The science of the summer solstice

Midnight Sun in Alaska In Alaska the sun remains visible in the night sky around the time of the summer solstice.

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Families across Britain enjoy the longest day of the year on Friday 21 June. It is the summer solstice of 2013.

We get the most hours of daylight because of the position of the Earth in relation to the Sun.

But the solstice does not necessarily fall on the same day each year. And in some parts of the world the Sun does not set at all.

While it is the day that has the most sunlight, Britain's weather typically does not become hotter until later in the summer.

Experts from five UK universities explain the science of the solstice.

What makes the solstice the longest day of the year?

Our planet does not spin on a vertical axis. It is titled at 23.4 degrees.

This means the amount of sunlight that reaches different regions of the Earth changes during the year as it orbits the Sun.

Stonehenge

Midsummer celebrations have been held in Britain at the time of the solstice for thousands of years.

Ancient stone circles like Stonehenge are still the focal point for such ceremonies today.

"Our summer solstice in the northern hemisphere is the point in the Earth's orbit when the North Pole is most inclined towards the Sun," says Manchester University's Dr Tim O'Brien, Associate Director at Jodrell Bank Observatory.

"The axis is tipped 23.4 degrees towards the plane of the Earth's orbit.

"Over the year the North Pole can be tipped towards the sun - summer in the northern hemisphere, or away from it, which is winter."

This tilt changes the path we see the sun take across the sky.

"On the summer solstice, the sun rises at its farthest point around the eastern horizon," adds Dr O'Brien.

"At noon the sun is as high above the horizon as it will ever get, and it sets at its farthest point around the west.

"So daylight lasts longer than on any other day in the year."

Earth's axis tilts 23.4 degrees

Why doesn't the sun set in some parts of the world?

Around the time of the summer solstice areas of Norway, Finland, Greenland, Alaska and other polar regions experience 'midnight sun'.

In the Arctic Circle the sun does not set at all. Again it comes down to the tilt of the earth's axis.

Iceland Midnight Sun During summer Iceland experiences 'midnight sun'

"The sun shines on the hemisphere of Earth that faces it," says Coel Hellier, Professor of Astrophysics at Keele University. "Our summer solstice is the point in Earth's orbit when the North Pole tilts most directly towards the sun."

"The polar regions are continually illuminated and there is 24-hour daylight throughout the Arctic Circle. This is down to a latitude 23 degrees from the pole, matching the angle of Earth's tilt."

While the north enjoys constant daylight the opposite occurs at the South Pole.

"When the Northern hemisphere is tilted towards the sun the Southern Hemisphere is tilted away," adds Prof Hellier. "Anyone in the Antarctic Circle would experience 24-hour darkness."

Why is Australia's summer solstice in December?

While June 21 is the summer solstice in Britain, for Australia and countries in the southern hemisphere this date marks the winter solstice.

"They have their summer solstice in the middle of our winter," says Martin Hendry, Professor of Gravitational Astrophysics at Glasgow University.

Start Quote

Oceans act as huge storage heaters and absorb heat all through the summer”

End Quote Dr Simon Boxall

"It comes in late December when their part of the Earth is leaning most towards the Sun."

Seasons are determined by the 23.4 degree tilt in the Earth's axis.

"Midway between the summer and winter solstices the axis is neither leaning directly towards, nor directly away from the Sun," adds Prof Hendry.

"We refer to these dates as the 'Equinoxes' - when the Sun spends about the same amount of time above and below the horizon.

"In the northern hemisphere the spring equinox occurs around March 21 and the autumn equinox around September 21.

"This is when the length of day and the length of night are about equal everywhere on the Earth."

Earth's orbit of the Sun

Why doesn't Britain get hotter until later in summer?

The northern hemisphere has the longest hours of sunlight around the solstice. But Britain usually sees higher temperatures in July and August.

It comes down to the way our planet retains heat.

Callanish Stones, Isle of Lewis
  • The four seasons of the year are caused by the 23.4 degree tilt in the Earth's axis.
  • The longest hours of sunlight come at the time of the June solstice for the northern hemisphere.
  • The seas around Britain store heat, bringing warmer temperatures later in summer.

"The oceans, atmosphere and the land all receive solar radiation," says Dr Simon Boxall, Lecturer in Oceanography at Southampton University.

"It provides over 99 per cent of the heat on the surface of Earth.

"The atmosphere and land can respond quickly to changes. If the earth was ocean free we would expect maximum temperatures closer to midsummer's day.

"But over 70 per cent of the planet is ocean, and water takes a while to warm up and cool down."

This effect is noticeable in Britain because we are surrounded by sea.

"The oceans act as huge storage heaters absorbing heat all through the summer," adds Dr Boxall. "Think about how long it takes to boil a pan of water and how long it stays warm afterwards.

"The oceans around us hit their peak usually at the end of August or beginning of September."

Why doesn't the solstice fall on the same day each year?

The summer solstice falls on June 21 in 2013, but the exact time changes each year. And every leap year it comes on June 20.

"A year of 365 days is only how we humans have chosen to divide time into convenient chunks," says Dr Somak Raychaudhury, Reader in Astrophysics at Birmingham University.

Changing times of the solstice

The solstice falls around six hours later each year, until a leap year when it jumps back to June 20.

  • 2013 June 21 05:04
  • 2014 June 21 10:51
  • 2015 June 21 16:38
  • 2016 June 20 22:34 - Leap year
  • 2017 June 21 04:24
  • 2018 June 21 10.07
  • 2019 June 21 15:54
  • 2020 June 20 21:44 - Leap year

"The average duration of a year is approximately 365 days five hours 48 minutes and 45 seconds.

"Even this varies by a few seconds every year, since the Earth's motion is not just caused by the Sun's pull of gravity.

"It is perturbed by the pull of the planets and moons in the Solar System. The relative positions of these change from year to year."

This means the solstice occurs around six hours later every year.

To resolve the difference between our calendar year and the actual time it takes the Earth to orbit the sun, we add an extra day at the end of February every four years.

"This makes the June solstice jump back to the previous date for each leap year," adds Dr Raychaudhury.

Celebrate the solstice with Midsummer Live on BBC Two Scotland at 7.30pm on Friday 21 June 2013, or watch online across the UK.

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