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Here Comes The Sun: The Science of the Summer Solstice

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Given the weather in the UK during most of May, you could be forgiven for thinking it was still spring (although, technically it was)! However, Sunday 21st June 2015 sees the summer solstice dawn upon us. So, what exactly is the summer solstice and have we really approached midsummer already?

The word ‘solstice’ derives from the Latin ‘sol’ meaning ‘sun’ and ‘sistere’ meaning ‘to stand still’. This standing still refers to the path of the sun and ‘declination’ which is a means of measuring the angle of a celestial body on the ‘celestial sphere’. This is an imaginary sphere around earth upon which all objects in the sky can be thought of as projected onto the underside of the sphere or dome. Similar to the latitude and longitude system used here on earth, it is a means by which astronomers can describe the position of stars and planets relative to a co-ordinate system. Declination specifies a position in the sky which is relative to the equator and the poles.

The earth doesn’t spin on an axis which is vertical, instead, the angle of tilt of the north/south axis is approximately 23.40 and this gives rises to the seasonal variations we experience on earth relative to the amount of sunlight we receive. At the Northern Hemisphere during the summer solstice, the North Pole is tilted towards the sun.

Factors affecting the exact date

The way in which a year is measured is not exactly an accurate science- it’s why we have leap-years. A year actually last for 365.24 days, so every four years we catch up with the extra day on February 29th. This means of measuring time also has an effect on the dates and exact times of each of the solstice and equinox dates. This year, it’s the 21st June, whereas in 2016 it will fall on the 20th June. The last time there was a summer solstice on the 22nd June was back in 1971.Other factors also influence the exact timing of the summer solstice. For example, the earth does not rotate at a constant speed around its elliptical orbit.

Axial precession

Another factor which occurs over a vast time scale is known as the ‘precession of the equinoxes’ (or ‘axial precession’). The earth is spheroid in shape and not a prefect sphere- indeed the equatorial diameter of the earth is 43 Km larger than the polar diameter. As described above, the earth it tilted in its orbit and this means that the ‘bulge’ around the equator is off-centre relative to the gravitational pull of the sun. This results in a small amount of torque as the gravitational force of the suns pulls harder on one side of the earth relative to the distant side. It’s not just the sun which exerts a gravitational pull on the earth- our own moon and other planets in the solar system also exert a pull on us. If the earth was a perfect sphere, precession would not occur.

If you extend an imaginary line through the centre of the earth (titled at 23.40), then this would trace a circle as the earth moves through the cycle of precession. One complete precession cycle takes 25,772 years. This cycle changes the exact times and dates of the solstices and equinoxes. Another interesting point is that the exact positions of stars in our sky also change through the cycle of precession. Polaris is our current ‘Pole Star’, however, 13,000 years from now, this will change and Vega (in the constellation of Lyra) will become our Pole Star. Polaris will get another turn at being the Pole Star in approximately 26,000 years from now!

Midsummer? But we’ve only just started!

The exact date of the solstice and what we know as midsummer do not fall upon the same days. Although midsummer festivities and celebrations are pagan in origin and would have fallen on the equinox days, the Christian church took over this festival and designated the date as the 24th June and is associated with the nativity of John the Baptist.

However, midsummer doesn’t mean the middle of summer. In meteorological terms, summer starts with the equinox and lasts for the months of June, July and August. In astronomical terms, the solstice marks the beginning of summer and it ends with the autumnal equinox on September 23rd. The average highest temperatures in the UK are usually in the months of July and August. This is partly due to the way that earth retains heat from the sun and the fact that we are an island. The oceans surrounding us act as a huge ‘heat sink’, absorbing and re-radiating the heat from the sun. Although the earth absorbs the most intense of the sun’s rays around the date of the solstice, it takes several weeks for the heat to be released, hence the fact that our hottest days are usually in July or August. If there were no oceans on earth, our hottest days would fall around the date of the solstice.

Depending on the location on earth, the summer solstice is generally regarded by those of us living in the Northern Hemisphere as the ‘longest day’. At the South Pole there will be 24 hours of darkness, whereas the North Pole will receive 24 hours of daylight. At the equator, there is approximately 12 hours of daylight. Here in the UK, the day length varies with London seeing around 16 hours and 38 minutes of daylight at the solstice, whereas in Edinburgh, the day length is approximately 17 hours and 36 minutes. Up in Shetland, they will see a day as long as 18 hours and 55 minutes.

Megalithic monuments and the solstice

Perhaps the most famous of the megalithic sites in the UK which is associated with the solstice is Stonehenge, although there are many other ancient sites in this country and around the world which are aligned to astronomical events.

Archaeologists have found evidence of very early wooden structures at the Stonehenge site which date back around 10,000 years ago. Even then, the post holes were aligned east/west which may have signified a link to the movements of the sun.

The earliest monumental building phase at Stonehenge was around 3,100 BC and consisted of a 110 metre circular bank and ditch with a gap facing the north east. The latter stages of monument building at the site maintained this NE/SW alignment. It was the English antiquarian and archaeologist William Stukeley (1687-1765) who first noted and recorded the rising of the summer solstice sun above the Heel Stone in the summer of 1720. The sun doesn’t rise exactly above the Heel Stone when viewed from the centre of the circle, but this is unsurprising when taking into account the vast time period and factors such as precession. Evidence has also shown that the Heel Stone was only one of a pair of such stones and the solstice sunrise would have been framed by these stones at the time of the building.

After much controversy and legal battles, English Heritage now grant access to Stonehenge at the times of the solstices and equinoxes (the stones are roped off to visitors at other times of the year);

http://www.english-heritage.org.uk/visit/places/stonehenge/plan-your-visit/summer-solstice/#

Author: Martin Wilson

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