Astronomy Picture of the Day . ... 23.09.2007 . ... Credit: STS-68 Crew , NASA Explanation: Today, the Sun crosses the celestial equator heading south at 0951 UT . Known as the equinox, the astronomical event marks the first day of autumn in the northern hemisphere and spring in the south . Equinox means equal night and with the Sun on the celestial equator, Earth dwellers will experience nearly 12 hours of daylight and 12 hours of darkness. ... Publications with keywords: equinox . ...