The sunset time is defined in astronomy as the moment when the trailing edge of the sun disc disappeared below the horizon. Close to the horizon, the atmospheric refraction causes the path of rays of light from the Sun to be distorted in such a way that geometrically the Sun disk is already about one diameter below the horizon when the sun sets is observed.
The sun sets differ from the twilight, which has three phases, the first being the civilian twilight, which begins after the sun disappears beneath the horizon, and continues until it drops to 6 degrees below the horizon; the second phase is the dusk, between 6 and 12 degrees below the horizon; and the third is the twilight of astronomy, which is the period when the Sun is between 12 and 18 degrees below the horizon. [1] Dusk is at the edge of the twilight of astronomy, and is the darkest night of dusk just before night. [2] Night occurs when the Sun reaches 18 degrees below the horizon and no longer shines the sky.