Winter solstice 2020 in Southern Hemisphere will be at 7:43 am on Sunday, 21 June. Of course for our Northern Hemisphere friends it is their Summer Solstice and official beginning of Summer.
Solstices and equinoxes are the reason for the seasons.
The Earth has seasons simply because its axis is tilted. It rotates on its axis as it orbits the Sun, but the axis always points in the same direction. Mars also have seasons as it is tilted as well.
We rotate around an ‘axis’, an imaginary line between the north and south poles passing through the centre of the Earth. This occurs once every 24 hours, and we also orbit the Sun once every year.
Our rotation axis is tilted relative to the plane of our orbit around the Sun.
As we orbit the Sun, this tilt angles the southern hemisphere towards or away from the Sun at different times of the year.
From the surface of the Earth, this means the midday Sun appears further north or south in the sky depending on the time of year.
It also means that each 24-hour rotation of Earth leaves Australia in daylight for more or less time, depending on where we are in our orbit around the Sun.
The solstices are the two times each year when the tilt in Earth’s axis lines up most with the direction of the Sun, creating the maximum difference between daylight and nighttime hours.
The two equinoxes, in between, are when the tilt of Earth’s axis is side-on to the Sun, so that our north and south poles are the same distance from the Sun. On those dates, the Sun appears directly above Earth’s equator.
After the March equinox, the tilt of Earth’s axis angles the southern hemisphere further away from the Sun, so days in Australia become shorter than nights. The Sun continues to move north in the sky until it’s over the Tropic of Cancer in late June—the southern hemisphere’s winter solstice.
This is when you can also see the both the Southern Cross and Polaris at the same time if you are on the Tropic of Cancer.
At the winter solstice, days are at their shortest. How many hours of daylight you’ll see depends on your latitude (how far north or south you are).
Hobart, for example, only sees around 9 hours of daylight at the winter solstice—but 15 hours of daylight at the summer solstice.
Darwin, meanwhile, only varies between about 11½ hours of daylight at the winter solstice and about 12½ hours of daylight at the summer solstice.