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Why did Julius Caesar make the year 46 BCE 445 days long?

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

Julius Caesar extended the year 46 BCE to 445 days to correct a significant discrepancy between the Roman civic calendar and the solar year, which had drifted out of alignment with the seasons. By the mid-1st century BCE, the Roman republican calendar—which was based on lunar cycles—had become so misaligned that it was roughly three months ahead of the solar calendar.12

To rectify this, Caesar consulted the astronomer Sosigenes of Alexandria and implemented a two-part adjustment to bring the calendar back into step with the equinoxes:1

Standard Intercalation: He included the customary 23-day intercalation that was already due for that year, following February 23.1
Additional Adjustment: He inserted two extra months between November and December, totaling 67 additional days.1

This massive correction, which resulted in a 445-day year, allowed the subsequent year to begin in alignment with the solar cycle, marking the transition to the Julian calendar. Because of this extreme length, 46 BCE is historically referred to as the year of confusion.123