What Causes Twilight?

When the sun disappears from sight in the evening, it continues for a while to illuminate the upper layers of the atmosphere. Earth's atmosphere, with its many dust particles, continues to reflect sunlight, and supplies the earth with indirect light, or twilight. Thus darkness comes on gradually as the sun sinks farther below the horizon and illuminates less of the atmosphere above. The same phenomenon, but in reverse, occurs in the morning before sunrise. This, too, is called twilight, or, more often, the dawn.
Not all people on earth experience twilight to the same extent. This is because its duration varies at different latitudes. For example, in equatorial regions, at lower latitudes, the sun rises and sets almost vertically in relation to the horizon. Therefore it quickly "travels" down below the horizon until its rays do not touch even the highest layers of earth's atmosphere. So twilight is very short near the equator.

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