23 Sep 2010
At the times when the Sun is crossing the celestial equator (the projection of the Earth's equator onto the sky), day and night are of nearly equal length at all latitudes so we call these dates the equinoxes (= 'equal night').
In fact, night and day are not exactly equal at the time of the equinox. This is because the Earth's orbit around the Sun is elliptical not circular, and the Earth moves fastest when it is closest to the Sun and slowest when furthest away. This causes variations in the length of the solar day and in time sof sunrise and sunset, so the Sun is not at its highest in the sky at precisely local noon each day.
Find out more about equinoxes and solstices in our fact file.