In much of Europe, 5 October 1582 never happened: the Julian calendar, leaping every fourth year without exception, had run 10 days ahead of the seasons, so the count jumped straight from the 4th to the 15th. Its replacement — a leap year if divisible by 4, except by 100, except again by 400 — yields an average year of 365.2425 days against a true tropical year of 365.2422. Earth’s axis also wobbles like a spinning top once every 25,770 years, so the pole star has passed from Thuban to Polaris and will one day be Vega.
In much of Europe, 5 October 1582 never happened: the Julian calendar, leaping every fourth year without exception, had run 10 days ahead of the seasons, so the count jumped straight from the 4th to the 15th. Its replacement — a leap year if divisible by 4, except by 100, except again by 400 — yields an average year of 365.2425 days against a true tropical year of 365.2422. Earth’s axis also wobbles like a spinning top once every 25,770 years, so the pole star has passed from Thuban to Polaris and will one day be Vega.
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