22, 23...? I don't know what the equation is to find out.
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Wednesday, September 22nd
http://www.wolframalpha.com/input/?i=Aut…
http://www.wolframalpha.com/input/?i=Aut…
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There isn't a particularly straightforward equation because the calculation involves the motion of the Earth moving at non-uniform speed in its elliptical orbit about the Sun.
However, a table of predicted dates and times gives the date as 22nd September 2088 at 00:18 Universal (Greenwich) time. That means for most of the western hemisphere (including the USA) the date will be 21st September.
Note that at this time the seasons will be almost at their earliest. They reach their earliest for a few centuries in 2096, then 7 years without a leap year will make them reach almost their latest by 2103.
However, a table of predicted dates and times gives the date as 22nd September 2088 at 00:18 Universal (Greenwich) time. That means for most of the western hemisphere (including the USA) the date will be 21st September.
Note that at this time the seasons will be almost at their earliest. They reach their earliest for a few centuries in 2096, then 7 years without a leap year will make them reach almost their latest by 2103.