The Science of the Winter Solstice

On December 21st at 6.30 a.m. the Sun will rise exactly between two specific pillars at the Temple at Karnak, North Luxor, Egypt. It has done this once every year for the last 3000 years, ever since the temple was built by the ancient Pharaohs’ to the God Amun – Re.

Amun – Re was the King of Gods. He was a fusion between Amun the God of Thebes and Re, or Ra the Sun God. The temple stands on the east Earth-lighting-winter-solstice_LAbank of the River Nile facing towards the Theban Hills and is lined up at the correct angle for the sunrise known as the Winter Solstice.

This event occurs because the Sun rises at different points on the horizon throughout the year as a result of the Earth rotating on a tilted axis relative to its orbit of the Sun. This tilt of 23.5⁰ also gives rise to the seasons. On December 21st the northern hemisphere is at its maximum tilt away from the Sun, receiving its minimum amount of sunlight for the year and putting the north polar region into complete darkness. On this date, all places above the latitude of 66.5⁰ north (the Arctic Polar Circle) are in darkness. The opposite situation applies for the southern hemisphere at this time.

During this time of year the intensity of solar radiation is at its lowest in the northern hemisphere partly because that area of the Earth’s surface is slightly further away from the Sun, but also because the Sun’s rays impinge at an oblique angle. This causes the Sun’s rays to pass through a greater thickness of atmosphere and also spreads them over a wider area of ground, weakening their impact.

Another noticeable feature of this oblique angle of the Sun is longer and more spectacular sunsets.

The Winter Solstice occurs annually between Dec 20th and Dec 23rd in the Gregorian calendar, in which Dec 21st and Dec 22nd are more frequent dates. The next solstice on Dec 23rd will be in 2303, the last having occurred in 1903. A Dec 20th solstice is very rare, and the next one is in 2080. This is mainly due to the calendar system. The Gregorian calendar has 365 days in a year and 366 days in a leap year. However, the tropical year is different to the calendar year. The tropical year is the length of time the Sun takes to return to the same position in the seasons’ cycle (as seen from Earth). In other words the time from one winter solstice to the next winter solstice is different to the calendar year. The tropical year is approximately 365.242199 days but additionally varies from year to year because of the influence of other planets. The exact orbital and daily rotational motion of the Earth, such as the “wobble” in the Earth’s axis (called precession), also contributes to the changing solstice dates.

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