Every four years, the addition of one day – that is, February 29 – brings us back closer to a more accurate location in the Earth’s orbit with respect to the stars.

Getting to our current calendar system has been a complicated process, mostly because of three important celestial motions: the Earth’s spin (or rotation), which gives us the day; the orbit of the Earth around the Sun, which gives us the year; and the orbit of the Moon around the Earth, the basis for the month.

During the reign of Julius Caesar (46 – 44 B.C.), the Egyptians were some of the best astronomers. They could see that the Earth’s orbit was very close to 365.25 days. Caesar used their knowledge to bring order to the Roman calendar. He established that every fourth year would be a leap year, tacking one day onto the last month – this is the basis of the Julian Calendar. Later, in 45 B.C., he moved the beginning of the year from March 1 to January 1. This is why the “seventh” month (September; Latin septem) through “tenth” month (December; Latin decim) are now the ninth through twelfth positions in the calendar. But before that move, the practice of adding the leap day at the end of the then year – February – had been established. (By the way, the month of July was later named for Julius Caesar after his death.)

In the late 1500s, astronomers under Pope Gregory XIII (for whom the modern Gregorian Calendar is named) realized 365.25 days wasn’t exactly right. The Earth’s orbit is actually 365.242199 days long. After hundreds of years using the Julian Calendar, spring had begun moving into winter. The Catholic Church did not want the celebration of Easter to occur in the winter. The astronomers worked out a new formula to skip some leap days to stay closer to an exact measurement. Certain hundred-year dates are leap years, such as 2000, whereas others (2100) will not be leap years. The Catholic Church also took ten days out of the calendar in 1582 to get it more closely synced with respect to its orbital position and the stars: October 4, 1582, was followed by October 15, 1582. Later, the British Empire, in 1752, did the same calendar adjustment but had to remove eleven days.

The Gregorian Calendar will work for a long time — at least for the next 3,300 years before an extra day is needed. Then a new calculation must be derived to get our calendar corrected with the Earth’s orbit. Today’s astronomers aren’t sure what that will be, but they’ve decided to let future astronomers figure it out.