Some angles on twilights


THE Return of the Native, by Thomas Hardy, contains a rather nice description of the twilight. "The heaven being spread with this pallid screen and the earth with the darkest vegetation, their meeting line at the horizon was clearly marked. In such contrast the heath wore the appearance of an instalment of night which had taken up its place before its astronomical hour had come darkness had to a great extent arrived hereon, while day stood distinct in the sky."

Literally, the word twilight means "two lights". It describes the light which is reflected or scattered by the atmosphere when the sun is a short distance below the horizon. It is a concept, moreover, like the beginning of spring, where different factions choose their own definition to suit their own purposes. There are, in fact, three twilights.

Civil twilight lasts from sunset until the sun's centre is six degrees below the horizon. By the time it ends, it has become too dull for convenient outdoor work or play without artificial illumination, and in a legal context its end marks the time when operations requiring daylight must cease. The concept of "lighting up time", for example - the time during which lights are required on vehicles - is based on a crude approximation to the end of civil twilight, and Hardy's observation, also, was obviously made around this time.

Nautical twilight, on the other hand, ends when the sun's centre is 12 degrees below the horizon, by which time, for all practical purposes, it is completely dark. The constellations can be distinguished overhead, and a distant horizon is no longer visible, except in moonlight. The criterion for astronomical twilight, however, is more rigid - or more liberal, depending how you look at it: astronomical twilight ends when it is theoretically impossible for any of the sun's light to be visible, which turns out to be when the sun is 18 degrees below the horizon.

The length of each of these twilight varies throughout the year, depending on the angle at which the path of the setting sun meets the horizon. At a large angle of incidence such as occurs near the equinoxes - the sun shoots straight down behind the horizon with no nonsense, and civil twilight lasts for only half an hour or so. But at the solstices in June and December, the path of the setting sun is at a relatively shallow angle to the horizon, and this means that it takes considerably longer for it to reach a position where it is six degrees below for this reason civil twilight at these times lasts for over 60 minutes.