Afternoon is the time of the day between noon and evening. It is the time when the sun is descending from its peak in the sky to somewhat before its terminus at the horizon in the west. In human life, it occupies roughly the latter half of the standard work and school day. In literal terms, it refers to a time specifically after noon. The equivalent of Earth's afternoon on another planet would refer to the time the principal star of that planetary system would be in descent from its prime meridian, as seen from the planet's surface.
