Yet, the concept of time has baffled philosophers and scientists for millennia and we are still unsure of exactly what time is; although the work of Einstein and others has led us some way in its understanding.

However, exactly what time is does not really matter in the running of our day-to-day lives, but measuring its passing has preoccupied people for thousands of years. Calendars have been around for millennia, agricultural, religious and social reasons have made them essential in forecasting when to harvest crops or when to celebrate a religious event.

The majority of all calendar systems have been based on the movement of the Earth or Moon. A complete rotation is a day; the Moon's orbit of the Earth is a month; and an orbit of the Sun is a year.

Calendars based on the movement of Moon are known as lunar calendars whilst those based around Earth's orbit of the Sun are called solar calendars. Because the number of days in a year is not a whole number (the Earth takes 365 days and six hours to orbit the Sun) solar calendars have to fudge the figures, usually by adding an extra day every few years (a leap day) making a leap year a day longer than the rest of the years.

Problems arise with lunar calendars too. While the Moon takes 28 days to circle the Earth, which can be divided into seven (four weeks) a year can't be divided into equal lunar cycles, so months have to have a different number of days (the moon actually goes around the Sun 13 times in 364 days).

The baseline for calendars (the date they start counting) depends on cultural or religious reasons. The Gregorian calendar, adopted in Europe throughout the middle ages, used the birth of Christ; whilst a year in Japan is based on the current emperor's reign (2008 is year 18 of the Emperor Akihito).

The main use of calendars has always been to identify events and in modern times they are often combined and used along with time to create a complete timescale. A calendar based on the movement of the Earth or Moon are less relevant today since the advent of accurate timepieces and modern technologies such atomic clocks, NTP servers (Network Time protocol) and GPS (Global Positioning System). These have allowed the development of a global standardised time scale (known as UTC - Coordinated Universal Time).

However, thanks to these technologies, we now know that the movement of the Earth is not as accurate as our modern clocks (an atomic clock is 1,000,000 times more stable than the Earth's rotation). The Earth actually slows down (and sometimes accelerates) in its orbit. If nothing was done to compensate for this, eventually noon would become midnight and vice versa (albeit in many millennia) so leap seconds are added to the standard time scale just as days are added in a leap year.

In modern times calendars are still used. The Gregorian calendar is widespread throughout the West and other calendars have been developed such as fiscal calendar, developed by business as a way of comparing productivity or profit from month to month and year to year. For this reason fiscal calendars have a fixed number of weeks in a month, January may have five weeks while March may have four. Other calendars exist too such as those used by schools or sports.

Copyright (c) 2008 Richard Williams