The Southern hemisphere Autumn Equinox for 2021 will occur at 8:37pm AEDT or 9:37am UT on Saturday March 20.

Many cultures around the world celebrate the whole day as the March equinox. However, in reality, the equinox occurs at a specific moment in time when the Sun crosses the celestial equator, the imaginary line in the sky above Earth’s Equator, from south to north.

At this moment, Earth’s axis is tilted neither away from nor towards the Sun, but is rather perpendicular to the Sun’s rays, like the illustration below shows.

Earth in relation to the Sun’s rays. (Not to scale)

This is when we have approximately equal lengths of day and night.

The day and night are only nearly equal on the September equinox and the March equinox. In fact, most locations on Earth tend to enjoy more daylight hours than nighttime hours on these two days of the year, thanks to the Earth’s atmosphere and our definition of sunrise and sunset.

In Latin, the word equinox, means equal night, giving the impression that the night and day on the equinox are exactly 12 hours long. And even though this is common wisdom, it isn’t entirely accurate.

Even if day and night aren’t exactly equal on the day of the equinox, there are days when sunlight hours and nighttime hours are both very close to 12 hours. This day is known as the equilux, and its date depends on a location’s latitude and can occur several days to weeks before or after an equinox.

For Sydney’s latitude it will occur on March 24 this year.

Of course if you are in the Northern hemisphere it will be the Spring equinox.

So what is an Equinox?

The equinox is generally accepted as occurring at the instant of time when the plane of Earth’s equator passes through the centre of the Sun.

This occurs twice each year: around March 20 and September 23. In other words, it is the moment at which the centre of the visible Sun is directly above the Equator.

In the northern hemisphere, it is commonly understood that the first day of Spring, Summer, Autumn and Winter occur with the relevant equinox (for Spring and Autumn) or solstice (for Summer and Winter).

However in Australia and New Zealand it is conventional to say that each season begins on the 1st day of a month so Spring started on Sept 1.

Of course as we are all well aware the weather rarely follows either convention.

However, what is consistent across the countries is that the increase in the length of daylight each day reaches a peak at Spring Equinox when daylight and night are of equal length.