Calendars have been used throughout the world for centuries as a means of scheduling events, planning for
agricultural seasons, or simply counting days. Calendars were invented independently in many cultures because they gave leaders the power to predict events (seasons, monsoons, etc.). Although the earliest known calendar was used by the Egyptians around
4236 B.C.E, archaeologists have found evidence of carvings on bones dating as far back as 11,000 years ago. Many
other ancient civilizations maintained calendars as well. Before 2,000 B.C.E, the Babylonians used a lunar calendar
with twelve months alternating between 29 and 30 days, totaling 354 days for the year. In contrast, the Egyptian calendar was based on the solar year and included 365 days. On the other side of the
world, the Mayans of Central America maintained not only a calendar based on the sun, but also a calendar based on
the planet Venus.
Most calendars measure a count of years after an initial "epoch" or an important event. The current calendar that
is accepted throughout the modern world, the Gregorian calendar, is based on the birth of Christ. The Islamic
calendar, still used in many Middle Eastern countries today, is based on the Hegira, when Muhammad emigrated from
Mecca to Medina (approximately A.D. 622). The Chinese calendar, on the other hand, does not count years
sequentially. Rather, the years rotate names every sixty years. For civil purposes, however, most nations use
the Gregorian calendar.
Although not scientifically perfect, the Gregorian calendar is pretty much able to stay aligned with the tropical
year (it has only misaligned itself by an hour and twenty minutes since its creation in 1582). The tropical year is
defined as the average interval between vernal equinoxes and is approximately 365 days, 5 hours, and 49 minutes. In
order to make up for that extra few hours every year that are not counted, the Gregorian calendar, like many previous
calendars, uses a leap year. The rule is that there is a leap year for every year divisible by four except for years
that are both divisible by 100 and not divisible by 400.
Other cultures have dealt with the leap year quite
differently. For example, while the Egyptians had a 365-day year, they knew that there was about an extra quarter of
a day; but instead of using the Gregorian leap year system, they let that quarter of a day accumulate for 1,460
years, or four periods of 365 days, and had an entire leap year after that period.
The idea for the leap year was really the basis of the reform for the calendar by Pope Gregory XIII in 1582. Father
Christopher Clavius had instructed him that the Julian calendar had fallen off by
about a week since it was adopted in 45 B.C. To realign the calendar the Pope ordered that Thursday, October 4, 1582, would be the last
day of the Julian calendar and that the next day would be Friday, October 15, the first day recorded on the new Gregorian calendar. This had the extraordinary effect of eliminating a total of ten days from
history. Similarly, in 1752, the British Parliament removed days to synchronize their calendar with the modern one,
taking out the span starting on September 3 and ending on September 13.
In 1972, Atomic Time replaced Earth Time as the world's official scientific time standard. The current official
definition of the second is the time it takes for 9,192,631,770 oscillations of the Cesium atom at zero magnetic
field. As for the calendar, suggestions have been made for reform, but the Gregorian calendar is still standard.
Some problems that exist with it, other than the fact that it is slightly misaligned with the tropical year, are that
a year is not divisible by an exact number of weeks, the number of days for each month is haphazard, and it cannot
be divided into equal halves or quarters; however, it looks like our current calendar is here to stay for a few more