In 1582 Pope Gregory XIII sponsored a new solar calendar to replace an older calendar in effect since 46 BC, that had been introduced by what person?

The calendar system that prevailed across the Roman Empire and much of the Western world for over 1,600 years was indeed introduced by Julius Caesar. In 46 BC, as *pontifex maximus*, or chief priest, Caesar initiated a significant reform of the chaotic Roman lunar calendar, which had become largely out of sync with the solar year due to political manipulation and imprecise calculations. With the guidance of the Alexandrian astronomer Sosigenes, Caesar established a solar calendar featuring 365 days, with an extra "leap day" every four years to account for the Earth's orbital period. This innovative system, known as the Julian calendar, brought much-needed order and became the foundation (Review) for timekeeping for centuries.

However, the Julian calendar, while a remarkable improvement, contained a subtle flaw. It approximated the average solar year as exactly 365.25 days. The true length of a tropical year, the time it takes for the Earth to complete one orbit around the Sun, is slightly shorter, at approximately 365.2422 days. This seemingly minor difference of about 11 minutes per year accumulated over time, causing the calendar to drift by roughly one day every 128 years.

By the 16th century, this accumulated error had become significant, with the calendar being about ten days ahead of the actual solar events, particularly affecting the accurate calculation of Easter. To rectify this growing misalignment, Pope Gregory XIII sponsored a new calendar in 1582. The Gregorian calendar refined the leap year rule: while still adding a leap day every four years, it omitted leap years for century years not divisible by 400 (e.g., 1700, 1800, 1900 were not leap years, but 2000 was). To correct the existing discrepancy, ten days were famously skipped in October 1582, with October 4th being followed directly by October 15th. This adjustment brought the calendar back into closer alignment with the seasons, and the Gregorian calendar is the system predominantly used worldwide today.