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The Summer Solstice Topical Science
In this Issue
The Summer Solstice in the Northern Hemisphere This year, for those of us who live in the Northern Hemisphere, the Summer solstice occurs on June 21st.
Page 1. The Summer Solstice Page 2. The Solstice & the Earth’s tilt. Page 3. Stonhenge and the Solstice Page 4. The Date of the Solstice
This is the day when the rays of the sun fall directly on the Tropic of Cancer, the midday sun is at its highest position in the sky and everywhere north of the Equator has the longest day of the year Inside the Arctic Circle, the sun does not set below the horizon at all on this day and people who live in these northern latitudes enjoy the so-called ‘Midnight Sun’. In many northern countries, festivals are held to celebrate the coming of Summer.
KEY DATES June 21st 2018
Sept. 23rd 2018
Dec. 21st 2018
The Northern Hemisphere celebrates the Summer Solstice and the greatest number of hours of daylight.
The Equinox; day and night are equal; we have equal hours of daylight and darkness all over the globe of Earth.
This is the Winter Solstice in the Northern Hemisphere; we have the shortest day and the longest night of the year.
TOPICAL SCIENCE
JUNE 2018
The Scientific Explanation for the Solstice The word ‘Solstice’ is derived from the Latin and it literally means ‘sun stands still’. From our viewpoint on planet Earth, it seems to us that the sun stands still at the so-called ‘Solstice’. On that day, which falls on June 21st in 2018, the midday sun will be at its highest position in the sky, as its rays fall directly on the Tropic of Cancer. It then appears to stop in its tracks, before beginning its apparent ‘journey’ south, until it is directly over the equator at the September equinox. It continues on its apparent journey south until its rays are directly over the Tropic of Capricorn, at the December Solstice. After that, the sun seems to begin to travel northward again. But this is not what really happens. Yes, those of us living in the Northern hemisphere will enjoy the greatest number of hours of daylight on June 21st and from then until the December Solstice, we will see the evenings getting slightly shorter, until we reach the shortest day of the year. So why does this happen? The cause of the seasonal variation in the hours of daylight and the seasonal changes in our weather is not due to any movement of the sun. What causes these seasonal changes is the tilt of the Earth’s axis. The Earth has two separate types of motion. During the course of one year, our planet travels in an elliptical orbit around the sun, a journey that takes 365.24 days. At the same, our planet rotates on its axis, like a spinning top. If the axis of rotation were perpendicular to the plane of the Earth’s orbit, then the sun’s rays would always fall directly on the equator, day and night would be equal all the year round, all over the world and we would have no seasons. But the axis of rotation is inclined at an angle of 23.4 degrees to the plane of the Earth’s orbit around the sun. The axis is an imaginary line, running through the centre of the earth from pole to pole. On our timescale, it points in the same direction, relative to the distant stars. But it does not always point in the same direction relative to the sun. At the Summer Solstice, the North Pole is tilted towards the sun. The significance of the Tropic of Cancer it that its latitude is 23.4 degrees north of the equator, the same as the angle of tilt, so this is why the direct rays of the sun fall on the Tropic of Cancer at the June Solstice. The North Pole is at latitude 90 degrees N and the Arctic Circle is at a latitude of 66.6 degrees N (or 23.4 degrees from the North Pole). This is why, at the June Solstice, all places within the Arctic Circle have 24 hours of daylight and the sun does not set at all below the horizon on this day. The opposite occurs in the southern hemisphere. The sun does not rise at all for 24 hours within the Antarctic Circle at the June solstice. The situation will be reversed on December 21st, when the northern hemisphere will have the shortest day. The diagram on the right shows the situation at the June Solstice. Because the Earth’s axis is tilted (with the North Pole pointing towards the sun, while the South Pole points away from it), all places within the Arctic Circle have 24 hours of daylight. At the same time, the diagram shows that all places within the Antarctic Circle are
in darkness. 2
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TOPICAL SCIENCE
JUNE 2018
Stonehenge and the Summer Solstice The Stonehenge Neolithic monument in Wiltshire in the south of England is now a UNESCO World Heritage site. It consists of a circle of tall standing stones and lintels and dates from between 3,000 and 1500 years before the Christian Era. Archaeologists have estimated that the structures were built in phases, over a period of hundreds of years. The first phase, which was constructed around 5,000 years ago, consisted of earthworks, making a large circular embankment. A circle of standing stones was built within the embankment about 500 years later. Later, an outer stone circle was added. The tallest stones, known as sarcens, are almost 9 metres tall and weigh up to 50 tons. In prehistoric times, to transport them to the site and erect them in their standing position was a great feat of engineering and required much heavy labour. What is extraordinary about Stonehenge is that it is aligned with the sunrise at the June solstice. One possible explanation is that the people who constructed it were sun-worshippers. Because of this, many modern druids come to Stonehenge each year, to celebrate midsummer’s day.
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TOPICAL SCIENCE
JUNE 2018
The date of the Solstice The June solstice occurs on June 21st in 2018. This is the most common date for the solstice in non-leap years. However, because it takes the earth more than 365 days to reach the same point in its annual orbit around the sun, the actual moment of the solstice occurs almost six hours later each year. In a leap year, there is an extra day in the month of February, which causes the solstice to occur on June 20th.
Dates of the June Solstice 2015 - 2021 Year 2015 2016 2017 2018 2019 2020 2021
Date of Solstice June 21 June 20 June 21 June 21 June 21 June 20 June 21
Day of Week Sunday Monday Wednesday Thursday Friday Saturday Monday
The table above shows the dates of the June solstice for the years from 2015 to 2021. In 2015, the solstice fell on June 21st, which was a Sunday. The following year, 2016, was a leap year, so the solstice occurred on June 20th, because of the extra day in February. In 2017 and 2018, it is back to June 21st again and it will be on June 21st in 2019 as well. 2020 will be another leap year and so the solstice will fall on June 20th, as it did in 2016. In 2021, the solstice will again fall on its more usual date of June 21st. There are other factors such as the slight precession, or ‘wobble’ of the earth’s axis, that can affect the date of the solstice, but the most dominant factor in the variation in the date is due to our Gregorian calendar system, which adds an extra day to the month of February in leap years.
About the Author The author, Margaret Franklin, is a retired chemistry lecturer. Her column, entitled ‘Topical Science’ appeared in ‘The Westmeath Independent’ (a local weekly newspaper) for over three years. Its aim was to explain, for the readers, the science behind various news stories that were topical at the time. Margaret was also a regular contributor to ‘Science Spin’ magazine. Topical Science is now published monthly, in electronic format, in ISSUU, aimed at a wider readership.
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