Starwatch: equinox, when the sun sits on the equator

Summer officially ends in the northern hemisphere tomorrow, and we enter the autumn. This is the time of year when the length of day and night are exactly equal. Known as the September equinox, it always takes place sometime around the 21st of the month. This year, it takes place on 22 September at 1431 BST.

At this moment the sun will be located directly on the celestial equator, the projection of Earth’s equator into the sky. This means the sun will pretty much rise due east and set due west on that day. For the next three months, the sun will rise and set ever more southwards along the horizon.

For the northern hemisphere, this means it will follow a shorter path across the sky, leading to shorter days and longer nights. The difference in length of day and night across the year drives the different seasons and is the result of Earth’s tilted rotation axis. In the southern hemisphere, the September solstice marks the turning point from winter into spring.