Julian calendar

The Julian calendar is a solar calendar introduced under Julius Caesar in 45 BCE, with 365-day years and a leap day every four years. Its average year is slightly longer than the tropical year, so calendar dates gradually drift against the seasons.

Tropical year: The average interval between successive passages of the Sun through the March equinox, about 365.2422 days. The Julian year's 365.25-day average exceeds this seasonal cycle.
Julius Caesar: A Roman general and dictator who ruled during the Republic's final years and initiated major political reforms. He ordered the calendar reform that bears his name.
Gregorian calendar: The solar calendar introduced by Pope Gregory XIII in 1582, with a revised leap-year rule. It corrected the seasonal drift inherited from the Julian calendar.
Revised Julian calendar: A calendar adopted by some Orthodox churches, using a leap-year rule that closely approximates the Gregorian calendar. It retains Julian month structure while changing the leap-year calculation.
Leap year: A calendar year containing an extra day to keep its dates aligned with the astronomical year. The Julian calendar adds February 29 every fourth year.
Roman calendar: The system of months and days used to organize civic and religious time in ancient Rome. The Julian calendar replaced the late Roman calendar's irregular year.
Adoption of the Gregorian calendar: The gradual replacement of the Julian calendar by the Gregorian calendar across countries and institutions. Different adoption dates left countries using different civil calendars for centuries.
Hebrew calendar: A lunisolar calendar that coordinates lunar months with the solar year through intercalation. Unlike the Julian calendar, it keeps months aligned with lunar phases.
February 29: The extra calendar date inserted into February during a leap year. It is the Julian calendar's recurring correction day.
Sosigenes of Alexandria: An astronomer associated with Alexandria who advised Julius Caesar on calendar reform. Ancient accounts credit his astronomical advice in shaping the Julian scheme.

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