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Why did the Julian calendar gradually move out of alignment with the seasons?

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

The Julian calendar gradually moved out of alignment with the seasons because it relied on an inaccurate calculation of the solar year. When Julius Caesar introduced the Julian calendar in 46 BCE, he worked with the astronomer Sosigenes to set the length of the year at 365.25 days.12

However, the actual tropical solar year is approximately 365.242199 days long. By overestimating the length of the year by 11 minutes and 14 seconds, the calendar accumulated a discrepancy of about one day every 130 years. Over several centuries, this error caused the calendar dates to drift away from the astronomical seasons. By the mid-1500s, this cumulative shift amounted to approximately 10 days, prompting Pope Gregory XIII to introduce the Gregorian calendar in 1582 to restore the proper alignment.21