SkyEye

"You are about to enter another dimension, a dimension not only of sight and sound but of mind. A journey into a wondrous land of imagination. Next stop, the Twilight Zone!" — Rod Serling, "The Twilight Zone", CBS Television, 1959

What is twilight?

When the Sun drops below the horizon at sunset, complete darkness does not fall instantly. For a period, which varies according to latitude and time of year, there is still enough natural light to carry on with daytime activities such as reading or playing sports. Eventually, however, the sky darkens and the first stars begin to appear. This is commonly known as twilight or dusk, an interval of fading light between daylight and night-time which has inspired poets and artists through the centuries.

Astronomers take a slightly more prosaic view of twilight, dividing it into three stages defined by the position of the Sun below the horizon.

Evening civil twilight begins at sunset, which is the instant when the upper limb, or edge, of the Sun is touching the horizon. If we were able to remove the Earth's atmosphere for a moment, we would find that the centre of the Sun's disk is actually 50 arc-minutes below the horizon at the moment of sunset, due to a combination of two factors: the finite size of the Sun’s disk and the refraction of the Sun's light by the atmosphere, which acts like a giant lens. Rigorous methods for calculating the time of sunset (and sunrise) assume that the geometric centre of the Sun's disk is 50 arc-minutes below the horizon.

Civil twilight ends when the geometric centre of the Sun's disk is 6° below the horizon. At this time, the western sky is still quite bright, but there is no longer enough natural light for activities that require good lighting, such as reading a book or playing cricket.

The brightest stars are beginning to become visible, and this marks the beginning of evening nautical twilight. At sea, it is now sufficiently dark to make sightings of stars and the brighter planets with a sextant, using the sea horizon as a reference. Before the wide availability of satellite-based global positioning systems, this was the only reliable way to determine a ship's position at sea once it was out of sight of land.

Nautical twilight ends when the geometric centre of the Sun's disk is 12° below the horizon. The sky has become dark enough that it is no longer possible to discern the horizon at sea, so this marks the end of the period when navigational sightings can be made with a sextant. On land, the end of nautical twilight is the beginning of full darkness for most practical purposes, and coincides with the start of evening astronomical twilight.

The sky continues to darken during astronomical twilight, and fainter stars become visible. When the geometric centre of the Sun's disk is 18° below the horizon, the sky is as dark as it can get, and this marks the end of astronomical twilight. At most large astronomical observatories, this is the time when the night's observing programme can begin.

Before dawn, the sequence of twilights occurs in reverse. Morning astronomical twilight begins when the Sun is 18° below the eastern horizon. Astronomers using the world's largest and most powerful telescopes will begin drawing their night's work to a close.

When the Sun is 12° below the horizon, morning nautical twilight begins. At sea, mariners can once again discern the horizon, allowing them to make navigational sightings with their sextants. On land, the first glimmerings of dawn will be noticed in the eastern sky by anyone who is out early enough.

Morning civil twilight starts when the Sun is 6° below the eastern horizon, and marks the time when (provided the sky is clear and unobstructed) there is enough natural light to resume daytime activities. Sunrise follows, when the upper edge of the Sun's disk appears on the horizon.

How long does twilight last?

The length of each period of twilight depends on the time of year and the latitude. In the tropics, the Sun sets almost vertically, and darkness falls very quickly. Civil twilight can be as short as 21 minutes, and the other twilights are almost as brief.

By contrast, in latitudes north of +49° or south of −49°, it is possible for astronomical twilight to continue throughout the night during the mid-summer months because the Sun never drops far enough below the horizon to reach the −18° threshold. In London, for example, there is permanent astronomical twilight all night from late May to late July. For astronomers, this means that the sky never becomes fully dark.

At higher latitudes still, nautical twilight and civil twilight can also continue through the whole night. Within the Arctic and Antarctic Circles, there are periods when the Sun never sets, a phenomenon known as the 'midnight Sun'.

In the diagram above, the position of the Sun is depicted around the time of sunset and during evening twilight for three different latitudes. The values for the local mean solar times of sunset and the ends of the various twilights for 20 June 2020 (solstice) are as follows:

It is summer in the northern hemisphere and sunset at 50° north latitude is late in the evening. Civil twilight lasts 45 minutes and nautical twilight lasts 65 minutes. However, astronomical twilight persists throughout the night, not ending until the beginning of nautical twilight the next morning. The sky never becomes completely dark. In the tropics, however, the length of twilight is typically quite short. On this day at 10° north latitude, civil twilight is just 23 minutes long, nautical twilight is two minutes longer than that and astronomical twilight lasts only 28 minutes. The southern hemisphere is experiencing winter in June. Sunset is early and at 30° south latitude, civil twilight lasts 26 minutes, with nautical twilight and astronomical twilight each a little longer at 30 minutes.