This page last modified 2011 June 04

Twilight

Questions about the twilight arise regularly on astronomy newsgroups. This document is an attempt to answer some of these questions.

What is Twilight?

Twilight is the period between the Sun disappearing behind the horizon and the sky becoming fully dark (light pollution excepted, of course). Twilight is divided into three phases:

Why is twilight shorter near the equator?

Twilight depends on the angular distance of the Sun below the horizon. The Sun appears to move along its line of declination. At the equator, lines of declination are perpendicular to the horizon so, after the Earth has rotated 18o after sunset, to a first approximation (i.e. ignoring the curvature of the lines of declination) the Sun will be 18o (or nearly so) below the horizon:

equator

Away from the equator, declination lines intersect the horizon at an angle equal to the co-latitude. E.g. at 51o, they will interesect the horizon at 90o - 51o = 39o:

Dec lines at 51*N

Hence after the Earth has rotated 18o, to a first approximation (i.e. ignoring the curvature of the lines of declination) the Sun will only be 18cos(latitude)o below the horizon. At 51oN, this is 18cos51o = 11o below the horizon.

Why is twilight longer in summer than in winter?

Twilight depends on the angle of the Sun below the horizon. The Sun appears to move along its line of declination. In the summer, when the Sun is in the north of the celestial sphere, its declination line curves up towards the horizon:

summer

This has the effect of gradually decreasing the angle at which the Sun appears to travel with respect to the horizon, thus increasing the time it takes to attain a given angle below the horizon.

In winter, when the Sun is in the south of the celestial sphere, its declination line curves down away from the horizon. This has the effect of gradually increasing the angle at which the Sun appears to travel with respect to the horizon, thus decreasing the time it takes to attain a given angle below the horizon.

winter