Leap Year

In ancient Rome, Julius Caesar established the $365-$day calendar to match the changing seasons created by the Earth's movement around the sun. Caesar recognized a problem, though: the Earth actually takes $365\frac{1}{4}$ days to circle the sun. The extra one$-$quarter day would gradually shift the calendar away from the seasons. Caesar's solution was to add an extra day every four years, in what are now called leap years. Caesar's math wasn't quite right, though. His approach added too many days. So$,$ in $1582,$ Pope Gregory XIII improved on Caesar's work. He proposed that century years should not be leap years unless they can be evenly divided by four hundred. Thus, $1600$ would be a leap year, while $1700$ would not. Pope Gregory's calendar, called the Gregorian calendar, is still in use today.

What is the main, or central, idea of the passage?

Julius Caesar created the leap year by adding an extra day to the $365-$day calendar once every four years.

Pope Gregory XIII improved the calendar by making century years leap years only if they could be divided by four hundred.

The leap year was created in ancient Rome to keep the calendar aligned with the Earth's movements and was later modified by Pope Gregory XIII.