# How does the Astronomical Twilight depend on the night length or the day length?

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?

• Could you source Some astronomers say that the Astronomical Twilight starts at the last 1/8th of the night length,. I've never heard that, only the 18 degree definition which is the one NOAA uses: weather.gov/fsd/twilight – user21 May 10 at 13:25
• @barrycarter from some Arabic references such as: 38162 7626 – geek11 May 10 at 16:10
• @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 May 11 at 14:45
• 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 Given that, there's no way the 1/8 of the night claim can be accurate for all locations all year long. – notovny May 12 at 13:41