The Lunar Month Index

The lunar month

The Moon gives out no light of its own, but it reflects the light of the Sun and this enables us to see it. The only part of the Moon we can see is the part facing both the Sun and the Earth.

The Moon goes round the Earth. Allowing for the movement of the Earth round the Sun, the Moon passes across, or more usually just over or just under (which is why we do not get an eclipse of the Sun every month), a line between the Earth and the Sun about once every 29½ days. As it crosses the line between the Earth and the Sun no part of the Moon faces both the Earth and the Sun, so no part of the Moon is visible from any point on the Earth. This is the astronomical New Moon. It is of course just a moment in time, and to someone in London (or anywhere else), because of the rotation of the Earth on its axis it can occur at any time of the day or night. So the first time we can actually see the New Moon, as a very thin crescent, is just after the Sunset following the astronomical New Moon. Our first sighting of the New Moon will therefore be just after Sunset either twenty nine days or thirty days days after our first sighting of the last one. The time from the first sighting of one New Moon to the first sighting of the next is a lunar month, and will be either twenty nine or thirty days. There are approximately 12½ lunar months in a solar year. Note that the length of the lunar month is predictable but does not follow a simple pattern: a twenty nine day lunar month is not always followed by one of thirty days.

As the Moon moves away from the line between the Earth and Sun more of it faces both the Earth and the Sun and about fourteen days after the New Moon we can see all of one lunar hemisphere - this is the Full Moon.

New Moons are easy to observe and easy to count (and a much more obvious beginning than a Full Moon), so most of the earliest calendars were based upon lunar months, each month starting in the evening of the first observance of the New Moon.

On the evening of the twenty ninth day after the last observation of the New Moon the priest would go to the place appointed and look for the New Moon. If he saw it he would declare the start of a new month, but if he did not see it he would declare the following evening the start of the new month - he knew that a month had to have twenty nine or thirty days so he did not have to observe the New Moon on the thirtieth day (Reference 3). See also Note.

Unusually, although the earliest Egyptian calendar was a lunar calendar, the new month started on the first day when the waning crescent was not visible in the Eastern dawn sky rather than the first day in which the waxing crescent was first visible in the Western evening sky. (Reference 11) There is a simple extended account, mainly for children, of the various Egyptian calendars in the Egypt section of this web site.

The time from one astronomical New Moon to the next is called a lunation. Individual lunations vary in length by several minutes, but the average length is about twenty nine and a half days - this is called the synodic period. The synodic period is gradually getting longer. Neither the variation in the length of individual lunations nor the long-term change in the synodic period will produce an observed lunar month of more than thirty or less than twenty nine days for several thousand years.

In some very early writings it is difficult for us to be certain whether the unit of time being referred to is the lunar month or the solar year. For example, in the fifth chapter of Genesis, the first Book in the Bible, we read (in translation of course!) that Methuselah became a father when he was one hundred and eighty seven years old, and died when he was nine hundred and sixty nine years old. His claim to fame is that he is the oldest man ever to have lived. But if the word that we have translated as year actually means not a solar year but a lunar month we find that he became a father when he was fifteen years old and lived to be seventy seven years old, which is not so remarkable.

Note that because as well as the Moon going round the Earth the Earth is going round the Sun, a lunar month is slightly longer than the time taken for the Moon to go round the Earth. The Moon actually goes round the Earth about once every twenty eight days.

The Moon also rotates on its axis, at exactly the same rate that it revolves round the Earth. This means that the same side of the Moon always faces the Earth: all Full Moons are the same and we can never see the other side of the Moon from the Earth.


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