Sunrise is the time at which the first part of the Sun appears above the horizon in the east. Sunrise shoul not be confused with dawn, which is the (variously defined) point at which the sky begins to lighten, some time before the sun itself appears, ending twilight. Because atmospheric refraction causes the sun to be seen while it is still below the horizon, both sunrise and sunset are, from one point of view, optical illusions. The sun also appears larger on the horizon, but this is another optical illusion, similar to the moon illusion.