<p>Many Internet sources define civil twilight (hereinafter CT) as follows:</p>
<p>Any point in time at which the 'geometric center of the sun' is between 0°, inclusively, and 6°, exclusively, below the horizon relative to a fixed position and orientation on Earth's surface.</p>
<p>The same sources state that evening CT (ECT) begins with sunset. Sunset is defined, in effect, as 'the sun is desdcending and its upper limb just on the horizon'.</p>
<p>How can ECT begin with sunset then ?</p>
<p>If the sun is descending and its 'geometric center' passes the horizon, then ECT begins, but there's still some way to go until sunset, isn't it ?</p>

CC BY-SA 4.0
2022-06-21T01:02:14.737Z
I agree that twilight cannot start before sunset. Can you provide links to many of these internet sources?
JohnHoltz
CC BY-SA 4.0
2022-06-21T01:19:29.377Z
Actually, twilight can begin before sunset. If you define sunset and twilight as in my post, namely sunset := upper limb of descending sun is on the horizon', twilight begin := 'geometric center of decending sun is on the horizon'.
Peterש
CC BY-SA 4.0
2022-06-21T01:44:34.453Z
I've studied the Internet sources again. It was all a misconcerption on my part. The definition of civil twilight beginning IS sunset. I just didn't want to believe it because the definition of civil twilight END (civil dusk) refers to the 'geometric center of the sun'. Thought the same should apply to civil twilight BEGINNING, too. Instead, the geometric center of the sun is ignored in the definition of civil twilight beginning. But that makes it hard to calculate the angle of the sun's geometric center at the beginning of twilight. I found out this angle is 90° + 50 arcminutes, approx.
Peterש
CC BY-SA 4.0
2022-06-21T02:05:09.470Z
Please write that up as an answer. Then you can mark it as an accepted answer.
JohnHoltz
CC BY-SA 4.0
2022-06-21T03:23:27.590Z
Just FYI, the +50 arcmin is accounting for atmospheric refraction (and the diameter of the sun).  But you have also referenced the "geometric" position of the Sun many times.  The geometric position does not account for refraction.  It generally doesn't matter much which you use, but make sure you're consistent, otherwise you might have some head scratching later on.
Greg Miller
CC BY-SA 4.0
2022-06-21T07:05:24.890Z
Some info on the horizon & refraction: https://www.siranah.de/html/sail040o.htm
PM 2Ring
<observational-astronomy>
civil twilight & sunset
Peterש
# civil twilight & sunset

(Asked by: username_0 on 2022-06-20)

Many Internet sources define civil twilight (hereinafter CT) as follows:

Any point in time at which the 'geometric center of the sun' is between 0°, inclusively, and 6°, exclusively, below the horizon relative to a fixed position and orientation on Earth's surface.

The same sources state that evening CT (ECT) begins with sunset. Sunset is defined, in effect, as 'the sun is desdcending and its upper limb just on the horizon'.

How can ECT begin with sunset then ?

If the sun is descending and its 'geometric center' passes the horizon, then ECT begins, but there's still some way to go until sunset, isn't it ?

**Tags:** observational-astronomy
> **Comments:**
> **username_1**: I agree that twilight cannot start before sunset. Can you provide links to many of these internet sources?  
> **username_0**: Actually, twilight can begin before sunset. If you define sunset and twilight as in my post, namely sunset := upper limb of descending sun is on the horizon', twilight begin := 'geometric center of decending sun is on the horizon'.  
> **username_0**: I've studied the Internet sources again. It was all a misconcerption on my part. The definition of civil twilight beginning IS sunset. I just didn't want to believe it because the definition of civil twilight END (civil dusk) refers to the 'geometric center of the sun'. Thought the same should apply to civil twilight BEGINNING, too. Instead, the geometric center of the sun is ignored in the definition of civil twilight beginning. But that makes it hard to calculate the angle of the sun's geometric center at the beginning of twilight. I found out this angle is 90° + 50 arcminutes, approx.  
> **username_1**: Please write that up as an answer. Then you can mark it as an accepted answer.  
> **username_2**: Just FYI, the +50 arcmin is accounting for atmospheric refraction (and the diameter of the sun).  But you have also referenced the "geometric" position of the Sun many times.  The geometric position does not account for refraction.  It generally doesn't matter much which you use, but make sure you're consistent, otherwise you might have some head scratching later on.  
> **username_3**: Some info on the horizon & refraction: https://www.siranah.de/html/sail040o.htm  

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