Astronomy Picture of the Day . ... Credit: Expedition 15 Crew , NASA Explanation: Today, the Sun crosses the celestial equator heading north at 0548 UT . Known as the equinox, the geocentric astronomical event marks the first day of spring in the northern hemisphere and autumn 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 - sunset . ...