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.
Remember the Y2K bug suffered by legacy computer systems? Well, there's a Y8K bug built into the Gregorian calendar, as it will likely fail to observe the vernal equinox anywhere near March 21 of the year 8000.
The average marginal difference between the actual astronomical year and the assumed astronomical year is 0.000125 days. Astronomer John Herschel famously proposed intercalating an extra leap day in the year 4000 to compensate for the Y8K calendar bug, but this solution was not adopted — and for more reasons than simply having almost a couple of millennia yet to make the decision.
The astronomical year is not a static interval of time, as there are tiny, random variances in the length of days and years. First and foremost, days are generally getting longer, as the braking force of Earth's tides is slowing the rotation of the planet. Major earthquakes have a similar effect. While the changes are usually measured in microseconds, the aggregate variance over the course of centuries cannot be accurately modeled, let alone predicted.
The complex gravitational interplay of the planets in our local solar system also causes acceleration and deceleration of Earth's progress around the sun. Add to that the gradual loss of mass by the sun itself, and the complex interactions of solar wind, gravitational radiation, and non-planetary objects make the collective force exerted on the Earth impossible to model over multi-century timespans. It should go without mentioning that, while these forces generally make years and days longer, they occasionally speed them up, as well.
Put more simply, the Year 8000 bug in the Gregorian calendar may not matter because there is enough error in the system already that we may need to add or even subtract a day from the calendar long before then, or perhaps make no changes at all, as the errors may cancel out over time.
That's not just a confusingly counter-intuitive chronology; it's an astronomically asymmetrical adage of Geek Trivia.
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