Stonehenge, situated on Salisbury Plain in southern England, is one of the finest examples of a prehistoric stone circle in the world. Considering the primitive state of European technology at the time it was built (between about 2200 B.C. and 1600 B.C), this megalithic “observatory” is an impressive achievement.
About 80 large stones (some weighing more than 25 tons) were transported from Marlborough Downs, a distance of more than 19 miles (30 kilometers). Many of the smaller stones (called bluestones) were taken from southwest Wales, even farther away. Moreover, many of the stones were shaped, dressed smooth, and carefully positioned to mark important astronomical events, such as the rising and setting of the sun and moon.
From the dawn of civilization, when people first began to contemplate the heavens, the sun, moon, stars, planets, comets, and meteors have been objects of wonder, mystery, and awe. Later, but still early in the development of human civilization, people began to study the heavens in a more scientific way. This was an important step toward understanding the natural world and using this knowledge to modify it to advantage. Then, as astronomical knowledge increased (helped by, and also helping, progress in other areas of study) our comprehension and mastery of nature also increased. This knowledge eventually led to the huge body of learning and the sophisticated technology of our modern world. Despite these prodigious advances, however, we still have much to discover about the heavenly bodies and the phenomena that can be observed in the universe.
Imagine what our way of life would have been like if we had been unable to see the sky because it was permanently overcast with clouds. There would probably have been no way of telling the time, for which the sun is invaluable, nor are we likely to have developed calendars. Calendars were essential for determining the best time to plant crops, for hunting and moving animal herds, and for organizing community life. Similarly, the development of navigation notably the concepts of latitude and longitude and the exploration of the earth would, at the very least, have been greatly retarded. Moreover, these are only the more obvious of the probable results of a world without astronomical knowledge; the full implications are much more complex and far-reaching. Nevertheless, it is probably a justifiable simplification to postulate that human beings would not have developed far beyond the stage of the Paleolithic cave dwellers, depending for food on what they could hunt or find growing naturally, if they had not studied the heavens and made practical use of the knowledge gained from them.
The ancient astronomers of prehistoric megalithic cultures had a surprisingly good understanding of the motions of the celestial bodies and of practical geometry. Evidence that they possessed these skills comes from the mysterious groups of large standing stones, or megaliths (some weighing more than 25 tons), arranged in regular geometric patterns that are found in many parts of the world. Some of these stone circles found in Europe (Stonehenge in Britain and Carnac in France, for example) were so arranged that they mark the rising and setting of the sun and moon at specific times throughout the year. In particular, they mark the eight extreme positions of the moon during its changes in declination in the 29.5-day cycle from one full moon to the next.
Stonehenge is probably the best known and finest example of the many prehistoric stone circle “observatories.” It was constructed in several phases over a period of approximately 600 years, between about 2200 B.C. and 1600 B.C., and most of the individual stones were sited in relation to the sun and moon rather than the positions of the stars. This plan was probably adopted because the declination and positions of the stars in the sky change gradually and unpredictably an effect that would be noticeable over the many centuries it took to build Stonehenge. Whereas, the declination of the sun and moon change in predictable cycles. Stonehenge was built on the 51 st degree of latitude and took account of the fact that the angle between the point at which the sun rises above the horizon at the summer solstice (approximately June 21, when the sun reaches its greatest declination) and the southernmost point at which the moon rises is a right angle. Divided into 56 segments, the surrounding circle could, therefore, be used to determine the position of the moon throughout the year. It could also be used to ascertain the dates of midsummer and midwinter and to predict the occurrence of solar eclipses.
Stone circles, therefore, provided prehistoric people with a comparatively reliable calendar—an essential requirement when they settled into organized agricultural and hunting communities after the last Ice Age, in about 10,000 B.C But although early people gradually learned to use the heavens as a means of regulating life, they also stood in awe of the celestial bodies. They regarded them as dwelling places or even as manifestations of powerful gods who controlled them and all worldly events. Thus began one aspect of the study of the heavens, now known as astrology, which paid particular attention to the relationship between the movements of stars and planets and the supposed influence of these upon human affairs, an interest maintained today.
The pyramids at Giza are sited about 6 miles (10 kilometers) west of the Nile River and about 9 miles (15 kilometers) southwest of Cairo. Dating from between about 2600 B.C and 2500 B.C, they are still among the largest buildings ever erected. Square in transverse cross-section, almost all of the many Egyptian pyramids were carefully positioned with their faces aligned north-south and east-west Other structural features of the pyramids also have astronomical significance. For example, the entrance passage of the Khufu (Cheops) pyramid pointed directly toward Alpha Draconis, the star closest to the north celestial pole at the time the pyramid was built.