１年は３６５日でもなく、３６６日でもない。

　今年は２月が２９日まであります。これは４年に一度、閏年を入れ、１年を３６６日としてます。人類の先達は１年が３６５日のままでは「季節のずれ」が起こることを知り、後世の人類が困ることがないよう閏年を入れ、暦を調整しておいてくれました。

　これにより、現代を生きる我々はその恩恵に浴し暮らしてます。閏年がなければ、遠い将来には梅の花が８月に咲くことにもなりかねません。

　ご存じの通り、閏年は通常４年に一度あります。４年に一度と言うことは１００年で２５回であり、４００年では１００回の閏年となる筈です。ところが、もし、この４年に一度の閏年を今後も永遠に続けて行くと、これでは閏年が多過ぎて、遠い将来には再度、季節がズレることを計算してました。

　このため、４００年に１００回でなく、４００年に９７回の閏年になってます。近年では２０００年が閏年の筈でしたが、平年でした。元々１年とは、地球が公転軌道を一周することです。それにかかる時間は365日5時間48分45.168秒と計算され、一年は３６５日では短過ぎ、３６６日では長過ぎるのです。

　調べてみると、１月１日午前０時に真南にあるシリウスの位置は毎年ほんの少し東にずれてます。つまり、３６５日では地球は公転軌道上を一周できてないことが分かります。そして４年に一度の閏年を加えることで、シリウスは元の位置に戻ります。

Dear my overseas friends

　The simultaneous interpretation for your understanding will be after my breakfast in Japan time.

　...............................

　This year, February has 29 days. This year is a leap year which is set once every four years, making 366 days in the year. Humankind's predecessors knew that if 365 days continue from now on, "seasonal shifts" would occur, so they added leap years and adjusted the calendar so that future generations would not have problems.

　As a result, we who live in modern times are living with the benefits of their wisdom. Without a leap year, in the distant future we might end up with snow in July and plum blossoms in August.

　As you know, leap years usually occur once every four years. Once every four years means 25 times in 100 years, and in 400 years there will be 100 leap years. However, if this leap year that occurs once every four years were to continue forever, there would be too many leap years, and it was calculated that the seasons would shift again in the distant future.

　Therefore, instead of 100 leap years in 400 years, there were 97 leap years in 400 years. In recent years, 2000 was supposed to be a leap year, but it turned out to be a normal year. Originally, a year means the Earth completes one revolution in its orbit. The time it takes is calculated to be 365 days, 5 hours, 48 minutes, and 45.168 seconds. This means that 365 days is too short for a year, and 366 days is too long.

　When I looked into it, the position of the brightest Sirius which is due south at midnight on January 1st, shifts slightly to the east year by year. In other words, we can understand that the Earth cannot complete one revolution in its orbit in 365 days. And by adding a leap year every four years, Sirius returns to his original position.

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