Astronomy

Astronomical Explanation of the Summer Solstice



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Solstices, believe it or not, are integral parts of our lives. Just recently when I was considering this topic it occurred to me that most people do not realise how important these times of year are to us and moreover have little to no idea why they occur. The Christian holiday of Christmas, the Jewish times of Passover and Sukkot, the Japanese Setsubun, the feast of St. John and many other holidays including Saturnailia, Hannukah, Yule and Litha are all shaped by varying solstices and equinoxes. Even our seasons are, for the most part, gauged on solstices, although this is debatable. I believe for these reasons it is important for us to understand the astronomical (not to be confused with astrological which involves star signs) explanation for these important occurrences. For the purpose of this article I will be considering the summer solstice, as it will be the next solstice to occur.

Put simply, the summer solstice occurs when the Earth's axis tilts the most towards the sun; it can not possibly tilt any closer during that yearly cycle. The meaning of the name is based on Latin, as sol' means sun and sistere' means to stand still (due to the fact that at the time of the solstice the sun stands still in declination and its movement north or south is very small). The reason for this is that planets do not spin perpendicular to their orbital plane, they actually rotates at an angle. This angle is hard to define as it varies, but Earth's is currently at an angle of roughly 23.26 degrees (it tends to fluctuate between 22.1 and 24.5 degrees because the world wobbles over time). This tilt is also expressed as an axial inclination' or an obliquity'.

As a result, for half of the year the northern hemisphere will be tipped towards the sun and during the other half the southern hemisphere will have its turn. The point at which the tilt can get no further away from the sun is the winter solstice, and respectively the point at which it can get no closer is the summer solstice which currently is expected to occur around mid-June (also known as midsummer, which gets its name from the point at which it could not possibly be any more summery!).

Due to the tilt of the axis, places that have a latitude of 23.26 degrees north during the northern solstice will see the sun directly overhead at noon this is known as the tropic of Cancer. Respectively, places that have a latitude of 23.26 degrees south during the southern solstice will see the same effect, and this is called the topic of Capricorn. The degree of tilt is directly proportionate to latitude at which one will see the sun exactly overhead.

This, in a nut shell and put relatively simply, is the astronomical reason for the occurrence of the summer solstice. It is remarkable how something that is so critical to our understanding of the world, our history and our culture is based on a concept that is so simple. Now the reader can enjoy their summer solstice (or their winter solstice of course!) knowing exactly how it occurs which makes it extra special!

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