Leap Year Calculation
by David Barth, 2007
CAESAR AND CALENDAR ISSUES
In 46 B.C., Caesar had had enough of the inaccurate calendar. A full 90 days had to be inserted into the calendar of that time in order to synchronize it with the
progression of the seasons. Caesar consulted the Greek astronomer, Sosigenes' whose calculations showed that the solar year was to last 365.24 days.
The calendar for the year 45 B.C. was the first to have 365 days. The missing six hours were to be saved up for three years and offset by an additional leap day
in the fourth year. Caesar assigned the ten extra days added to the new "ordinary year" to the individual months, resulting in them having 30 or 31 days. February,
because it was the leap month, had only 28 or 29 days.
THE MIDDLE AGES
It was not until the Middle Ages that Christians noticed that the date of Easter kept being pushed further and further back from year to year. At the behest of Pope
Gregory XIII, astronomers began their calculations once again. Ultimately, they realized that Caesar was off by 11 minutes and 14 seconds or 0.0078 days, per year,
resulting in an adjustment to the calendar.
THE POPE GREGORY XIII RULE
The Julian calendar was changed again on October 15, 1582. Before that, however, the additional ten days that had built up in the meantime would have to be
offset by doing away with the period from October 5-14, 1582. Gregory XIII formulated a new rule for the calendar: only the century years, 1600,1700, 1800 and
so on, that are divisible by 400, would be leap years. So despite the on-going 4-year cycle, the leap day was to be omitted in the years 1700, 1800, 1900, 2100,
2200, 2300, etc. because they are not divisible by 400. That is why we had a leap day in the year 2000.
IDENTIFYING LEAP YEARS
To determine if a year is a leap year, if it is evenly divisible by 4, the year is a leap year. The exception is if a year is a century year, 1600, 1700, 1800, etc.,
divide it by 400. If there is no remainder as a result of the division, it is a leap year.