The Moon

Just as much as the sun, the moon has always been an object of wonder and veneration. It is a spectacular night sky object, and has the advantage over the sun that it can always be looked at directly. Its cycle of phases during each month is obvious and those phases, and the months themselves, are a convenient way of dividing time and describing different periods of time in a pre-literate society. The diagrams show how the moon is seen over the twenty-nine day period from the first appearance as a new moon.

In Figure 3 the sun has set below the horizon in the west, and the first crescent of the moon is visible. It will soon also set.

The moon rises in the east about one hour later each following day, and the shape of the crescent changes during the next seven days to become a half moon, called the first quarter (Figure 4). At this time the moon spends the first half of the night in the sky.

The moon waxes further over the following seven days, until it becomes full, and rises just before sunset (Figure 5). The full moon will remain in the sky for the whole night.

Then the full moon begins to wane, and after the next seven days will become a half moon again, the period called the last quarter (Figure 6). At this time the moon is in the sky in the later part of the night.

As the days pass, the moon wanes again to a crescent, all the while getting closer to the sun, until at last it rises in the east shortly before the sun, and is visible for only a short period (Figure 7).

Finally the visible moon disappears from the sky entirely (Figure 8), the period of the new moon, and so the cycle repeats endlessly.

The diagrams and description represent a simplification of the lunar movements, because there is another cycle in the moon's behavior which is of vital interest to the present study. Anyone observing the moon even casually over the period of a year would inevitably notice several interesting facts.

The most important is probably that the twelve or thirteen full moons over the course of the year do not rise and set in the same positions on the horizon. The full moons in summer rise and set much further to the south compared with those in winter. This means that the full moons of summer are in the sky for a shorter period than those of winter, and reach a lower altitude in the sky. From the perspective of a pre-technological society, it would seem very lucky that the winter full moons rose earlier and set later, providing light through the long winter nights just when it was needed most. In fact, it probably seemed miraculous. The full moon nearest to midwinter is always the highest and longest shining full moon of the year, and everyone has experienced bright moonlit frosty nights around the winter holiday.

Once this seasonal difference in the full moons had been noticed, a further fact would probably have been observed as well, as the years passed. This is that from year to year the rising and setting positions of these winter and summer solstice moons themselves change. The change from one winter solstice or summer solstice to its equivalent a year later is about 3°. (This is the same as the width of six moon diameters, and the change would be plain to any careful observer.) This is the most obvious way in which the other cycle of the moon's movement reveals itself. The moon goes through a cycle of horizon positions which repeats itself every 18.6 years. Fundamentally, the cycle is of a widening and narrowing band of rising and setting positions which the moon at any phase can attain. At one extreme point of the cycle, called the 'major standstill' (following Alexander Thom), the moon will rise and set far to the north, well beyond the position of the sun at the summer solstice. Two weeks later, the moon will be rising and setting far to the south, only appearing in the sky for a very short period.

Figure 9 illustrates the very different paths of the full moon across the horizon at the winter solstice and the summer solstice, during the lunar major standstill period. The contrast in rising positions and the time that the moon is in the sky at those two times of year is very marked, and would not have been missed by people who spent most of their lives out of doors. During the period of the major standstill the moon at phases other than full will swing between the two limits shown; in other words a waxing or waning moon could appear in the extreme positions to north or to south. But the only full moons which will attain the extreme positions shown in the diagram are those closest to the summer and winter solstices.

Just over nine years later, during the 'minor standstill', the limits of the moon's path have contracted, and the positions of the solstice full moons will be as shown in Figure 10. It will be seen that now there is less of a contrast between the summer and winter full moon positions, though the winter full moon is still in the sky for a much longer period than the summer full moon. Again it must be remembered that at other times of year during the minor standstill the moon's phases will move between those limits, with a waxing or waning moon capable of reaching the limiting positions.

Such are the fundamentally simple movements of the sun and the moon across our skies. The key positions for the sun are the two solstices and the two equinoxes, and for the moon the positions of the full moon closest to the two solstices at the times of the standstills. The easiest way of defining and recording such events for a society with no writing or instruments is by erecting permanent markers which cause a viewer to look towards the positions on the horizon where the sun or the moon will rise or set at the times in question. It is now accepted that this is part of the purpose of many of the standing stones, stone circles and chambered cairns erected in the late stone age or Neolithic period, and in the early Bronze age.

Compliments of Stones of Wonder