... Credit Copyright: STS-68 Crew , NASA Explanation: Today the Sun crosses the celestial equator heading south -- marking the Autumnal Equinox, the first day of Autumn. Equinox means equal night and with the Sun on the celestial equator , Earthlings will experience 12 hours of daylight and 12 hours of darkness. ... A few weeks after the Autumnal Equinox of 1994, the Crew of the Shuttle Endeavor recorded this image of the Sun poised above the Earth's limb. ... NASA Official: Jay Norris. ...