What is the winter solstice?

Today is shortest day of the year (the winter solstice) for the Southern Hemisphere – an astronomical phenomenon that occurs each year in June as the Earth orbits the Sun.

But why is it the shortest day of the year?

“It all comes down to the fact that the Earth is tilted at about 23 degrees,” explains Geraint Lewis, Professor of Astrophysics at the Sydney Institute for Astronomy.

“So what happens is that orientation in space is fixed, and at some point the top of the Earth is closer to the Sun than the bottom.

“We are at the point where the Southern Hemisphere is further from the Sun.”

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The solstices relate to the tilt of the Earth. The Southern Hemisphere is currently tilted away from the Sun.

Each year the solstice falls between 20-22 June for the Southern Hemisphere, while the summer solstice – the longest day of the year – occurs between 20-22 December.

“The word ‘solstice’ means ‘Sun still’,” says Fred Watson, astronomer at the Australian Astronomy Observatory.

“The Sun stands still in the sense that it’s northern and southern journey stops.”

From today, the days will get longer as the Earth continues to orbit, the Southern Hemisphere getting closer to the Sun each day as it approaches the December summer solstice.

Two astronomical phenomena collide

While it is the shortest day of the year in the Southern Hemisphere, it was (literally) the polar opposite for the Northern Hemisphere, which had its longest day of the year yesterday.

It was a particularly special summer solstice however, as for the first time since 1967 – almost half a century ago – the June full Moon and summer solstice fell on the same date. While not astronomically significant, these phenomena rarely cross paths, as the Moon’s cycle and Sun’s orbit are independent processes.

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