June 27 1962 - Astronomy and the Ancients

Most people know at least something about the stone circles scattered here and there across Britain, and of which Stonehenge is the most famous. It is often maintained that these stone circles have an astronomical significance, though we remain uncertain of their exact purpose. In the 1962 June programme, a description was given of a fascinating theory by George Henderson, who carried out some additional research especially for the purpose. There is still discussion as to whether the theory is valid; I suggest that it would provide grounds for a thorough survey to be carried out by archaeologists and prehistorians.

Midsummer Day has always been associated with ceremonial. The Sun has reached its northernmost position in the sky, so that it rises farther north of east and sets farthest north of west; the nights are at their shortest. In Britain, there is no true darkness in June during the time when the Moon is gibbous or full.

In fact, the Earth is at its maximum distance from the Sun during northern summer. The distance amounts to 94J million miles, as against only 91J million miles in December. The Earth's orbit round the Sun is not perfectly circular, though the eccentricity amounts to only o-oi7. The seasons, however, are due to a different cause. The Earth's axis of rotation is inclined to the perpendicular by 23J degrees. During northern summer, the North Pole is tilted toward the Sun; during northern winter, the reverse is the case. The seasons owe little to the minor changes in the Earth—Sun distance.

In theory, the southern hemisphere should have summers which are slightly shorter and hotter than those in the northern hemi­sphere - while the southern winters should be longer and colder. On the Earth, however, such effects are largely masked by the unequal distribution of land and sea, and are not important. Conditions are different on the planet Mars, where the axial inclination is roughly the same (about 26^ degrees) and the orbital eccentricity is much greater, so that the distance between Mars and the Sun ranges between 139 million miles at perihelion and 153 million miles at aphelion. Measures show that on Mars the southern hemisphere does experience bigger temperature changes than the northern - and the southern polar cap has even been observed to vanish completely at Martian midsummer.

Needless to say, ancient peoples regulated their lives by the seasons. They had no choice; they depended upon hunting and farming, and it was essential for them to be able to predict what conditions to expect. For instance, they had to bear in mind the behaviour and movements of the animals which they hunted; and they had to know when to plant the appropriate crops for each year. The march of the seasons was, in fact, much more vital to our remote ancestors than it is to ourselves. Consequently, they had to be able to measure certain events of each year. The 'longest day' presented no real difficulties, and little equipment other than a pole capable of casting a shadow, but the moment of the vernal equinox, when the Sun passes from the southern to the northern hemisphere of the sky, was not so obvious. The only way to carry out such determinations was by means of astronomy, and there is a link here with the arrangement of the various 'stone circles' to be found here and there all over the British Isles.

Stonehenge photographed by the author in 1963

The most famous of these stone circles is Stonehenge. Stone-henge is impressive, and is in a good state of preservation. A great many people connect it with the Druids, the 'holy men' of Ancient Britain who were so savagely persecuted by the Romans during the occupation. The Druids themselves are regarded as sinister figures: bloodthirsty, white-bearded old men, practising human sacrifice together with rituals at least as horrible as anything which the modern world can offer, and keeping the common people in a state of constant terror.

Actually, this picture seems to be a long way from the truth. There is no evidence, for instance, that the Druids practised human sacrifice, and there is a great deal of indirect evidence that they did not; there is no reason to suppose that their religious ceremonies were savage. It is much more probable that they were remarkably enlightened by the standards of their time, and that they were teachers rather than oppressors. They believed passionately in free­dom - which is why the Romans objected to them so strongly. So long as Druidism remained a powerful force, there could be no security for the occupation forces in Britain; the famous Governor, Suetonius Paulinus, attacked them in their strongholds of Anglesey, and slaughtered many of them, though traces of the cult lingered on for centuries more.

Moreover, there is not the slightest cause to believe that Stone­henge is connected with the Druids in any way whatsoever. For one thing, the time-scale is wrong. The monument was probably erected in several stages, and it is not easy to give precise dates, but at any rate we may be sure that the building was complete well before 1000 b.c. The earliest mention of the Druids in Britain goes back only to about 200 B.C., and it is hardly likely that the cult was firmly established before 400 b.c. This means that Stonehenge existed a long time before the Druids did. In point of time, in fact, Stonehenge was as distant from the Druids as we ourselves are from King Canute.

It has been suggested that even though the Druids did not build Stonehenge, they took it over and used it as a place of worship until the monument was destroyed by the Romans in their anti- Druidical campaign. This, too, appears to be questionable. Stone­henge was not the sort of place to interest the Druids, whose centres of worship were sacred groves, and it is by no means certain that the Romans played any part in ruining the monument.

The second popular misconception concerns the Heel Stone, which stands well outside the main circle. It is said that if an ob­server stands at the centre of the monument on Midsummer's Day and watches the Sun rise, he will see the Sun appear straight over the Heel Stone - which acts as a check that the time really is precise midsummer. This is one of the main features of the modern cere­monies.

Actually, midsummer sunrise does not take place straight over the Heel Stone, and it never has. Moreover, at the time when Stonehenge was built, the Sun rose slightly farther north than it does now, and was already well clear of the horizon when it passed above the Heel Stone. This change is due to the phenomenon known as the Precession of the Equinoxes.

At present the Earth's axis points, in a northward direction, to a point marked approximately by the bright star Polaris (Alpha Ursae Minoris). This is the famous Pole Star, and is familiar to most people; any doubt as to its identity may be resolved by using two of the stars in the Great Bear, Merak and Dubhe, as pointers to it. In ancient times, however, the pole of the sky was in a different position, and was marked approximately by a fainter star, Thuban (Alpha Draconis). Thuban, then, was the pole star to the builders of Stonehenge.

The direction of the Earth's axis changes very slowly over the years; it may be said that the Earth spins rather in the manner of a child's top which is running down, though the comparison should not be taken too far. It takes roughly 25,000 years for the celestial pole to describe a full circle and return to its original position, so that the movement is very slow; but over a long period it becomes appreciable. In 12,000 years from now, the north polar star will be the brilliant bluish Vega (Alpha Lyrae). In our epoch, there is no conspicuous south polar star, the nearest naked-eye object being the very obscure Sigma Octantis. There is no mystery about this movement; it may be fully explained on astronomical grounds.

If the celestial pole shifts, the celestial equator inevitably shifts too, and this causes a perceptible change in the right ascensions and declinations of the stars - which is why all stellar catalogues are carefully dated. It also causes slight alterations in the rising and setting positions of the Sun and all other celestial bodies. This is why in the early days of Stonehenge the sunrise point was even farther away from the Heel Stone, on Midsummer Day, than it is now. In this sense, true sunrise over the Stone at midsummer will not take place until about a.d. 3260. Altogether, we can see that the picture of the white-robed Druids sitting in the middle of Stonehenge and waiting for midsummer sunrise over the Heel Stone is nothing more than an attractive legend.

This is not to suggest that Stonehenge lacks astronomical asso­ciations. There can be little doubt that it is basically astronomical, and was probably used as a solar temple (though not by the Druids). The same is true of the various other stone circles found in Britain, such as Callanish in the Isle of Lewis. Equally interesting is Avebury, in Wiltshire, where the circle surrounds the modern village. Then there is the Nine Stones monument at Winterbourne Abbas: and there are many more.

Rollright in Oxfordshire

One excellent example of a stone circle is to be found at Roll- right, in Oxfordshire. It appears to date from about 1500 b.c., so that it too is much older than the Druids and comparable with Stonehenge. This is one of the many circles studied in recent years by George Henderson, who has for a number of years farmed at Enstone, not far from Roll right. Henderson has been responsible for a most interesting theory which may possibly throw new light upon the uses of these ancient stone circles.

Henderson points out that for measuring rising (and setting) points a star is much more reliable than the Sun, simply because a star is a dot of light - whereas the Sun is a brilliant disk, and it is by no means easy to tell the exact moment when 'sunrise' takes place. He contends that, as well as being used as solar temples, the circles were associated with the stars, largely for the purpose of measuring dates such as the spring equinox which were extremely important to ancient peoples.

The scale of many of the circles is remarkably uniform; about 33 yards in diameter, with a 'guide stone' about 100 yards outside. And at Rollright and other circles the guide-stone is in a definite direction: N. 27 degrees E. from the centre of the circle. This is well beyond the limit at which the Sun could ever rise to an observer standing in the middle of the circle - and so whatever may be its purpose, the guide-stone cannot be a sunrise marker.

Henderson believes that the guide-stone at Rollright relates not to the Sun but to a specific star, the object being to time the moment of spring equinox. What is needed is to locate a bright star which just skirts the horizon at dawn at the time of spring equinox, and thus appears over the guide-stone. The choice is obvious enough: the star concerned can only be Capella.

Capella, or Alpha Aurigae, is one of the most brilliant stars in the sky; of the stars visible in the north hemisphere only Sirius, Arcturus, Vega, and Rigel are its equals or superiors. It is highly luminous, and has a candle-power 200 times that of the Sun although admittedly this is not a great deal when compared with Rigel, which has a candle-power at least 20,000 times that of the Sun and possibly as much as 50,000. It is yellow in hue, and is easily identifiable because of the three faint stars, the 'Haedi' or Kids, close beside it. During winter evenings it is to be seen almost at the zenith, while in summer evenings it is very low in the north. It never actually sets, and so is circumpolar in Britain. This also applies to Vega, which lies on the opposite side of the present Pole Star; Vega is almost overhead during summer evenings, low in the north during winter evenings.

Capella does rise at the correct angle at the correct time - of course, allowance has been made for precession - and if this is pure coincidence, it is a most remarkable one. This would be the case if Rollright were the sole example; but Henderson has shown that it is not. In other cases, he points out that natural features (gaps in hills, for instance) could serve instead of actual guide-stones; the Winterbourne Abbas circle is an example of this. And observations of Capella made in such a way would undoubtedly serve as a good check upon the time of spring equinox.

Naturally, there are many circles which do not have 'Capella markers' of this sort; Stonehenge is one. Yet we must always remem­ber that the circles are extremely ancient, and it is possible that in many cases the outer guides have been removed while the more obvious circle-stones have been preserved.

Much remains to be learned about the uses of the stone circles. Yet they do at least add proof, if proof were needed, that the ancient peoples did more than simply look up at the night skies. They had no real idea of the nature of the universe; they did not know that the Earth moved round the Sun, and they would have refused to believe that our own glorious Sun is nothing more than a normal star, far inferior to Capella and others in size and luminosity. All the same, it may be said that they were astronomers, even if primi­tive ones, instead of mere star-gazers; and in these days of space rockets and planetary probes, it is well to look back sometimes and remind ourselves that astronomy is the oldest science in the world.