So what can we notice if we watch the moon?
The first, most obvious, feature is that its shape changes, and then, if we watch it regularly, it soon becomes clear that it doesn’t appear in the sky at the same times each day. After keeping an eye on it from time to time for a while, these changes are predictable. At the time of writing (20th July 2012), the moon was what we call “new” the day before yesterday, so it’s not yet visible to my eyes, but I can imagine how it looks and where it is, a tiny thin sliver of a crescent, lost in the glare of the sun, trailing a little behind that brighter body as they both cross the sky from east to west. As the sun sets tonight, I shall keep watch: with the solar brilliance slipping below the horizon, I may just be able to discern the moon’s silvery line. If not tonight, surely tomorrow I will see it (weather and cloud cover permitting). A couple of days ago, the moon rose and set at pretty much the same time as the sun, although we couldn’t see it, lost as it was in the sun’s glare, but each day the moon lags behind by between a bit under an hour and a little over an hour and a half, depending on where it is in its cycle. So each day for about the next ten days, the moon will be rising a bit later, further behind the sun, and will be appearing bigger and brighter in the sky. By the time the moon is full, It will have got so far behind the sun that it will be rising about the time that the sun is setting, and setting around sunrise. Funny, isn’t it, how we think of the moon as a feature of the night, when actually it can rise and set at any time of the day or night, including, at new moon, around the same time as the sun.
But I’m getting ahead of myself. This time, between what we now call “new moon”, and the day when we can first observe the tiny sliver with our own eyes, is a good moment to reflect on how the sky appears when it is empty of an observable moon. The nights are dark, and many earlier societies used to keep watch for the return of the moon, and start a new time-keeping period (cycle, month) when it appeared. We get our word “calendar” from the Latin “Kalends”, the name for the first day of each month. This word comes from a Greek word meaning “to announce”, after the practice of announcing the first appearance of the new moon in the west after the sun had set. It is thought that, originally, this announcement would be the trigger to change from one month to the next, but pretty soon the ability to predict this date was so secure that the Romans counted down towards it: they didn’t call their dates “three days after Kalends”, for example, but, once Ides (which probably originally corresponded to full moon) was past, they called the days “fifteen before Kalends”, or “five before Kalends”, and so on.
In fact, if we were in very early Roman times, an official would be charged with looking out for the first visible sliver of moon just after sunset tonight, and, if he saw it, would “announce” it, so that today would be Kalends, the first day of the new month, to coincide with the first sighting of the new moon. The same official, who had plenty of experience in judging the phases of the moon, would observe it each day, and, by the size and shape of it, would be able to tell how many days there were left until Nones, the day when exactly half the moon would be visible (what we now call the First Quarter, because it’s a quarter of the way through the moon’s monthly cycle). So tomorrow, he might say it was “five before Nones”, and the next day “four before Nones”, and so on. Incidentally, Nones comes from the word for nine: the Romans counted inclusively, and Nones is nine days before Ides, if you count both the day of Nones itself and that of Ides.
Of course, pretty quickly, the Romans needed a system that didn’t rely on local officials making the same estimate, so they instituted (probably under Numa Pompilius, one of the early kings of Rome) a system where each month followed another on pre-set days, whatever the moon was doing. They kept the names for the calendar points, though, even once Kalends could come around at any old point of the moon’s cycle.
This is a pretty common pattern: there are many ancient and classical societies where a period of time that was originally prompted by and closely allied to the phases of the moon, ends up staying approximately the right length, but starting at pre-determined moments that may no longer follow the moon. Perhaps I’ll explore some of those another time!