Lunar Standstill

Where did the sun rise this morning? In the east. Actually, the sun has a very wide band of rising positions in Scotland's latitudes. The rising and setting position of the sun varies with the seasons. At midwinter, about December 21st or 22nd, it rises in the south-east, climbs to a low position in the southern sky at noon, and sets in the south-west. It is above the horizon for only about 7 hours. This means it is completely dark for over 15 hours of the day. At the spring equinox, March 20th or 21st, and the autumn equinox, September 22nd or 23rd, the sun rises exactly in the east and sets exactly in the west, giving a day equally divided between day and night, twelve hours each. At midsummer, usually June 21st, the sun will rise in the north-east and be above the horizon for 17 hours or more before setting in the north-west. In the far north of Scotland the sun is such a short distance below the horizon at midnight that it is never fully dark. This dramatic variation in the length of the day was probably part of the reason for interest in the movements of the sun in prehistoric Scotland. In more southerly latitudes the variation between winter and summer daylight is not so marked and would not have made such an impact on daily life.

Figure 1 illustrates the different rising and setting positions at those four times of year, with the altitude or vertical position of the sun exaggerated (it is never overhead in Scotland). From its position at midwinter the rising sun moves further north day by day, until after six months it is at its extreme northerly rising position for midsummer. During the following six months it moves slowly south again, back to its southerly extreme for midwinter. These extreme positions are called the solstices, the times of year when the sun appears to stand on the horizon at the same place. There is a well-known analogy with a pendulum -

'Suppose we take a series of rapid-fire snapshots of a swinging pendulum as it reaches the top of its swing. First, it begins to slow down gradually, then it comes to a complete standstill just before reversing its direction. This is exactly what the rising and setting sun does on its annual cycle. As viewed along the horizon, it executes a seasonal turnabout. In fact, the term solstice, which defines the first day of summer, means "sun-stand" '. [Aveni 1989, page 74.]

So about the time of the solstices the sun's rising or setting position on the horizon does not change much from day to day compared with other times of year; by comparison, the movement on the horizon is greatest at the equinoxes, when the sun moves along the horizon from one day to the next by a distance more than its own diameter.

The winter and summer solstices and the spring and autumn equinoxes divide the year into four parts of nearly equal lengths. As mentioned above, during the Iron Age the Celtic peoples of Europe had additional periods for their festivities. Important celebrations occurred in early November (Samhuinn), early February (Imbolg), early May (Bealtuinn) and early August (Lunasda). There is the possibility that these festivals were inherited from an earlier era. These days occur half-way between the solstices and equinoxes, and are the more critical times of year for a subsistence agricultural and pastoral society. Samhuinn (Martinmas) and Imbolg (Candlemas) are sometimes referred to as the winter quarter days. The sun will rise and set at about the same position on the horizon for those two dates, once in November when approaching the winter solstice extreme, and once after it, in February. Likewise the sun at two summer quarter days will be in the same position in the sky, at Bealtuinn (Whitsun) before midsummer and at Lunasda (Lammas) after. The quarter days also divide the year into four parts. But critically, unlike the solstice/equinox divisions, these four parts closely coincide with the real seasons of spring, summer, autumn and winter. This is no surprise since they are derived from the needs of an agricultural society.

So, for the sun, there are ten horizon points which we may assume had some interest for skywatchers in prehistoric Scotland, five rising points and five setting. These positions are those which the sun has at the summer solstice, the winter solstice, the equinoxes, the winter quarter days, and the summer quarter days. These positions are illustrated in Figure 2. Observing the position of the sun on the horizon would allow a basic calendrical system without the need for written records or a calendar as we know it. Though such practices are long gone from Europe, the use of the sun's position on the horizon as a calendar marker is known from modern studies of non-literate cultures. One example is from the south-western USA, where the Hopi used such a system to regulate planting, harvesting and ceremonies; one individual, called the Sun Watcher, was responsible for observing every sunrise and telling the people of the arrival of the important days [Renfrew 1973, page 263].

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.

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 diagrams and description represent a simplification of the lunar movements, because there is another cycle in the moon's behaviour 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.