A leap year occurs every four years to help synchronize the calendar year with the astronomical year, specifically the Earth’s orbit around the Sun.
The reason for this adjustment lies in the fact that a
complete orbit of the Earth around the Sun doesn’t take exactly 365 days, but
rather about 365.2422 days.
This small fraction of a day adds up over time, causing the
calendar to slowly drift out of alignment with the seasons.
To correct this, an extra day is added to the calendar every
four years—February 29—bringing the calendar back into sync with Earth’s orbit.
The system we use today, known as the Gregorian calendar,
was introduced in 1582 by Pope Gregory XIII as a refinement of the earlier
Julian calendar.
The Julian calendar, which had been in use since 46 BCE,
accounted for the extra time by adding a leap year every four years.
However, this system still caused the calendar to drift
slightly over centuries, since it didn’t fully account for the exact length of
the astronomical year.
The Gregorian calendar addressed this by introducing a more
accurate rule: a year is a leap year if it is divisible by 4, but years
divisible by 100 are not leap years unless they are also divisible by 400.
This rule ensures that the calendar remains accurate over
long periods.
The addition of a leap day has practical effects beyond
keeping the calendar aligned with Earth’s orbit. It ensures that seasonal
events, such as solstices and equinoxes, occur at roughly the same time each
year.
Without leap years, over time, the calendar would fall out
of sync, causing seasons to shift gradually. For example, without this
correction, after a few centuries, summer in the Northern Hemisphere would
eventually start in what we currently consider the spring months.
In essence, leap years are a clever mathematical adjustment
that keeps our calendar accurate, ensuring that the natural rhythms of the
Earth and Sun align with the way we organize our time.