Astronomical measurements of the month The origin of the month Months and weeks The future of months

One month was originally measured by the time required for the Moon to go through a full cycle or one orbit of the Earth, known as a lunation. One lunation takes approximately 29.5 days, so twelve lunations equals 354 days, 11 days (or 12 days in a leap year) short of one tropical year. In 46 BC Julius Caesar ruled that months would no longer be based on the moon, rather he had them altered so that they would fit more evenly into one year.

Astronomical measurements of the month
The length of a one lunation depends on the observer's perspective. An observer on Earth is able to see that the Moon takes 29.5 days to do a complete cycle, while measurements made by an astronaut observing the movement of the Moon around the Earth from deep space would find that it takes 27.3 days for the Moon to complete one orbit of the Earth. This already gives rise to two definitions of a month, one based on observations of the phases of the Moon and the other on the passage of the Moon through space.

The sidereal month of 27 days 7 hours 43 minutes 12 seconds is the time taken by the Moon to make an orbit and return to the same place against the background of the stars.

The Earthbound observer's month, or synodic month, of 29 days 12 hours 44 minutes 3 seconds is based on the observation of a complete cycle of phases of the Moon from New Moon to New Moon. This month is made up of the time taken for the Moon to orbit the Earth once as well as to catch up with the progress of the Earth around the Sun.

Synodic month

The difference between synodic and sidereal months is due to the movement of the Earth and Moon in orbit around the Sun. That is, by the time the Moon has made one complete orbit, the Earth has moved on in its revolution around the Sun and the Moon needs another 2 days and 5 hours to come back to the same phase as seen from the Earth.

The origin of the month
Our current 12 month calendar (known as the Gregorian Calendar) originated from the 10 month Roman calendar, believed to be first introduced by an early Roman King, Romulus, in 738 BC. Each month was based on one cycle of the Moon and named after various Roman gods and numbers.

In this early calendar, two Moon cycles, and 11 days, in the middle of winter were left nameless, possibly because during this time, there was little agriculture activity. Later in the same century (700s BC) another Roman King, Numa Pompilius, added two more months to account for the two extra Moon cycles. This still left 11 days unaccounted for.

By the time Julius Caesar was ruler, winter was in September when it should have been in December! Based on the suggestions of the astronomer Sosigenes, Julius Caesar changed the calendar in 46 BC so that the months were no longer based on the cycle of the moon. Each month was now 30 or 31 days except for February, which had 29 days (one of which was moved to August later on), gaining a day every four years. Finally, to realign the calendar with the seasons, Julius Caesar declared that the year 46 BC would have 445 days. Apparently the Romans called this year the "Year of confusion"!

Although very similar to the calendar we use today, the Julian Calendar was still inaccurate, being about 11 minutes and 14 seconds longer than one tropical year. By 1580 AD the Julian Calendar was 10 days behind. In 1582, Pope Gregory XIII revised the calendar yet again, the numbering of the years was changed so that they were based on the birth of Jesus Christ and the leap year rule was changed (see Years and years for more information on leap years).

Months and weeks
Days, months and year are all based on natural occurences, but weeks are not. The idea to group seven days into one week comes from the bible:

Remember the Sabbath day, to keep it holy. Six days shalt thou labour, and do all thy work but the seventh day is the Sabbath of the Lord thy God. [Exodus 20:8]

In our current calendar, weeks do not divide evenly into one month.

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