The culmination is the instant at which a celestial object reaches its highest altitude above the horizon as Earth's rotation carries it across the sky; in other words, the moment the object crosses the observer's meridian (the north-south line in the sky). Upper culmination (also known as culmination above pole for circumpolar stars and the Moon) is the crossing closer to the observer's zenith; lower culmination (or culmination below pole) is the crossing farther from the zenith. Alternative names for culmination are meridian passage and transit.