The reason why the Gregorian Calendar was adopted in place of the Julian Calendar was because, quite simply, the Earth doesn’t really take 365 and a quarter days to get around the Sun. It’s a tad short of that figure, and problems began to appear with the alignment of the vernal equinox. This story traces a little of the history, and then introduces Christian Zeller, the mathematician who is understood to be the first to find a formula for calculating the elapsed days of the year.

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