What Time Is Winter Solstice and When Is the Longest Night of the Year?

The northern hemisphere's winter solstice occurs this week marking the first day of winter and the shortest day of the year.

On the winter solstice, also known as the December solstice, the northern hemisphere will experience fewer hours of daylight than on any other day of the year and the sun is at the lowest point in the sky that it will appear at all year.

The winter solstice happens because of the axial tilt of the Earth. On December 21 or 22 every year, the planet's north pole is tilted as far away from the sun as it will be all year.

At 10:59 am ET on Tuesday, December 21, 2021, the north pole will be orientated away from our star by around 23 degrees, marking the moment of the winter solstice and the astronomical beginning of winter.

Winter solstice
A diagram showing how the earth is tilted during its solstices. The winter solstice sees the north pole tilted away from the sun. NASA

From this point, days in the northern hemisphere begin to become longer once more, providing more hours of daylight up until Tuesday, June 21. This marks the northern hemisphere's summer solstice.

At 5:13 am ET on this day the north pole will have its maximum tilt towards the sun, which will appear at the highest point in the sky. From here, the days will become shorter again in the northern hemisphere leading up to 2022's winter solstice.

As this is occurring in Earth's northern hemisphere, exactly the opposite is happening in the southern hemisphere. For countries below the equator, the December solstice marks the summer solstice and the longest day of the year.

This is because as the north pole is at its maximum tilt away from the sun, the south pole must be angled towards our star at its maximum.

This maximum axial tilt towards the sun for the southern hemisphere, including countries and regions like Australia, New Zealand, South America and southern Africa, will occur at exactly the same time, at around 10:59 a.m. ET.

December 21 marks the first day of the astronomical winter, but many organizations, like the U.K.'s weather forecasting Met Office, use the meteorological definition of winter, which sees the season begin on December 1.

Winter in the northern hemisphere will last until March 20, when the spring equinox occurs, marking the first day of that season. On that day, the sun will cross the equator line moving toward the north.

The March equinox will happen on March 20 at 11:33 a.m. The March equinox, or vernal equinox, marks the beginning of autumn for the southern hemisphere.

Winter Tree
A file photo of the sun low in the sky behind a tree stripped of leaves. Winter solstice, the astronomical start of winter, occurs on December 21. buxtree/Getty