As of 2021, nearly all humans who have ever lived have not personally observed the horizon of any celestial body other than Earth's horizon, with the only exceptions being those Apollo astronauts who have traveled to the Moon and have thus also observed the lunar horizon in addition to the terrestrial horizon. In addition the horizons of several other celestial bodies in the Solar System, especially Mars, have been filmed by uncrewed spacecraft launched from Earth. Except where noted, the remainder of this article will exclusively discuss Earth's horizon.