Equinox and the Iron Sun . ... Credit Copyright: NASA / Goddard / SDO AIA Team . Explanation: Today, the Sun crosses the celestial equator heading south at 03:09 Universal Time . Known as an equinox, this astronomical event marks the first day of autumn in the northern hemisphere and spring in the south. Equinox means equal night . With the Sun on the celestial equator, Earth dwellers will experience nearly 12 hours of daylight and 12 hours of darkness. ... NASA Official: Jay Norris. ...