History of Science: The Astronomy of the winter solstice - Berliner Morgenpost

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22:12:13, 01:35

Until the reform of the calendar, Christmas was the shortest day of the year

By Norbert Lossau

As of today, the days are getting longer. The difference from the previous day may be tiny, but already in the Stone Age were people able to identify the day of the winter solstice, such as the worship of Stonehege occupied. The significance of the winter solstice was once charged religious, today combine most of these day with the feeling that the sun again from now spread more light.

In pagan times it was natural to celebrate the return of the life-giving sun. On the day of the winter solstice – 2000 years ago on the 25th December – by the Christians then their Christmas. The calendar reform of Pope Gregory XIII. tore apart the collapse of Christmas and the winter solstice. The 25th December was retained as the Christmas deadline, but the shortest day of the year shifted to 21 December.

For the people of the 21st Century is the winter solstice primarily an astronomical event, it is clear from the geometry of the Earth’s orbit around the sun and the tilt of Earth’s axis relative to the orbital plane. The level at which the Earth’s axis is perpendicular (“ecliptic”) is inclined to the orbital plane in which the Earth orbits the Sun at 23.4 degrees. Therefore go in the winter in the northern hemisphere the sun later on, and formerly as in summer – the days are shorter

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Twice a year passes through the earth a position in the orbital plane and the ecliptic intersect. These are the 21 March and the 23 September, in which the day as long as the night is exactly (“Day and night are equally”). Does the earth of these two positions 90 degrees on its annual path, they reached the points of the summer and the winter solstice. At the winter solstice, the sun is in the constellation Sagittarius, the summer solstice between Taurus and Gemini.

This was not always so, and that’s changing again. The Earth’s axis “coasts” namely a bit, so that changed the inclination between the ecliptic and the orbital plane. In each 25,780 years the sun goes through the time of the winter solstice once the entire zodiac. Thus, in ancient times the sun was at the winter solstice in Capricorn, in 300 years, people will see the sun in the zodiac sign Ophiuchus.

But why the day of the winter solstice does not always fall on 21st December? The solar year (the time for a railway revolution of the Earth around the Sun) is almost six hours longer than 365 calendar days. Therefore, the time of the winter solstice shifts annually by about six hours ahead. In a leap year, then returns a time jump of about 18 hours. Therefore, the winter solstice is already at a 20 or 22 Take place in December.

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