One year = 365,25 days

A calendar is a system invented by man to keep track of time. It is based on observations of periodic phenomena (alternation of day and night, phases of the Moon, position of constellations).

There are 2 main types of calendar:

• Solar calendars: A date corresponds to the position of the Earth in its orbit around the Sun. The solar calendar is seasonal (a season corresponds to the same months each year). 
• Lunar calendars: A lunar cycle (lunation) defines the unit of the month. However, lunar calendars don't allow us to accurately track the periodicity of the seasons, and many lunar calendars have evolved into “lunisolar” calendars.

The calendar most widely used today is the Gregorian calendar (solar calendar). A year is divided into twelve months of unequal length (28, 29, 30 or 31 days). A common year has 365 days. But since an astronomical year (the Earth's period of revolution around the Sun) is just over 365.25 days, a new day must be added every 4 years to compensate for the shift that occurs with each revolution: this is the 366th day of a “leap” year.

In this animation, the distance scale is not respected for the sake of legibility.