File:Twilight zones.jpg

After sunset the sky remains light for some distance over the earth. 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 off the 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 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.

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. The total area of twilight (between 0° and 18° away from the "sunset line" is about 15% of the earth's surface).

The arrows on this diagram show a few of the very many paths that light can take through the atmosphere to spread the illumination of the sun into the area of twilight on the night side of the earth.

The "dashed" narrow cylinders indicate the rotational axis of the earth, the location of the North Pole. Because the earth is tilted, this does not coincide with the "sunset line" except once in spring and once in autumn.

The thickness of the atmosphere is vastly exaggerated in this illustration. Half the mass of the atmosphere is below 5.6 km / 3.5 miles from sea level on the earth (although small traces of the atmosphere do extend very far out into space, causing - for example - satellite drag). By comparison, the diameter of the earth is 12,700 km / 7,890 miles.