Winter Solstice: The shortest day of the year is almost here. This is why it happens

The days are getting shorter and colder and the afternoons we spent sunbathing are really behind us, especially in the southern states.

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

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

And while we are 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 boils down to Earth being a little shaky on its axis.

Instead of rotating perfectly vertically, our planet is tilted about 23 degrees.

This tilt gives us the four seasons: winter, spring, summer and fall.

Moreover, the earth does not move in a perfect circle around the sun, but in a football-shaped ellipse.

As the Earth completes one orbiting the sun every year, different parts of the planet are tilted toward our star at different times and receive the most direct sunlight.

Illustration of the Earth orbiting the Sun
The Earth’s axial tilt and its elliptical orbit give 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 away from the sun, leading to cold temperatures and longer nights. That’s winter.

“During winter, our part of the earth is away from the sun and therefore [the Sun] isn’t above the horizon every day for that long,” 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 from the sun.

In the southern hemisphere, this falls somewhere between June 20 and 22, while in the northern hemisphere it usually occurs on December 21 or 22.

The spring and fall equinoxes occur when the sun appears directly over the equator, making days and nights of approximately equal length.

Why doesn’t it happen on the same day every year?

While our reliable calendars help us chart the year, they don’t sync perfectly with the sun’s movements.

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