How did leap years work in the Julian calendar?

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Kalendarium (“Calendar”) by Regiomontanus — calendar First complete printed title page for the *Kalendarium* (“Calendar”) by Regiomontanus, 1476.

The Julian calendar, introduced by Julius Caesar in 46 BCE, established a system to account for the discrepancy between the calendar year and the solar year. Commissioned by Caesar and designed by the astronomer Sosigenes, the Julian calendar utilized a 365-day year with an added leap year every four years.1

This system was based on the calculation that the tropical solar year is 365.25 days long. However, because the actual solar year is approximately 365.242199 days, the Julian calendar was slightly inaccurate. This minor error resulted in a drift of one day every 130 years, eventually necessitating the transition to the Gregorian calendar in 1582 to correct the accumulated 10 days missing from the calendar cycle.1