February 29, 1996
Consider this: the iron in your blood was forged in a supernova explosion billions of years ago.
Julius Caesar and Leap Days
Birthday · February 29, 1996
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APOD published February 29, 1996. Full image credits are on the original APOD entry below. A publication-date match, not a photo taken on your date. Independent of NASA.
Today, February 29th, is a leap day - a relatively rare occurrence. Advised by Alexandrian astronomer Sosigenes, Roman dictator Julius Caesar, pictured above in a self-decreed minted coin, created a calender system in 46 BC that contained one leap day every four years. The reason for adding leap days was that a year - defined by the time it takes the Earth to circle the Sun - does not actually take an exact integer number of days - defined by the time it takes for the Earth to rotate once. In fact, one year by these astronomical definitions is about 365.24219 days. If all calendar years contained 365 days, they would drift from the actual year by about 1 day every 4 years. Eventually July (named posthumously for Julius Caesar himself) would occur during the northern hemisphere winter! By making most years 365 days but every fourth year 366 days, the calendar year and the actual year remained more nearly in step. This "Julian" calender system was used until the year 1582 when Pope Gregory XIII added that leap days should not occur in years ending in "00" except if divisible by 400, providing a further fine tuning. This "Gregorian" calender system is the one in most common use today.