File:Twilight Zones - 2.jpg

This chart shows the relationship between the angular elevation of the sun relative to the horizon and levels of twilight. The values are for the 24 hours between 1pm BST 12th August 2010 and 1pm BST 13th August 2010, and for the location Brighton in East Sussex (50.8° N 0.1° W). A full listing of data for the <a href="https://www.flickr.com/photos/dominicspics/sets/72157624781879918/">two nights I tried to photograph Perseid meteors</a> is at the foot of this description.

In equatorial regions, night and day breaks very much more quickly. In Brighton on this late summer day, sunset to full darkness took 2 hours and 22 minutes. By comparison, in Singapore (near the equator) the process took 1 hour 11 minutes.

At the other extreme, in northerly Reykjavík, Iceland (64° North) neither Astronomical nor Nautical dusk was ever achieved. Sunset to sunrise lasted just over 7 hours, with Civil Twilight making up 2 hours 16 minutes of this time, the rest of the night being Nautical Twilight. Scandinavian people are real experts at <a href="http://en.wikipedia.org/wiki/Gloaming" rel="nofollow">gloaming</a>.

After sunset the sky remains light for some time. This is caused by diffraction and <a href="http://en.wikipedia.org/wiki/Scattering" rel="nofollow">scattering</a> of light through the atmosphere and also reflection of light back into the sky off the earth's surface. This enables light from the sun to appear to "bend" round the earth and to continue to illuminate the sky, albeit at reduced intensity. This illumination is described by the term <a href="http://en.wikipedia.org/wiki/Twilight" rel="nofollow">twilight</a>.

Although of course occurring as a continuous gradation, three zones of twilight are commonly described. <a href="http://en.wikipedia.org/wiki/Twilight#Civil_twilight" rel="nofollow">Civil Twilight</a> is when there is still enough light to read or work without artificial illumination. <a href="http://en.wikipedia.org/wiki/Twilight#Nautical_twilight" rel="nofollow">Nautical Twilight</a> is when there is still enough light for mariners see the horizon and - together with an accurate clock - thus be able to navigate by measuring the altitude of the stars against the horizon. During <a href="http://en.wikipedia.org/wiki/Twilight#Astronomical_twilight" rel="nofollow">Astronomical Twilight</a> there is still enough light to hamper astronomical observations of the more faint objects in the firmament. These zones are more clearly illustrated <a href="https://www.flickr.com/photos/dominicspics/4915198623/in/photostream/">here.</a>

In Brighton, England, for a few weeks in mid-summer it never gets dark enough for Astronomical Twilight to end, even though it "feels" dark by midnight.

The boundary of each zone is defined by the sun setting a further 6° below the horizon.

Here are the full sunset to sunrise figures in Brighton for night of 11/12th August 2010: All times are BST (British Summer Time) ≅ GMT / UTC / UT1 + 1 hour) The figures are taken from <a href="http://www.timeanddate.com/" rel="nofollow">timeanddate[dot]com</a>

Sunset 20:29 Civil Dusk 21:06 Nautical Dusk 21:55 Astronomical Dusk 22:53 Astronomical Dawn 3:19 Nautical Dawn 4:17 Civil Dawn 5:05 Sunrise 5:43

Here are the full sunset to sunrise figures in Brighton for night of 12/13th August 2010: Sunset 20:27 Civil Dusk 21:04 Nautical Dusk 21:52 Astronomical Dusk 22:50 Astronomical Dawn 3:22 Nautical Dawn 4:19 Civil Dawn 5:07 Sunrise 5:45