One of the more intriguing aspects of astronomy is that it teaches us a year is not actually a year. More specifically, a calendar year (365 days) is not equivalent to an astronomical year (roughly 365.25 days), which is why we must (triple word score alert) intercalate an additional day into the calendar every four years to make up the difference. Thus, for those of us who observe the modern Gregorian calendar, the quadrennial appearance of February 29 will go down in just six days.
Except that, strictly speaking, February 29 isn’t a quadrennial event. A Gregorian leap year is observed every four years except in century years. Thus, the year 1900 was not a leap year. However, every fourth century year is a leap year, so the year 2000 included a February 29. Why all the conditional intercalation? Because an astronomical year isn’t precisely 365.25 days — it’s closer to 365.2425 days, or 365 days, 5 hours, 49 minutes, and 12 seconds. The intercalated adjustments are made to ensure that the vernal equinox stays on or about March 21 of every year.
Yet for all these allowances, the modern Gregorian calendar is still inaccurate, as the astronomical year isn’t precisely 365.2425 days long. A recurring margin of error is building in the leap year system that suggests it will fail — as in, the vernal equinox will be more than a day removed from March 21 — at a particular date in the future.
IN WHAT YEAR WILL THE GREGORIAN LEAP YEAR SYSTEM “FAIL?”