Even though the
standard calendar year is 365 days, the Earth actually takes 365 days, five hours, 48
minutes and 46 seconds to travel around the sun. This is called a solar
year. To keep the calendar cycle synchronized with the seasons, one
extra day --- February 29 --- is added every four years.
The Julian Calendar,
established by Julius Caesar in 46 BCE (Before Common Era), introduced the Egyptian solar calendar
to the Roman world, standardized the 365-day year, and created the predecessor to
our current leap year. February 29 was not reflected on the Julian Calendar,
rather February 23 was repeated every four years.
You may be asking “The
solar year is not a full 365 days and 6 hours, so what about those extra 11 minutes
and 14 seconds?” An additional calendar reformation
in the 1500s added a special rule to adjust for this discrepancy. In 1582, Pope
Gregory XIII created a slightly modified calendar to better account for leap years.
Called the Gregorian Calendar,
this new system said that no century year (like 1900) would be a leap year
except for centuries divisible by 400 (like 2000). In order to correct the calendar,
the Pope eliminated October 5 – 14, 1582. The calendar moved directly from the
fourth to the fifteenth to align the dates with the seasons again. It feels
almost like science fiction to think that 10 full days were removed from the calendar
in the year 1582.
But where does the
phrase leap year originate?
In 365-day years, known
as common years, fixed dates advance one day in the week per year. For example,
Christmas fell on a Tuesday in 2018 and on a Wednesday in 2019. With the insertion
of a leap day, dates (following February) advance two days instead of one. In
2020, Christmas will leap over Thursday to fall on a Friday.
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