Starwatch: equinox marks the changing of the seasons

Night and day will be of equal length at the end of this week as the sun crosses the celestial equator from north to south

The September equinox takes place this week. Occurring on 23 September, it marks the end of summer and the beginning of autumn for the northern hemisphere and is called the autumnal equinox at northern latitudes. In the southern hemisphere, the situation is reversed and the moment marks the end of winter and the beginning of spring. The equinox is defined as the place in the sky where the sun's path, called the ecliptic, crosses the celestial equator, which is the projection of Earth's equator up into the sky. This crossing occurs in Virgo and the sun will cross this point at 0850 BST on 23 September, as shown in the chart. At this moment, the sun will be moving from the northern sky into the southern sky. On this day around the world, everywhere will experience an almost even split of 12 hours day and 12 hours night. After this, the days shorten at northern latitudes and lengthen in the southern hemisphere. The next equinox will take place at 0350 GMT on 20 March 2020.