The equinox sunset from Sydney Observatory and how its position changes from day to day.

It’s the September Equinox today, Friday September 23. This is the Spring Equinox for the southern hemisphere, but in an international context it’s better to say “Septemebr Equinox”!

Although it’s often said that the length of daytime and nighttime are equal on this day, that is not quite true.

Sunrise is defined to be when the top edge of the Sun first appears above the horizon, and sunset is when the top edge dips below the horizon so there are just a few more minutes of day than night.

Equinox in fact is when the (centre of the) Sun crosses the equator of the celestial sphere, moving (today) from the northern to the southern hemisphere. This happens at precisely 11:04am AEST.

From now until the Summer Solstice our days get longer, and warmer.

Happy Equinox!