Did you know? Twilight

This Month . . . . Twilight. There are three types of twilight, and they are all defined by the angle of the Sun.

Civil twilight: Occurs when the Sun is six degrees below the horizon. It ends at sunrise in the morning or follows sunset in the evening. There is enough light to distinguish physical objects without artificial lighting. Only the brightest stars and planets, like Venus and Jupiter, can be seen with the naked eye.

Nautical twilight: Occurs when the Sun is between six and twelve degrees below the horizon. The horizon is faintly visible during this phase, many of the brighter stars can be seen, making it possible to use the position of the stars to navigate at sea, hence the name nautical twilight.

Astronomical twilight: Occurs when the Sun is between twelve and eighteen degrees below the horizon and ends when all traces of sky glow are gone. Astronomers can begin to observe the stars - assuming no clouds are in the way!

The photograph below is by Geoff Wyatt, taken at twilight in 2007 showing a faint but unmistakable Comet McNaught.

If you could see twilight from outer space, you would find that it isn’t marked by a sharp boundary on the Earth’s surface. Instead, the shadow line on Earth – the terminator - is spread over a wide area and shows the gradual transition to darkness.

The duration of each twilight phase depends on the latitude and the time of the year. Locations where the Sun is directly overhead at noon, for example the Equator during the equinoxes, have swift transitions between night and day and relatively short twilight phases.

In the polar circles the Sun does not set at all in the summer, so there is no twilight during that time of the year. However, during the winter months, the Sun may reach an angle of 12-18° below the horizon causing a short daily period of astronomical twilight.

Astronomical twilight is a good time to observe and photograph northern and southern lights as well as other atmospheric phenomena like false dawn.