Monday marks the date of the spring equinox in Australia, with days to be longer than nights for the next six months.

This year's spring equinox occurred at 07:50am UTC (5:50pm Sydney time) on Monday, September 23rd. 

The equinox in September, which is known as the spring equinox in the southern hemisphere, marks the moment the sun is positioned directly above Earth's equator. 

Image: The solar terminator running perpendicular to Earth's equator just over one hour before the southern hemisphere's spring equinox on 23rd September, 2019. Source: RAMMB/CIRA

On the day of the equinox, the solar terminator - which is the line between day and night - runs perpendicular to the Earth's equator. This causes day and night to be roughly equal in length around the world on the date of the equinox.

After the southern hemisphere's spring equinox, the south pole gradually tilts towards the sun and causes days to slowly become longer than nights. This imbalance persists until the autumn equinox in late March, at which point nights will start to become longer than days once again.