<p>Some references say that the Astronomical Twilight starts at the last <strong>1/8th of the night length</strong>, so it is <strong>long in Winter and short in Summer</strong>;</p>

<p>But by using the <strong>18 angle</strong> (sun elevation below the horizon) the twilight is <strong>long in Summer and short in Winter</strong>!</p>

<p>What would be the correct method proven by observations for all days and all locations?</p>

CC BY-SA 4.0
2020-05-10T16:10:13.027Z
@barrycarter from some Arabic references such as: 

[38162](https://shamela.ws/index.php/book/38162)

[7626](https://shamela.ws/index.php/book/7626)
geek11
CC BY-SA 4.0
2020-05-11T14:45:20.403Z
@geek11 The first step is to decide what is your definition of twilight. If it is based on some measurement of the decreasing or increasing light, then definitions based on the elevation of the Sun are more accurate.
JohnHoltz
CC BY-SA 4.0
2020-05-12T13:41:30.503Z
There are definitely parts of the planet where the Sun never gets lower than 18° below the horizon near the Summer Solstice, and never gets above 18° below the horizon near the Winter Solstice. [Wikipedia](https://en.wikipedia.org/wiki/Twilight#/media/File:Twilight_latitudes_world_map.svg) Given that, there's no way the 1/8 of the night claim can be accurate for all locations all year long.
notovny
<observational-astronomy><night-sky><twilight>
How does the Astronomical Twilight depend on the night length or the day length?
geek11
# How does the Astronomical Twilight depend on the night length or the day length?

(Asked by: username_0 on 2020-05-10)

Some references say that the Astronomical Twilight starts at the last **1/8th of the night length**, so it is **long in Winter and short in Summer**;

But by using the **18 angle** (sun elevation below the horizon) the twilight is **long in Summer and short in Winter**!

What would be the correct method proven by observations for all days and all locations?

**Tags:** observational-astronomy, night-sky, twilight
> **Comments:**
> **username_0**: @barrycarter from some Arabic references such as:   [38162](https://shamela.ws/index.php/book/38162)  [7626](https://shamela.ws/index.php/book/7626)  
> **username_1**: @geek11 The first step is to decide what is your definition of twilight. If it is based on some measurement of the decreasing or increasing light, then definitions based on the elevation of the Sun are more accurate.  
> **username_2**: There are definitely parts of the planet where the Sun never gets lower than 18° below the horizon near the Summer Solstice, and never gets above 18° below the horizon near the Winter Solstice. [Wikipedia](https://en.wikipedia.org/wiki/Twilight#/media/File:Twilight_latitudes_world_map.svg) Given that, there's no way the 1/8 of the night claim can be accurate for all locations all year long.  

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## 0 answers

