What's a lunar month?

What is a lunar month?

Well, you can have a quick look at Wikipedia and get thoroughly confused with unpronounceable words. Or we can have a little think and get some idea of what the problems are with trying to answer that question.

Start with the Sun, use an orange or a melon or a vase in the middle of your dining room table to represent it. If you want to be a bit more abstract stick a dot in the middle of a piece of A4 paper. Put an apple about half a metre from the Sun, this represents the earth. Finally following the fruit thing to the end, use a plum to represent the moon and put it about ten centimetres from the earth, sorry apple. This is NOT to scale, and has no intention of being, it just helps the ideas. Line up the three pieces of fruit, with the moon-plum between the earth-apple and sun-melon. This represents a new moon, that pretty little crescent visible just after sunset once a month or so.

Now move the earth round the sun a bit, about a twelfth of the total circle that would be made in a complete circuit. This would take about a month, so send the moon all round the earth at the same time until the three are back in line for another new moon. Can you see that the moon has done actually done rather more than a full orbit of the earth in that time, actually close to one and one twelfth? So in a full year, we’ll get twelve new moons but the moon has had to go round the earth thirteen times to achieve that. Is it just the planets and moons spinning or has your head joined in?

We do have a possible candidate for a lunar month though, the time from new moon to new moon. Is there another? Well, yes, there is. Whatever we may say we think of astrology, there are not many of us that don’t know the name of our birth sign, Aquarius, Taurus, Scorpio and so on, even though we may not actually recognize it in the night sky. As the year progresses, these are the constellations that the sun appears to rise in. A moment’s thought will convince you that the moon, when new, appears to be in each of these constellations in a year, at least to a first approximation. For example, the moon is in Leo at the moment, so tomorrow’s new moon will also appear to be in Leo. Another important measure of lunar progress is, therefore, how long it takes to be apparently (this word being used here in its literal sense) back in the same place, relative to the constellations or fixed stars. A little bit of playing about with the bits of fruit will convince you the moon will have to go round the earth about thirteen of these circuits in a full year. Each one of these circuits of the earth is called a sidereal month.

OK, I confess, it’s been simplified a bit, but we have two candidates for the title of lunar month, one about twenty-nine and half days long, the other about twenty-seven.

Actually, it gets more complicated yet, as the moon is subject to little bits of gravitational pull from the other planets in the solar system and it also appears to wobble in its orbit, for reasons way outside the scope of this little piece.

Now what has all this to do with living in Lebanon? Ramadan starts tomorrow, or at least is widely expected to, but the new moon being sighted is needed to confirm it. Perhaps this little discourse explains why that old fashioned method still holds sway.