The history of calendars is the story of civilizations’ search for a reliable way to measure time. From the first observations of the Sun and Moon to modern calendars, this evolution spans more than 5,000 years.
Prehistoric Calendar
Humans have always felt the need to develop tools to manage time. Early humans observed natural cycles such as the alternation of day and night, the phases of the moon, and the changing seasons. These observations were essential for organizing their activities, such as hunting, agriculture, and even religious rituals.
In 1950, the Belgian geologist and archaeologist Jean de Heinzelin de Braucourt discovered an engraved bone in Central Africa dating back more than 20,000 years. Known as the Ishango Bone, this engraved bone may have been used to count days or the phases of the moon. The Ishango Bone is considered one of the oldest known examples of human mathematical activity.
Lunar Calendar
Early calendars were often based on the phases of the moon. Lunar cycles provided a relatively easy-to-observe measure of time, which led to the creation of lunar calendars in many ancient cultures.
The Julian calendar
The first Roman calendar, introduced around the 7th century BC, divided a 304-day year beginning with March into ten months. January and February were added later. As the months only contained 29 or 30 days, another month had to be inserted, roughly every other year. The months were designated by counting backwards from three pivotal dates: the calendas at the beginning of the month, the ides in the middle and the nones, which fell on the ninth day before the ides. This calendar became hopelessly confused when the Roman rulers who were responsible for setting the days and months to be added abused their authority to extend their term of office or change the date of elections.
In 46 BC. Julius Caesar decided, on the advice of the Greek astronomer Sosigenes, to establish a new calendar. This calendar, known as the Julian calendar, set the length of a normal year at 365 days and that of a leap year, every 4 years, at 366 days, with 24 February as the double day. Caesar also changed the start of the year from March 1 to January 1.
Gregorian calendar
The Julian calendar was a great step forward, but it kept getting out of step with the spring equinox. It was in 1582 that Pope Gregory XIII decided to correct the error of the Julian calendar. He removed ten days from the calendar to realign the spring equinox with 21 March, and corrected the calculation of leap years. The Gregorian calendar is still used today in most countries of the world.
