Days will only be longer than nights in Australia for another three weeks, before darkness outlasts daylight for six months.

You may have noticed that days have gradually been getting shorter in Australia since late-December.

If you live in Sydney, you've actually lost about one and a half hours of daylight since the start of the year. In Melbourne, the day's length has dropped by just over 100 minutes so far this year.

These shorter days and longer nights are a clear sign that the seasons are about to change.

Australia loses daylight at the beginning of every calendar year as the southern hemisphere gradually becomes more tilted towards the sun. By late March, nights get become longer than days across the Southern Hemisphere, which drives the transition from summer into autumn.

The autumnal equinox marks the date the sun appears to pass over Earth's equator. From our perspective here on Earth, the sun appears to move from being above the southern hemisphere to the northern hemisphere on the date of the equinox.

Image: The sun appears to be positioned above the Earth's equator on the date of the autumnal equinox. This makes day and night roughly equal in length in both the northern and southern hemsiphere.

Both day and night are roughly equal in length on the date of the equinox, which this year occurs at 7:37pm (AEST) on Saturday March 20th.

Following the equinox, days will be shorter than nights in the southern hemisphere for six months, until the spring equinox in late September.