Months
and 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.
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.
The future of months
In the future, our year may be divided into 13 months of exactly the
same length. An idea currently being put forward is that the year would
be divided into 13 months of exactly 28 days (four weeks). This type
of calendar would leave one day, placed at the end of the year, belonging
to no week or month. Every four years one day would be added just before
July 1. Every month would begin on a sunday and end on a Saturday.
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