Every March and September, we experience an event called an equinox. The word equinox comes from the Latin word aequinoctium, meaning ‘equal night’.
Each equinox takes place at a particular moment in time, when the plane of Earth’s equator, if extended into space, would pass through the centre of the Sun. In other words, if you were standing on the equator at the moment of the equinox, and at the right longitude to have the Sun at its highest point in the sky, it would be exactly overhead. In 2025 this moment is on 20 March at 8:02 pm Tasmanian Summer Time.
The variation in the maximum height of the Sun in the sky over the year is caused by the tilt of Earth’s axis, which gives rise to the seasons.
The two equinoxes are called the vernal (spring) equinox and the autumnal equinox. The March equinox marks the astronomical beginning of spring in the northern hemisphere, but autumn for us in the south. So, it is better to simply call these the March and September equinoxes.
Broadly speaking, the equinoxes are when day and night are of equal length (12 hours each) all over our planet Earth. However, on examining a published table of sunrise and sunset times, you will notice that on the day of the equinox, the period between sunrise and sunset is slightly greater than 12 hours. For Ulverstone on 20 March, this period is 12 hours, 9 minutes and 24 seconds. Here at the Ulverstone Planetarium we have received several questions about that over the years!
The reason for these extra bits of daylight is that we regard sunrise and sunset as occurring when the Sun’s upper edge, not its centre, is exactly on the horizon. In addition, the bending of light in the atmosphere — called refraction — causes the Sun (and other objects) to appear slightly higher in the sky than they actually are, especially when near the horizon. These effects add together to produce several extra minutes of sunlight!
Illustration caption:
Earth as seen from the Sun at the moment of the 2025 March equinox, showing lines of latitude and longitude. The equator (in bold) will be aimed exactly at the Sun at that moment. At that time, the Sun will be exactly overhead as seen from a point in the Indian Ocean near Somalia.