The equinox is being celebrated around the world – heralding autumn in the northern hemisphere and spring in the south. What is an equinox and how does it work, asks Justin Parkinson.

The name equinox means “equal night” in Latin. It’s theoretically the day of the year when all points on the earth’s surface experience the same lengths of daylight and darkness – 12 hours of each. The autumn equinox in the northern hemisphere (it’s the spring equinox for the southern hemisphere) always falls on 22, 23 or 24 September.

Imagine the earth orbiting the sun in a flat plane – ie as if it was pinned on the edge of…