When is the winter solstice? Everything you need to know about the shortest day of the year

The days are getting shorter and colder and afternoons spent soaking up the sun are well and truly behind us — especially in the southern states.

But if you think the mornings are dark and gloomy now, wait until we hit the winter solstice.

Next Tuesday, June 21, the southern hemisphere will experience its shortest day and longest night of the year.

And as we’re plunged into darkness, the northern hemisphere will enjoy the summer solstice, the longest day and shortest night of the year. 

So, what’s behind this astronomical phenomenon?

Why do we have the winter solstice? 

It all comes down to Earth being a little wonky on its axis.

Rather than rotating perfectly vertically, our planet is tilted at about 23 degrees. 

This tilt is what gives us the four seasons: winter, spring, summer, and autumn.

On top of that, the Earth doesn’t travel around the Sun in a perfect circle, but in a football-shaped ellipse. 

As the Earth completes one lap around the Sun each year, different parts of the planet are tilted towards our star at different times and get the most direct sunlight.

Illustration of Earth orbiting Sun
The axial tilt of the Earth and its elliptical orbit gives rise to the seasons, solstices and equinoxes.(Getty Images: Stocktrek Images/Photon Illustration)

This results in warm weather and short nights. Welcome to summer!

At the same time, the other side of the world is tilted away from the Sun, leading to chilly temperatures and longer nights. That’s winter.

“During winter, our part of the Earth is tilted away from the Sun and therefore [the Sun] isn’t above the horizon for as long every day,” said Tim Bedding, an astronomer at the University of Sydney.

When is the winter solstice?

The winter solstice marks the point at which the southern or northern hemisphere is tilted furthest away from the Sun. 

In the southern hemisphere, this falls sometime between June 20-22 (June 21 this year), while in the northern hemisphere it usually occurs on either December 21 or 22.

The spring and autumn equinoxes occur when the Sun appears directly above the equator, leading to days and nights being roughly equal in length.

Why doesn’t it occur on the same day each year?

While our trusty calendars help us map out the year, they don’t sync up perfectly with the Sun’s movements.

The Earth takes about 365 and a quarter days to whip around the Sun, which is why we have leap years, Professor Bedding said.

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