The Julian calendar was found defective and the Gregorian calendar replaced it. One would think, with modern technology, we can measure the year more accurately than astronomers in Pope Gregory's time (1582) to replace the Gregorian calendar.
Turns out we can't, surprisingly because of chaos theory. The length of the year, i.e., Earth's orbit around the Sun, is a 3 body (or more) problem (Sun, Jupiter, Earth), and by chaos theory, inherently unpredictable over the long term. Furthermore, the length of a day, changes due to tides, earthquakes, etc.
That which will replace the Gregorian calendar will be an observational one: every few thousand years, probably skip (because the day is definitely getting longer) a Gregorian leap day. It'll be Y2k all over again. This is similar in spirit to observational lunar calendars used today.
How many microseconds long was last year, from say vernal equinox to vernal equinox? How does it compare to previous years?
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