The southern hemisphere's spring equinox will occur tonight, meaning days will be longer than nights in Australia for the next six months.

The spring equinox will occur at 11:31pm AEST tonight, marking the moment the sun appears to be positioned directly above Earth's equator. This orientation of the sun relative to Earth makes day and night roughly equal in length around our planet near the date of the equinox.

Image: Visible satellite image on Tuesday morning showing the solar terminator (line between day and night) running roughly perpendicular to the equator.

Following the equinox in September (the other one is in March), the southern hemisphere becomes tilted more towards the sun for the next six months. This makes days longer than nights in Australia until the next equinox, which will occur on March 20th in 2021.

Australia's longest day of the year will be in about three months time, on the date of the summer solstice, which is December 21st this year.