Summer Solstice – How Does It Work?
Sunrise over Stonehenge. Credit: Pedro Vera via flickr

Summer Solstice – How Does It Work?

20/06/2018Written by Tamela Maciel

In 2018 the Summer Solstice occurs at 11:07 BST on 21 June. Ever wondered what’s really going on?

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Welcome to the longest day of the year!

This week marks the Summer Solstice, a time of long daylight and short night. A moment to pause and celebrate the official start of summer. That is, if you’re reading this from the Northern Hemisphere.

If you’re south of the equator, enjoy those crisp winter days!

Ancient calendars

Ancient calendars
Summer solstice at Stonehenge. Credit:

Humans have known about the Summer and Winter 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 Summer Solstice the Sun rises directly above one of the standing stones, known as the Heel Stone, marking the start of summer.

What does a solstice really mean?

Credit: NASA

Credit: NASA

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

And despite common misconception, the solstice really is a moment, rather than a day. This year that moment is at 11:07 British Summer Time on 21 June 2018.

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.

Reason for the seasons

Reason for the seasons
Credit: Jecowa via Wikimedia Commons

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. This is what occurs during the winter months.

The same thing happens in summer when the Sun’s light hits the Northern Hemisphere from overhead, heating the Earth more intensely. The longer days also mean that there is more time to receive the heat before the Sun sets again.

Happy start of summer!

About the author: Dr Tamela Maciel is the Space Communications Manager at the National Space Centre.