Stonehenge
The megalithic monument of Stonehenge, located in Salisbury Plain, in southern England is one of those constructions that most of us know at least in photography.
Furthermore, thinking of this construction as a structure linked to astronomy is not a strange thing, indeed this association is very spontaneous.
The territory of Great Britain, Ireland, the Breton peninsula in France and many other places in northern Europe are scattered with these stone constructions, most often circles or mounds, alignments of monoliths, menhirs. However, it was only towards the end of the last century that archaeoastronomy work began, that is, to identify possible astronomical alignments in these monuments. Of course, archeoastronomy does not only deal with megalithic constructions but also with Greek temples, Egyptian monuments and much more.
Astronomy can be considered, without excessive presumption, probably the first science of man.
Anyone of us perceives the motions of the Sun, the Moon or the starry vault, perhaps not in an obvious way, but over time we realize this. We must therefore believe that in ancient times and in total lack of instruments for measuring time, primitive peoples sought in nature the equivalent of a clock. It will have taken a long time before man became aware of the periodicity of the motion of the Sun and the Moon, for example, but this has happened. Furthermore, given that the livelihood of these populations was mainly agriculture and farming, it became of primary importance to have a method at hand to predict the arrival of the seasons.
The Stonehenge monument therefore appears to be a huge calendar, useful every year, using poles driven into the ground or stones as a timepiece.
To better understand the complexity and accuracy of the arrangement of the architectural elements that make up the structure, it is essential to have some useful astronomical references at hand.
Motion of the Sun and the Moon
First of all we know that the movement that the Sun makes in the sky during the span of days is an apparent movement due to the two main motions of the Earth: the rotation on its axis, completed in 24 hours, and the revolution around the Sun, performed in one year. Of course we are not able, physically speaking, to perceive these motions during the course of a day.
By observing the shadows of objects we are only able to notice the daily or diurnal (apparent) motion of the Sun at consistent time intervals.
As for the annual (always apparent) motion of the Sun this is absolutely imperceptible during the course of a single day. Only after several days do we realize this, for example by noticing the lengthening or shortening of the days depending on the season.
A method that allows you to notice this without the use of the clock is to observe the Sun as it rises or sets at intervals of about 10 days, always at sunrise or always at sunset. By referring to fixed objects placed on the horizon it will be possible to verify that the Sun always rises and sets in different points. Therefore, claiming that the sun rises in the east and sets in the west is a misconception. The change in the point of sunrise or sunset has been known for millennia. We probably do not notice this phenomenon because we have lost a lot of the taste for observing nature and also because we have watches. Our ancestors had therefore noticed all this and probably had also associated the lengthening and shortening of the days to the various positions that the Sun assumes on the horizon.
When the days get longer, the Sun moves along the horizon, from south to north, passing through the east on March 21st (spring equinox); when they shorten, the Sun moves in the opposite direction passing through the east on September 23 (autumn equinox). On the days when the Sun reverses its gear we have the two solstices. On June 21, the summer solstice or commonly called the longest day of the year, the Sun stops moving from south to north. On December 21, the winter solstice or the shortest day of the year, the Sun interrupts the path from north to south. The winter solstice for megalithic civilizations and was not the most important date. It had been noticed that in those days the Sun was slowing down its run towards the south to stop it and resume it towards the north, in the opposite direction. The Sun began to rise again in the sky and this was a good luck sign: no one could know if after the solstice the Sun would resume its usual path. Many megalithic monuments have a preferential orientation towards the point on the horizon where the Sun rises on the day of the winter solstice.
Other very important astronomical references are the celestial equator and the ecliptic. The first owes its name to the simple fact that it is nothing more than a projection on the celestial vault of the terrestrial equator: this reference is exactly a semicircle. This circle is important because it shows the path of the sun during the equinoxes, when there are consequently twelve hours of light and as many hours of darkness. On all other days of the year, the daily path of the Sun is parallel to the celestial equator: in spring and summer there is a longer path than the celestial equator, so the day prevails over the night, in autumn and winter the exact opposite happens.
To clarify the concept of ecliptic, let's imagine that we can see, as if by magic, the fixed stars during the day. If this were the case, an acute observer might notice that as the months go by, the stars behind the Sun are gradually different. All these stars in turn form constellations, the famous twelve zodiacal constellations that determine a reference on the celestial vault: the ecliptic.
In other words, the ecliptic shows us the annual path of the Sun in the sky. The planets also move in the sky against the background of the constellation belt of the zodiac. Another definition of ecliptic, equally correct, defines it as the plane of the Earth-Sun orbit projected into the celestial vault.
The Earth's rotation axis is inclined by about 23° with respect to the vertical to the plane of the orbit. By transferring these data to the celestial vault we see that the celestial equator and the ecliptic are inclined by 23° relative to each other and this angle is called the declination of the Sun, + 23° at the summer solstice, -23° at the winter solstice.
The points of intersection between the celestial equator and the ecliptic are precisely the equinoxes: the spring equinox is the Aries point or gamma point, the autumn equinox is the balance point. All this will be useful to us in the context of the motions of the Moon that we are now going to define.
If primitive populations found the movements of the Sun on the celestial vault with some difficulty, it must have been much more difficult to discover the movements of the Moon, for a variety of reasons. The Moon orbits the Earth on an elliptical orbit (a slightly flattened circumference) whose plane does not coincide with the plane of the ecliptic.
If this were the case, an eclipse of the Sun would occur at each new Moon and an eclipse of the Moon at each full Moon. These two orbital planes are inclined by 5° 9' with respect to each other. The points of intersection between these two planes are called the nodes and the line joining them the line of nodes. Furthermore, the line of nodes is not fixed but slowly rotates in the opposite direction to the rotation of the Moon (retrograde motion) and completes a complete revolution in 18.6 years. When the line of nodes is perfectly aligned with the Earth-Sun junction and the Moon passes through one of the nodes, then the phenomenon of eclipses occurs: of the Sun when the Moon obscures the solar disk, of the Moon when it disappears in the cone of shadow of the Earth.
The Moon takes 27.3 days to circle the Earth and this period is called the sidereal month.
However, in this time the Earth has moved along its orbit so that after a sidereal month the Moon is not in the same conditions of illumination: if we count 27.3 days from the full Moon we do not find ourselves again in the full Moon phase. For this to happen, it takes 29.5 days in the synodic month.
The number 29.5 is one to keep in mind regarding Stonehenge. The position of the Moon during the synodic month determines the well-known phenomenon of the lunar phases, that is, the different illumination of the Moon by the Sun. The Sun always illuminates half of the Moon and this half is not always turned towards the Earth. When the Moon is on the side of the Sun we are unable to see the Moon at any time of the night. Later we begin to see a crescent moon with its hump facing west just after sunset, near the western horizon. As the nights go by, the Moon "grows" and rises higher and higher in the sky.
At a quarter turn the Moon is seen exactly in the middle, the phase of the first quarter and culminates in the sky when the sun sets. Then the phase increases until the full Moon when we see the entire lunar disk and then decreases until the last quarter (we still see exactly half of it, the left half) and then we return to the new Moon. Between two new moons, 29 and a half days have passed, that is a synodic month.
There is one last interesting phenomenon concerning the lunar motions with references to Stonehenge.
It has been said that the line of nodes moves and consequently the plane of the Earth-Moon orbit rotates. When the ascending node (the node crossed by the Moon when it passes from below to above the ecliptic) coincides with the Aries point (spring equinox), the Moon reaches its extreme positions on the horizon, i.e. the maximum and minimum declination: ( + 23° + 5°) and (-23° -5°).
On that day the Moon will rise on the horizon at the northernmost point possible, even further north of the rising point of the Sun on the summer solstice (upper stopping point).
After 15 days the opposite situation will occur and the Moon will rise as far south as possible (lower stopping point).
When the Moon is in the sky at its maximum positive declination (+ 29° at the time of the construction of Stonehenge) it will describe the longest path in the sky and the rising and setting points will be close to each other and close to the north.
When the Moon, on the other hand, is at its maximum negative declination (-29°), the rising and setting points will always be close to each other, but this time close to the south and the Moon's path in the sky will be the smallest.
At latitudes greater than 61° North (90° -29°) the Moon during its maximum path will be circumpolar, that is, it will not set in that particular night and 15 days later it will not rise at all because its minimum path will be found all under the 'horizon.
There are also two other stop points called minor that occur when the Aries point coincides with the descending node. In this case the declinations will be + 18° (+ 23° -5°) and -18° (-23° + 5°). These four points, which are very difficult to identify with observations, will be reviewed at Stonehenge.
Stonehenge
The place where Stonehenge was built appears to have been actively used in a period ranging from 3100 BC to 2600 BC followed by a 500 year period in which it was covered by vegetation to be reused again between 2100 BC and 1400 BC when the site was permanently abandoned.
The complex was built in three different stages that will be gradually analyzed.
Starting from the outside we see its various parts. Stonehenge has a circular plan formed by a moat and an embankment probably made up of the material excavated from the moat. The diameter of the embankment is around one hundred meters.
According to some authors, the Aubrey holes could be used to predict eclipses by periodically moving the markers corresponding to the Sun, the Moon and the nodes.
Immediately inside the embankment, arranged along the circular boundary, there is a series of holes, called the Aubrey holes, from the name of the English antiquarian who discovered the monument in 1649.
The holes in total are 56, equally spaced from each other and have an average depth of one meter. In a north-easterly direction from the embankment, the avenue starts an avenue just over half a kilometer long surrounded by two embankments and at the entrance of the avenue there is the Heelstone (heel stone), a monolith about five meters high.
Between the circular moat and the embankment of the avenue there are about forty holes for posts and it is thought that they were markers of the positions of the Moon.
In the circle of the Aubrey holes there are the 4 stations, named with the numbers 91, 92, 93 and 94 consisting of two stones and two small mounds arranged to form a perfect rectangle. The two short sides of the rectangle formed by the four stations are parallel to the axis of the monument which is the junction of the center with the Heelstone.
Between the Aubrey holes and the stone structure are two circles of holes, now covered, called Y and Z. They are about 90 centimeters deep and were not used for driving posts.
Circle Y has 30 holes, circle Z has 29.
The stone structure of the monument is divided into three parts: the circle of Sarsen, the five triliths and the circle of blue stones.
The circle of Sarsen (name of the place where the stones come from and about thirty kilometers away) has a diameter of 29 and a half meters and consists of 30 vertical stones 6-7 meters high topped by architraves so as to form a circle even with architraves. Of the 30 stones in the circle of Sarsen, one is half of the others.
Another characteristic detail concerns the construction technique of the circle.
In the upper part of each stone there is a protuberance that fits into a hollow of the overlying architrave: it is the so-called mortise joint. The architraves then have a joint in the contact face between them.
Inside the circle of Sarsen there is the enclosure of blue stones, coming from the north of England: there are in all 59 stones arranged in a horseshoe with the opening facing the driveway and therefore the Heelstone.
Inside this last enclosure there are the five triliths: two vertical stones surmounted by a horizontal one. Currently three are still intact. Finally, the altar stone is placed in the center of the triliths.
Archaeological studies show that the monument was built in three phases.
During the first phase, the external embankment, the Aubrey holes and the Heelstone were placed; during the second phase the avenue was added and finally in the third phase the stones of the circle of Sarsen, the triliths and the blue stones were erected.
Alignments and astronomical references
In 1740 the rev. William Stukeley notes that the monument's axis (joining center-heelstone) are headed northeast where the Sun rises on June 21st. Verification of this led to the search for other possible alignments as shown in the figure.
The short sides of the rectangle formed by the four stations also point both towards sunrise on June 21st and towards sunset on December 21st.
In addition, a whole other series of alignments show the directions of the four stop points (previously defined) of the Moon. These points, in order to be identified, require continuous observations and for prolonged periods. The Moon is found in these points once every 18.6 years.
By analyzing the quantity of stones and holes, we discover interesting randomness. The circles of holes called Y and Z are 59 in all, which is the double of 29.5 that is the number of days in the synodic month.
There are also 59 blue stones while the stones of the Sarsen circle are 30 of which one, as mentioned before, half of the others for which 29 and a half stones.
Finally the holes of Aubrey. As mentioned there are 56 in total and dividing by three we obtain about 18.6 which, as mentioned, is the time it takes for the knot line to complete a complete revolution. If we have 4 indicators: one for the Sun, one for the Moon and two for the two nodes and we arrange them so that the Sun is in hole 1, the Moon in hole 28 and the nodes in holes 14 and 42 can be simulate eclipses. Moving the Sun indicator clockwise by two holes every 13 days, the Moon (always clockwise) by two holes a day and the knots counterclockwise by 3 holes a year when the Sun indicator, that of the Moon and one of the two nodes are in the same hole an eclipse can really occur.
At this point it is good to make a consideration.
The alignments are a fact and they are many and such as to exclude coincidences.
This inevitably leads to consider Stonehenge as a luni-solar astronomical observatory. The numbers of the holes and the parallelism with the synodic month is obviously not proven, as well as the mechanism of forecasting the eclipses. There is no doubt that the phenomenon was known, however, to predict eclipses it is necessary to have the concrete possibility of observing as many of them as to extrapolate the mechanism. The skies and climate of southern England, as well as the latitude of the place do not seem to be suitable for such observations.
It must also be taken into account that the peoples who erected Stonehenge, in all three phases, did not know the writing so it is not clear how they handed down the information to be able to reproduce all the alignments seen.
Not to mention the construction techniques of the monument and the transport of the monoliths.
The series of mysteries concerning Stonehenge and other similar places scattered throughout northern Europe for now has no solutions even if the study of archeology and astronomy together have allowed to shed light on at least some things.
