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The Science Behind the Summer Solstice

On the night of the 20th, 21st or 22nd of June every year (depending on whether we have a leap year), people young and old congregate at Stonehenge to witness the arrival of summer. As the first fingers of sunlight slide over Salisbury Plain, they douse Stonehenge in the rays of a new season. The longest day has arrived – for the northern hemisphere anyway. This is when the hours of daylight are at their maximum: welcome to summer solstice.

Why Do We Have a Solstice?

Our home planet is not ‘upright’ as it completes its circuit around the sun. There is a slight varying tilt, which makes it wobble slightly as it goes through its year. Without it, there would be no winter or summer and no variation in the sunlight. The wobble effect creates the seasons and allows both hemispheres to have maximum and minimum daylight hours at opposite ends. Australasia, Argentina and South Africa all experience their summer solstice on 21st December when we are celebrating our winter solstice and vice versa.

Why Does the Solstice Date Change Each Year?

The planet Earth’s orbit is not 365 days exactly – that would be impressive. In fact, it is 365 days, 6 hours, 9 minutes and 10 seconds. As the years advance, each Equinox and solstice are adjusted slightly, pushed back until the Leap Year day is added. This correction starts the cycle all over again. Without the correction, the agricultural seasons would be out of kilter. This is less of an issue today, but some ancient societies found adding the leap day was necessary to stop the harvest times becoming out of synch.

What Is So Special about Stonehenge?

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Stonehenge is associated with the summer solstice all over the world.

The monument attracts millions of visitors from all over the globe annually. UNESCO (United Nations Educational, Scientific and Cultural Organisation) recognises it as the most important Neolithic monument in Europe. There is no greater monument associated with the solstice. Competing theories on its meaning still dominate academia and the public imagination but nobody knows for certain what it is. One theory that tends to dominate is that Stonehenge seems to align with the seasons. It’s possible that this is a stone calendar, but other theories challenge it.

Focusing on the monument means we miss everything else. The landscape around Stonehenge is equally interesting. Since the new visitor centre opened, heritage professionals have been keen to emphasise the important cultural landscape including burial mounds, the avenue and its proximity to ancient trackways. As you look around the visitor centre, you will see a bird’s eye view of the development of the site.

Other Solstice Monuments

Stonehenge may be the most famous of the solstice monuments, but it is not the only one. Several other stone circles throughout the British Isles have a clear indication that they were built to mark the seasons.

The first is New Grange in the Boyne Valley of Northern Ireland. The large oval structure was built as a domed tomb with a grassy roof between 3000BC and 2500BC. Inside are a series of tunnels; typically it looks like a tomb and probably was built for that function. As the sun rises on the winter solstice, sunlight pours through the doors and lights up the chambers. This alignment must be deliberate, celebrating the day that the days of sunlight have reached their minimum and will only get brighter.

Tulum in Mexico is one of the most impressive Mayan city ruins. Visitors are often impressed with its immense ruins on the coast. At the impressive temple ruins, the doors have been perfectly aligned to the solstice sunrises – from one side the summer, from the other side the winter. In each case, it creates an impressive star field effect. The Maya had a much more accurate solar calendar than we had in Europe at the time and were excellent astronomical observers.