Winter Solstice – How Does It Work?

Ancient calendars

Humans have known about the Winter and Summer Solstices for thousands of years thanks to careful observations of the Sun. Ancient astronomers noticed that in the summer the Sun traced a path much higher in the sky compared to its winter path. These changes were a convenient way of marking out the year to keep track of upcoming seasons such as when to mate animals and when to harvest crops.

Many civilisations built monuments to mark the solstices – early analogue versions of our calendar notifications today. Stonehenge, built in south-west England between 3,000 and 2,000 BC, is one of the most famous examples. Stonehenge is aligned with the Sun so that every Winter Solstice the sun sets directly between two of the standing stones, marking the depth of winter.

What does a solstice really mean?

Today we have an astronomical understanding of what causes the Winter Solstice. This day marks the moment in the year when the Earth’s north pole is tilted the furthest away from the Sun, meaning that the Sun reaches its southernmost point in the sky and the northern day is shortest.

And despite common misconception, the solstice really is a moment, rather than a day.

To visualise how this works, remember that the Earth’s pole is tilted with respect to the line between the Sun and the Earth. If it wasn’t tilted then as Earth orbited around the Sun, its Northern and Southern hemispheres would receive the same amount of Sun all year long and the lengths of days and nights would be constant. Seasons would cease to exist.

Instead, Earth’s pole is tilted about 23.5 degrees away from perpendicular to the Earth’s orbital plane.  This means that for half the year, the north pole points towards the Sun and for the other half the year, the north pole points away from the Sun. The moment of maximum tilting towards or away from the Sun is called a solstice and happens twice a year – the Summer Solstice in June and the Winter Solstice in December.

Earth’s seasons

This change of angle is what causes the Sun to appear high in the sky in summer and low in the sky in winter. It’s also what gives us our seasons.

Imagine the difference between shining a torch on a surface from directly overhead versus at an angle. If the torch is at an angle, then the light is spread out over a larger area and is less intense.

The same thing happens in winter when the sun’s light hits the Northern Hemisphere at an angle and is less intense, thus not heating the Earth as much. The shorter days also mean that there is less time to receive the heat before the sun sets again.