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Everything you need to know about the shortest day of the year

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

June 21, the southern hemisphere experienced 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.

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.

"A year isn't exactly an even number of days," he said.

And what about late sunrises and early sunsets?

Similarly, the length of a solar day isn't precisely 24 hours long.

This means that solar noon — the time at which the Sun reaches the highest point in the sky — doesn't occur at the moment your clock hits midday.

Around the winter solstice, solar noon occurs a few minutes later than the day before.

Sunrises and sunsets also gradually become later after the winter solstice.

This means that the earliest sunset will occur before the winter solstice, while the latest sunrise will happen after it's passed.

"What matters is not sunset and sunrise, but the difference between them is shortest [on the winter solstice]," Professor Bedding said.

Is the shortest day of the year also the coldest?

Not usually. This is because of a weird delay called seasonal lag.

"There's a delay because the Earth takes a while to cool down and heat up again," Professor Bedding said.

In Australia, the main culprit behind this is the ocean, which takes longer to cool down and heat up than the land due to its higher heat capacity.

Think of heating up a pot of water (or winter-warming soup) on your stove.

Rather than going from cold to boiling as soon as you turn the heat on, the liquid will take a few minutes to warm up.

Similarly, the pot will gradually cool down once the heat is turned off.

It's the same idea with the Earth. Even though we will gradually be getting more sunlight after the winter solstice passes, it will take some months for average temperatures to catch up.

Just how short will the day be?

It depends on where you are.

In Darwin, you will experience a bit less than 11.5 hours of daylight on the winter solstice, while Hobart will get around 9 hours of sun.

"The further from the equator you are, the more the difference is," Professor Bedding said.

"If you're in northern Queensland, you probably won't even notice a difference, but if you live in Tasmania, you will definitely notice a big difference."

But if you really wanted to feel the darkness of winter, look no further than the South Pole, Professor Bedding said.

From March, the South Pole witnesses its last sunset for a few months, as it passes into a period known as polar night.

"The Sun doesn't even get above the horizon — it's night for 24 hours."

The polar night gradually recedes after the winter solstice passes, but the South Pole still doesn't see a decent sunrise until September.

What will I be able to see in the night sky?

If the idea of spending most of the day in darkness is a downer, you might want to gaze up at the early morning sky — if you can brave the cold.

"There are beautiful planets visible in the morning sky," Professor Bedding said.

If you look east, you will catch bright Venus, Mars, Jupiter, and possibly even Mercury appearing in a straight line in the sky just before dawn.

"As far as astronomers are concerned, the winter solstice is a great time because we have the most night time and astronomers love night time," he said.

"So if you like looking at the stars, it's a perfect time of year."

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