June 20- The Solstice

The June solstice will fall on June 20 or June 21 this year, depending on where you are in the world.  It is the longest day in the northern hemisphere and the day when the Sun is at its highest in the midday sky (see note). The origin of the word solstice is from the Latin words sol, which means Sun, and sistere, to stand still, because around the time of the solstice the Sun stops getting higher, appears to stand still at the same height for a few days, and then gets lower in the midday sky.

The graph below show the maximum height, or elevation, of the Sun, measured in degrees above the horizon, during the month of June. The graph is for a place 50 degrees in latitude North, roughly the same latitude as the southern tip of the British Isles.  It shows how the elevation of the Sun changes around the solstice.

The fact that the Sun’s elevation changes only gradually means that the length of the days only changes very slowly around the time of the solstice. This is shown in the table below, which gives the sunrise and sunset times and the length of day in hours, minutes and seconds for June in London.  The amount of daylight on June 20 is only a fraction of a second more than on June 21.

Table of sunrise and sunset times for London (Time and Date 2015). Although June 20 is the day with the most daylight, the earliest sunrise occurs on June 16 and the latest sunset on June 24.

Precise definition of the solstice

The diagram above shows the Earth’s orbit around the Sun. For clarity the sizes of the Earth and Sun have been greatly exaggerated.

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• During June, marked as A in the diagram, the Earth’s North pole is tilted towards the Sun and the days are longer in the northern hemisphere.
• During December, marked as C in the diagram, the Earth’s South pole is tilted towards the Sun and days are longer in the southern hemisphere.
• At points B and D known as the equinoxes, neither pole is tilted towards the Sun and the amounts of daylight in the northern and southern hemisphere are equal.

The precise astronomical definition of the June solstice (also called the summer solstice in the northern hemisphere) is the exact point in time when the North pole is tilted furthest towards the Sun. The times when this will occur from now until 2020 are given in the table below – in Greenwich Mean Time, in Tokyo time (which is 9 hours ahead of GMT) and in Hawaiian time (which is 10 hours behind GMT).

 

As you can see, the time of the solstice varies from year to year. It can fall on 20, 21 or 22 June, depending on your longitude (and thus your time zone).

Importance of the solstice to early man

The solstice was of great importance to early man, and many prehistoric sites appear to have been built to celebrate it. The most famous of these is Stonehenge, which is located in Wiltshire, England. It is a set of concentric stone stone circles built between 4000 and 5000 years ago. It was an amazing feat of construction for stone age man. The stone circle is over 30 metres in diameter. The largest stones are more than 9 metres tall, weigh over 25 tonnes and were hauled over 30 km to the site. It is reckoned that the smaller stones were moved from western Wales, a distance of 225 km (Jarus 2014).

Image from Wikimedia commons 

At the centre of Stonehenge is a horseshoe arrangement of five sets of arches called triliths, each containing three stones.  The open side of the horseshoe points North East towards a large stone 80 metres away called the Heel Stone, shown below.

Image from Wikimedia-commons

The monument is arranged in such a way that, for a few days either side of the the June solstice and only at those dates, someone standing in the centre of the horse shoe and facing North East will see the Sun rise over the Heel stone.

How sunrise at the summer solstice at Stonehenge would have looked after the monument’s construction.

It is amazing that prehistoric man built such a large monument to line up with the June solstice. It clearly must have been a major event for a people living outdoors with only natural daylight, but in fact the solstice is still celebrated at Stonehenge today. Modern groups with ancient origins, such as Druids and Pagans, who revere the natural world more than many modern humans, join approximately 30,000 people who flock to Stonehenge to watch the Sun rise at the solstice each year.

For the BBC report on last year’s solstice celebrations click on the link below.

http://www.bbc.co.uk/news/uk-england-wiltshire-33211353

The southern hemisphere

To those of you who live in the southern hemisphere the June solstice is, of course, the winter solstice, when the midday Sun is at its lowest in the sky. After the solstice the days start getting gradually longer and the nights gradually shorter, although the change doesn’t really become noticeable until July.

Note

Strictly speaking it isn’t true that for the whole northern hemisphere the midday Sun is at its highest in the sky on the solstice. At the Tropic of Cancer, which is 23.5 degrees north, and is shown as the upper red line in diagram below, the Sun is directly overhead at midday on the June solstice. At low latitudes between the equator and the Tropic of Cancer the Sun is directly overhead at midday on two dates either side of the solstice. For example in San Juan, Puerto Rico, which lies only 18.5 degrees North of the equator, the Sun is overhead at midday on May 13 and July 30.

References

Jarus, O (2014) Stonehenge: Facts & Theories About Mysterious Monument, Available at: http://www.livescience.com/22427-stonehenge-facts.html (Accessed: 10 June 2016).

Time and Date (2015) London, ENG, United Kingdom — sunrise, sunset, and daylength, June 2016, Available at: http://www.timeanddate.com/sun/uk/london(Accessed: 10 June 2016).