Watching the sun, moon and stars
|A primer on the movements of the sun and moon from season to season. What are the solstices, the equinoxes, the quarter days and the major and minor standstills?|
The achievements and discoveries of astronomers over the past few centuries have been grand and exciting. But the impact of their work on the general public and on non-scientists has been limited, as one writer mischievously pointed out to a surprised scientific gathering -
' I am... a laborer on the arts-and-humanities side of the gulf that, we were assured decades ago by C.P. Snow, divides the realms of knowledge. The gulf is real. Just a few days ago, perhaps you saw, as I did, the item in [a quality newspaper] which revealed that twenty-one percent of... adults, according to a telephone poll, think the sun goes round the earth instead of the other way around, and seven more percent answered that they were undecided. Of the seventy-two percent who answered that the earth does orbit the sun, seventeen percent said that it takes one day, two percent one month, and nine percent could not guess at any time span. ' [Updike 1991, page 815.]
The reason for this confusion is possibly to be found in the very different explanations science and our human senses provide about what the sun and moon seem to do in the sky. This Web guidebook will take a purely observational, earth-centred universe method of describing the movements of the sun and the moon. This is not the usual textbook approach, which is to imagine the observer as outside in space, with the diagrammatic globe of the earth enhanced with the plane of the ecliptic, the tilt of the poles, and the rotating limits of the moon's orbit included.
For the purposes of description here, in contrast, we will assume again what was once universally believed, that the earth is still, and the heavenly bodies rotate around us. This is not to reject modern science and the truth of human knowledge, which is, as has often been said, the most precious thing human beings have; it is simply to acknowledge explicitly what our experience of the sky really is - we watch the sun rise and set, not the earthly horizon sink down in the east and sweep up in the west. Objectively, intellectually, we know what is really happening, but it is not what we see. We may 'know' we are on a huge rotating ball with the stars fixed, but we still see the stars move and the moon rise with them into the night sky. What we see today is what our ancestors saw when they erected the megaliths, and so that is what we must appreciate and become familiar with.
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.
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 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.
The Stars and Planets
Stars seem to be obvious candidates for human interest in the prehistoric past, just as much as in the present day. If anything, pre-technological societies have more pressing reasons to study the stars and the constellations, and anthropological studies have shown that modern individuals in such societies know the important facts about the night sky. For example, they know that any particular star will always rise at the same place on the horizon from night to night - vital for travel and navigation.
They know that the sun moves round the zodiac in the course of the year, so that different constellations and stars are visible in the night sky at particular seasons - useful for a simple calendar. At those particular seasons, stars rise and set in a regular sequence, thus creating a celestial clock to count off the hours of darkness. All societies have names for the patterns of stars or constellations in the night sky, often allied to the myths and legends of the people, to make the make the stars their own and the dread black night less terrifying and more familiar.
Despite this, it has almost become traditional when assessing the potential of prehistoric standing stone sites to dismiss the possibility of stellar alignments or orientations. The reasons usually given by those who argue this way are as follows:
1) The sun and the moon are always visible when they rise or set, given clear weather. However, because of the thickness of the atmosphere through which the much weaker light from a star has to pass at the horizon, stars are not visible when they rise or set, with the sole exception of Sirius, the brightest star of all, and even this star needs perfect conditions to be seen. (The thickness of the atmosphere at the horizon is also the reason we can look directly at the sun rising or setting while we cannot look directly at the sun high in the sky).
2) Since a particular star always rises on the eastern horizon at the same point, and sets on the western horizon at the same point, no markers are really needed for the stars - the horizon points simply have to be remembered.
3) Since the relationship of the star field or firmament to the earthly horizon is not fixed, but moves over a cycle lasting 26,000 years due to the wobble of he earth on its axis, referred to as the 'precession of the equinoxes', the rising and setting position of the stars change noticably even over the lifetime of a human being. Over longer periods of a few centuries, the changes are dramatic. For example, the azimuth (bearing in degrees from true north) of Sirius as it rose in 3000BC in Scotland was 138°. In 2500BC it had moved to133°, in 2000BC to 130° and by 1500BC it was rising at an azimuth of about 127°. (Today it rises at about 121°). Since the dates of the vast majority of the prehistoric megalithic sites are hardly known even to the nearest 500 years, then any supposed line could be due to chance. Furthermore, with such chronological flexibility, it it is almost always possible to find a bright star which will fit with a particular orientation, rendering the whole process meaningless. It is also suggested that prehistoric observers would soon have realised that the erection of stones to mark stellar positions was pointless, as the stars would eventually drift away from the marked lines.
These are strong arguments! However, if the prehistoric peoples of Scotland were closely observing the sun and moon, it seems unlikely they would have ignored the stars completely. At the very least they would have noticed there was a point around which the heavens appeared to revolve - the Pole. As for visibility, most horizons in Scotland are not at sea level, but are raised to half a degree or more by hills or mountains. Furthermore, a modern skywatcher has reported -
`During the writing of this book I visited Crete. The stars hang lower there ; you feel that you could strip them from the sky in handfuls like grapes from a vine. The Milky Way arches overhead like a tangled skein of phosphorescent wool. As the Earth turns you cannot fail to notice stars gradually sink into the placid sea on one horizon while others emerge from the deep on the other horizon.' [Ridpath 1988, page 6.]
It is likely that the clarity of the skies in prehistoric Scotland, which had a warmer climate and a less polluted environment, more closely matched those of modern Crete than modern Scotland.
A star does rise and set (in the short term at least) always in the same places, so at first sight the need to mark a stellar position is unclear. But the sun at midwinter always rises at the same point on the south-eastern horizon and sets at the same point on the south-western; it was still marked by permanent monuments, despite the fact that the hill or mountain over which the sun rose or set at midwinter would soon become well known to everyone who gathered to watch the event, so this argument against stellar lines is not conclusive.
The lack of proper dating of the prehistoric sites (though this will be achieved in time) and the multiplicity of available stellar lines are significant considerations. There is also a further problem in that some of the brightest stars in the period 3000BC to 1500BC (including Sirius) can also assume the declinations of the sun and moon at their extreme positions, and there is no way of knowing objectively which of the targets (sun or star / moon or star) was the intended one. However, these problems can be tackled.
For one thing, the problem of horizon visibility for stars itself limits the number of stars which can be considered as targets. Obviously, only the brightest stars will be visible at all at low altitudes, so these are the only ones which could be distinguished by orientated monuments. Of the thirty brightest stars visible from Earth, only twenty are actually visible from the latitude of Scotland. Of this twenty, two, namely Arcturus and Vega, are circumpolar, in other words they are in the sky always and never touch the horizon. Another pair, Deneb and Regulus, change their declinations by only half a degree over the Neolithic and Early Bronze Age period, and alignments to those stars may be apparent. As for the remaining sixteen bright stars, if there appears to be a prehistoric monument which is aligned to one of them this will be mentioned in the site description in the following pages, in the expectation that dating evidence from the site in the future will confirm or deny the significance of the orientation.
Another significant consideration is that prehistoric observers are more likely to have been aware of stars in the patterns or constellations which they form in the night sky. Just after sunset, as the sky fades from blue to black, and individual stars begin to appear, it is not possible to be sure which stars they are until the full pattern of the constellation becomes visible. So it may be that observers in the prehistoric past considered the whole constellation to be the object of interest, rather than simply its component stars. Because all constellations contain faint as well as bright stars, an orientation to a rising or setting constellation would require a very high horizon.
There are also the five planets known to the ancient civilizations, which are easily visible to the naked eye, namely Mercury, Venus, Mars, Jupiter and Saturn. These planets are certain to have been observed by the inhabitants of prehistoric Scotland. However, as seen from Earth, the motion of these planets appears irregular and unpredictable. This is due to the fact that these planets also orbit the sun, at different speeds and different distances. From Earth, the planets appear to move from east to west against the star field. But they can also appear to stop, and move backwards against the stars, and travel in loops. This is because their apparent motion is a combination of the movement of the Earth in its orbit and the movement of the planet itself in its own orbit. The ancient Greeks knew the planets as 'wanderers'. What the prehistoric peoples of Scotland made of the planets we do not know, but it is unlikely they were able to understand or predict most of what they saw, and it is also unlikely they erected permanent monuments to mark the positions of such wayward objects.