If what I've been told is true, then I'm looking for a way to calculate every day the accurate time between 18 degrees to 0.</p>

CC BY-SA 4.0
2021-06-03T07:50:51.037Z
This thread could be read togheter with the other question you have posted https://astronomy.stackexchange.com/questions/44134/astronomical-twilight-is-always-seen-when-the-sun-is-18-degrees-under-the-horizo as they complement each other.
Alchimista
<horizon><sunrise><twilight>
Is the appearance of the twilight not constant in the interval before the sunrise?
Reckless Glacier
# Is the appearance of the twilight not constant in the interval before the sunrise?

(Asked by: username_0 on 2021-06-02)

I've been told by an astronomer in past, that the appearance of the twilight isn't constant in the interval before the sunrise. That is to say that not every day the earth is going at the same speed, so for one day it takes 70 minutes for the earth to move from 18 degrees under the horizon (=astronomical twilight) to 0 degrees (horizon), and on other days it may be even 85 minutes. It depends on the speed of the earth circles the Sun. If what I've been told is true, then I'm looking for a way to calculate every day the accurate time between 18 degrees to 0.

**Tags:** horizon, sunrise, twilight
> **Comments:**
> **username_1**: This thread could be read togheter with the other question you have posted https://astronomy.stackexchange.com/questions/44134/astronomical-twilight-is-always-seen-when-the-sun-is-18-degrees-under-the-horizo as they complement each other.  

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## 1 answers

### Answer by username_2 on 2021-06-02. Score: 3 | Accepted answer

The length of Astronomical twilight isn't constant, but the main reason is the tilt of the Earth, and not the orbital speed of the Earth.

The change from day to night is a result of the Earth spinning. In summer the sun is high in the Northern Sky, as a result its angle relative to the horizon when it sets is shallow and twilight will take longer. Back when I lived in York, twilight was permanent in June. The sky was still slightly pale at midnight.

This image show the day of the year from left to right and time from midnight to midnight vertically. In the centre you can see that in York truely dark skies are never achieved at Midsummer.

[![enter image description here](https://i.sstatic.net/Kv9B7.png)](https://i.sstatic.net/Kv9B7.png)

The best way to find the length of twilight at anytime and date is to use a service like [time and date|](https://www.timeanddate.com/sun/uk/york). The calculations are not impossible, but not trivial either. Alternatively astronomical software such as [skyfield](https://rhodesmill.org/skyfield/) can calculate twilight times too.

> **Comments:**
> **username_0**: Thank you very much for the answer. Basically, I want to find minimal and maximal time for this period (18-0 degrees).  
> **username_2**: Well, it can last weeks in the arctic.  
> **username_0**: That's right... If I want to find the minimal or maximal time for this period in a specific country, can I use Time-and-date website and find these two specific times without checking 365 days a year seperately?  
> **username_2**: Should be possible  
> **username_0**: Great. Taking an example of London, using this site, can I say that in Oct this period is shortest (1:17 h) and in Jul this period is the longest (and takes 4:33 hours)?   https://www.timeanddate.com/sun/uk/london  Just to be sure I understood the system.  
> **username_2**: That looks correct.  
> **username_0**: But it seems that the times I mentioned refer only to the interval between 18-12 degrees, rather than until the sunrise (which is almost 0 degrees). Do I not have to add the time for nautical twilight and civil twilight, in order to get the interval time between 18-0 degrees?  
> **username_2**: I don't know what you want!  The time and date site will give you the times when the sun is 0, 6, 12 and 18 degrees below the horizon for every day of the year at any given location.  From that anyone can calculate the length between any two times. It is just a difference of times.  If you want to find the length of time from sunset to  the end of astronomical twilight you can work it out.  If you want to find the time from the start to the end of astronimical twilight, you can do that too.  

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