Moon Motions of

Just as the rising and setting positions of the sun move north and south along the eastern and western horizons over an annual cycle, those of the moon do the same over the course of a month. This means that, unlike the sun, the day-to-day change in the rising or setting position of the moon is considerable, since the passage from one "lunistice" to the other is completed in a period of just two weeks. Observing these lunar changes is not an easy matter, even from a place with perfect weather, because of the lunar phase cycle. The moon rises and sets approximately an hour later each day, meaning that over the course of the month, the aspiring observer will have to rise at all hours of the night. Furthermore, for half of the month the moon will be rising (or setting) in daylight, rendering the event almost certainly invisible.

The length of time it takes for the moon to complete a whole cycle from north to south and back again, known technically as the tropical month, is significantly shorter than the synodic (phase-cycle) month: 27.3 days as opposed to 29.5. This means that the phase cycle and the cycle to and fro along the horizon get significantly out of step. However, a pattern emerges over the course of a complete year. At the time of the June solstice, the moon is full when it is farthest south, and new when it is farthest north. At the time of the December solstice, on the other hand, the opposite is the case. This means that in the northern hemisphere, the midsummer full moon—or more precisely the full moon nearest to the summer solstice, which can occur up to two weeks before or after the solstice itself—will rise and set at, or very close to, the southern limit of its monthly motions. Conversely, the midwinter full moon will be at or close to the northern limit. Since the full moon is the most prominent phase, and also the phase at which the moon lights up the entire night, it may well be that it was the annual motions of the full moon northward and southward, rather than the monthly cycle to and fro, that was most important to many human cultures in the past.

From what has been said so far, the northward and southward swing of the rising or setting moon along the horizon would seem to be similar to that of the sun, but on a shorter time scale. However, there is another critical complication. The directions of sunrise and sunset at the solstices are effectively constant from year to year and only change very slowly over the centuries, owing to the gradual change in the obliquity of the ecliptic. The directions of moonrise and moonset at the monthly "lunistices," in contrast, vary noticeably from year to year. They themselves swing back and forth over a cycle lasting 18.6 years, which is known as the lunar node cycle. At one point in this cycle, the monthly swing of the moon from north to south is at its maximum and considerably wider than the annual swing of the sun. At such times, the most northerly monthly rising or setting path of the moon will reach beyond the most northerly annual rising or setting path of the sun by more than five degrees, or ten solar or lunar diameters; the most southerly path will do the same in the other direction. Some nine years later, in contrast, the monthly swing of the moon will be at its minimum, falling short of the annual swing of the sun at both ends by a similar amount. Alexander Thom coined the terms major standstill and minor standstill, respectively, to signify these times. Although the terms have been criticized as misleading, since nothing ever physically stands still, they are convenient and have become quite commonly used.

Again, it is likely to have been the changes in the position of the full moon that stood out most clearly to many human communities in the past. If they noticed the lunar node cycle at all, it may have been because of the changing path through the sky of the midwinter or midsummer full moon, rising and setting unusually far north or south for only one or two short periods during a lifetime. The effect would be most striking at latitudes of about sixty degrees, which is reached in the northern hemisphere, for example, in northern parts of Scotland. Here, in years close to a major standstill, the midsummer full moon would appear only briefly in the south, passing very low over the southern horizon, where its proximity to terrestrial landmarks would make it appear larger and closer.

See also:

Solstitial Directions; Thom, Alexander (1894-1985).

Lunar Phase Cycle; Obliquity of the Ecliptic; Solstices; Sun, Motions of.

References and further reading

Aveni, Anthony F. Stairways to the Stars: Skywatching in Three Great An-aent Cultures, 27-33. New York: Wiley, 1997.

Ruggles, Clive. Astronomy in Prehistoric Britain and Ireland, 36-37. New Haven: Yale University Press, 1999.

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