The Belt of Venus (also called the anti-twilight arch) is an atmospheric optical phenomenon that appears as a pink or orange band visible on the horizon opposite the sun during the minutes of civil twilight, just after sunset or just before sunrise. It sits above a dark bluish stripe called the Earth's shadow.
Optical mechanism
It is produced because sunlight, now very low-angle, passes through a large mass of atmosphere acting as a filter, removing blue wavelengths (Rayleigh scattering) and allowing reddish and pink tones to predominate. This reddish light illuminates the upper atmosphere in the direction opposite the sun. The dark blue band below is literally the shadow that the Earth casts on its own atmosphere.
It is an everyday and universal phenomenon that can be observed at every clear sunset and sunrise, yet goes unnoticed by most people. It is best seen from locations with an unobstructed horizon (coast, plains), clean skies, and without light pollution. The total duration is barely 10-15 minutes, and the effect gradually shifts upward as the sun descends further below the horizon.
In Spain, the Mediterranean coasts at sunset (looking west) and the Atlantic coasts at sunrise (looking east) offer magnificent views of the Belt of Venus. It is one of the most accessible optical phenomena and, paradoxically, one of the least known to the general public. See also: Brocken spectre, glory.