Jun 212015
 

At 17:39 today, the Sun reached the limit of its journey into the Northern Hemisphere and started on its return south. This is the June Solstice, the longest daylight hours in the Northern Hemisphere, and 9 hours, 21 minutes longer than on the December Solstice.

A solstice occurs when the Sun is at its furthest point from the equator. On the June solstice it reaches its northernmost point and the Earth’s North Pole tilts directly towards the sun, at about 23.5 degrees. It’s also known as the northern solstice because it occurs when the sun is directly over the Tropic of Cancer in the Northern Hemisphere.

Illumination of Earth by Sun at the northern s...

Illumination of Earth by Sun at the northern solstice. (Photo credit: Wikipedia)

According to timeanddate.com, ‘Solstice’ (from the Latin: ‘solstitium’) means ‘sun-stopping’. The point on the horizon where the sun appears to rise and set, stops and reverses direction after this day. On the solstice, the sun does not rise precisely in the east, but rises to the north of east and sets to the north of west, meaning it’s visible in the sky for a longer period of time.

Although the June solstice marks the first day of astronomical summer, it’s more common to use meteorological definitions of seasons, making the solstice midsummer or midwinter.

On the June solstice, the midnight sun is visible (weather permitting) throughout the night, in all areas from just south of the Arctic Circle to the North Pole. On the other side of the planet, south of the Antarctic Circle there’s Polar Night, meaning no Sunlight at all, on the June Solstice.

The midnight sun in Longyearbyen

The midnight sun in Longyearbyen (Photo credit: Wikipedia)

The hours of daylight will start to shorten until the December Solstice, which this year occurs on the 22nd at 04:49 in Nottingham. The difference will be barely noticeable at first, the daylight hours on the 22nd of June being just two seconds shorter than the 21st, but by July 1st, the difference each day will be a minute or more.

The Sun will set on the 21st at 21:34, and will continue to set at this time until July 1st. The daylight will shorten entirely in the morning during this period. This is due to ‘the equation of time’.

The equation of time is defined as the difference between apparent solar time and mean solar time. Apparent solar time is measured by the current position of the sun, whereas the mean solar time is measured by a clock set so that over the year its differences from apparent solar time average to zero. Apparent time can be ahead (fast) by as much as 16 minutes 33 seconds (around November 3rd), or behind (slow) by as much as 14 minutes 6 seconds (around February 12th).

Put simply, a day is not exactly 24 hours long, 23 hours, 56 minutes and 4 seconds to be precise, so Earth time gets out of step with the orbiting of the Sun. As a result of Earth’s elliptical orbit and the Earth’s axial tilt, apparent solar days are shorter in March (26–27) and September (12–13) than they are in June (18–19) or December (20–21).

If that’s too complicated to follow, pour a glass of your favourite tipple, step outside and enjoy the warm Summer evenings – Winter is on its way.

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