As Thursday, February 29, rolls around, it's not just another day—it's a leap day, a quirk of the calendar that only comes once every four years, according to 12News. The practice, which dates back to the time of Julius Caesar, keeps our modern-day Gregorian calendar in alignment with Earth's revolutions around the sun. It turns out the Earth doesn't orbit the sun in a neat 365 days, but takes about 365.2422 days. Hence the need to play catch-up with an extra day every four years.
This adjustment isn't as simple as tacking on an extra 24 hours every four years, due to the extra .2422 days adding up, so our calendar system includes a nuanced rule to keep things accurate over the centuries, only certain century years are leap years—they have to be divisible by 400. This means while the years 1700, 1800, and 1900 didn't get a February 29, the year 2000 did, Live Science explains. Without these additional measures, the calendar would drift and in a mere hundred years, we'd be off by 18 hours; even with a leap day every four years we're still gaining about 44 extra minutes, or a day every 129 years.
The "leap" in leap year comes from the idea that dates post-March "leap" over a day compared to the previous year; for instance, March 1 moved from being a Wednesday in 2023 to a Friday in 2024. Other calendar systems, such as the Hebrew and Chinese calendars, also incorporate their versions of leap years, though they differ in frequency and methodology. The idea of leap years has evolved over centuries, with the most recent tweak being the abolition of leap seconds from 2035 by the International Bureau of Weights and Measures (IBWM).
So why stick leap day onto February? Historical decisions dating back to the eighth century B.C., when Romans added January and February to a previously 10-month calendar, have left February as the shortest month even though, it was the last month of the year in the Roman calendar before being recognized as the start of the civil year around 450 B.C., Pope Gregory XIII placed the extra leap day there because it was already the shortest month making February's brevity a little less pronounced every four years, says Live Science. While the Gregorian calendar won't need a major revamp for another few thousand years, February 29 remains a calendar anomaly that ensures our concept of time stays consistent with the cosmic dance of our planet around the sun.
