The calendrical convention of the leap year is important to how we keep track of time. In the Gregorian calendar, adopted in 1582, a leap year is a year to which an extra day (February 29) is added. This extra day occurs in every year evenly divisible by 4. There is one exception to this rule. For years marking the turn of a century (e.g., 2000, 2100), if the year can be divided evenly by 400, it is considered a leap year. The year 2000 was a leap year, whereas 2100 will not be.
Leap years correct for the difference between a solar year and our calendar year. A solar year is the time (approximately 365.25 days) it takes the earth to complete a single orbit around the sun. Counting 365 days from January 1 to December 31 means that in four Earth orbits around the sun, the calendar difference (4 times .25) would have the New Year starting on January 2.
Early in history, the Egyptians, with sophisticated astronomical skills, realized the calendar would get out of date with the solar year. They therefore began adding a day every 4th year. Later, the Romans used a calendar with alternating months of 30 and 29 days. This produced a calendar year of 354 days, which came up short by 11.25 days a year, meaning in just 3 years the calendar became out of synchronization by more than a month. Their imperfect solution was to add a month every 2 or 3 years, thereby causing more confusion.
The emperor Julius Caesar solved the problem in 45 BCE by establishing a calendar of 365 days and adding 1 day every 4 years. In order to bring the calendar into alignment with the solar year, he added extra months to 1 year. This worked for a while, but the remaining inaccuracy led to a difference between the calendar beginning of spring and the vernal equinox, which defines spring.
The first day of spring occurs when the sun is directly above the equator while moving from south to north. This day, when the lengths of day and night are equal, is the vernal equinox. Similarly, in fall, the autumnal equinox occurs when the sun is again directly overhead but moving from north to south. The slow changes in calendar year versus the solar year meant that by the 16th century CE, the beginning of spring had moved from March 23 to March 11. Pope Gregory XIII ordained the change the calendar in 1582 to move the 1st day of spring to March 21. To do this, 10 days were eliminated, so the day after October 4 was October 15. Non-Catholic countries changed as well but not until 1752 or later. Other cultures use leap years but with different methods to calculate when to insert days or even months.
Even with the new adjustments, there is still a very small difference in the length of a calendar day and a solar day. However, it will take about 8,000 years before the difference will amount to a full day.
Charles R. Anderson
See also Calendar, Egyptian; Calendar, Gregorian; Calendar, Julian; Calendar, Roman; Earth, Revolution of; Time, Measurements of
Duncan, D. E. (1998). Calendar: Humanity’s epic struggle to determine a true and accurate year. New York: Avon.
Steele, D. (2000). Marking time: The epic quest to invent the perfect calendar. New York: Wiley.