A monument of giant stones stands at the edge of lonely Salisbury Plain in southern England. Some of the stones stand alone, some stand in pairs capped by a third slab making each pair into a great archway, some tilt to one side, and some lie on the ground. Encircling the stones at a distance is a ring of chalk-filled holes. Beyond these holes are a ditch and a bank of earth. Outside an entrance that breaks the ditch and bank stands a solitary large stone about sixteen feet high, called the heel stone.
            The age of Stonehenge has been placed at 3,800 years, or 1850 B.C. The stones must have been moved during those ancient times to where they now stand because there are no similar stones nearby. Without modern machines the ancient builders of Stonehenge must have had great difficulty in moving the stones. The heaviest stone weights fifty tons.
            Many people have tried to solve the mystery of Stonehenge. Part of the mystery – the origins of the stones used at Stonehenge – has been solved. There are two kinds of stones at Stonehenge. One kind is very hard and has a dark bluish tint, while the other kind is gritty and has a dark reddish color. Igneous rocks similar to the dark bluish rocks are found in Wales, 240 miles away from Stonehenge. The reddish rocks are sandstone and sometimes called sarsen stones. The word sarsen comes from Saracen. The Saracens were Moslems who invaded much of the Christian world in Asia, Africa, and Europe during the 7th & 8th centuries. Large masses of sandstone rocks resembling monuments erected by the Saracens are scattered over the countryside within twenty miles of Stonehenge. Englishmen of the Middle Ages called these sandstone rocks "Saracen stones" because they looked so much like monuments erected by the Saracens. Through the centuries the term Saracen stone has been shortened to sarsen stone.
            This hypothesis about the meaning of the monument doesn't really explain all the facts. Why were the stones grouped the way they were? They seemed to be placed with some regularity. What was the shceme that determined distances between them?
            An American astronomer, Dr. G. S. Hawkins, had a theory. It had long been observed that the stone called the heel stone outside the entrance of Stonehenge was so placed that an observer looking through the archway at the entrance would see the sun rising exactly over the heel stone on the longest day of the year, usually June 21. Dr. Hawkins believed that the positions of the other stones had some astronomical significance. He observed that the archways are only about twelve inches wide, and that each archway lines up with a bordering archway, thus controlling the direction of observation as if together they formed a sighting instrument. A sighting instrument aids the eye in aiming. For example, if you want to see whether or not a row of fence posts is in line, you bend down and look with one eye along the top of the first post. You can then see how well the other fence posts line up. The paired archways at Stonehenge served to aim the eye toward a certain part of the sky. Why was the direction of viewing so important to the early men of Stonehenge?
            Could Stonehenge have been built as a kind of observatory for viewing the sun, moon, planets, and stars? Suppose you lived back in 1850 B.C. Which heavenly events would be important to you? You would have had to know the best time for planting, harvesting, hunting, fishing, and other activities. Remember, you would not have had our modern calendar to mark off the seasons of the year. A brief span of warm weather would not necessarily mean that the growing season had begun. Sometimes warm January days will be followed by months of wintry weather in parts of England. You would want to know when the days began to lengthen and when the longest day of the year occurred so that you could be sure that the frosts were over. It would also be helpful to know when to expect the shortest day of the year, so that you'd know that winter weather was due.
            Data about the sun's apparent movements would also have been helpful to Stonehenge men. We say that the sun rises in the east and sets in the west, but only on two days a year are the directions exactly east and west – on or near March 21 and September 21. the points on the horizon at which the sun rises and sets change a little every day, as the earth revolves around the sun. In most of the United States, from March 21  sunrises gradually shift to the north, so that by June 21 the sun rises from the northeast. Then it shifts again, a little southward each day, back to due east by September 21, and continues until it rises from the southeast by December 21.

The Path of the Sun

            The ancient builders of Stonehenge placed the heel stone so that it marked the spot where the sun rises on the longest day of the year. This day is called the summer solstice. Could there be other stones to mark other important points in the journey of the sun? And could some stones line up with the travels of the moon, planets, or stars?
            Dr. Hawkins wanted to find out on which heavenly objects each pair of sighting points was aligned. He used a computer to find the answers, for the job would have taken too much time to do by ordinary figuring. The computer was programmed with data on the location of the sighting points (latitude and longitude). Then it was instructed to act as if it were standing at each of the sighting points, to sight across each of the other pairs of points to the horizon, and to report what it saw in the sky.
            In less than a minute the computer determined that some of the stones were placed to align at sunrise on the longest day of the year, while another alignment pointed out sunrise on the shortest day. There were also alignments that pointed out sunset on both these days. The high and low points in the winter and summer orbits of the moon are located by other alignments of stones. Positions of the planets and stars were not indicated by the stones.
            Continuing his decoding of Stonehenge, Dr. Hawkins discovered that the positions of the stones and holes made it possible to make certain predictions. For example, the stones could be used to keep track of when the sun, moon, and earth were in line. Thus, Stonehenge could be used as a computing machine to predict eclipses. Although we do not know for certain that Stonehenge was used as an astronomical observatory, Dr. Hawkins' belief that it was is the best explanation to date.
            Man's ingenuity and patient observation made possible the creation of Stonehenge. Even in the Stone Age, man made careful observations of the movement of the sun and moon and used these observations to make predictions.
            Astronomy, perhaps the earliest science in man's history, advanced as men learned to make their observations of the sky more exact. Before these observations could be made precise, a plan for mapping the heavens had to be devised. Such a map would indicate the location of each object in the heavens regardless of the position of an observer on the earth. We do not know in what detail Stonehenge man was able to map the sky, but as early as 120 B.C., the Greek astronomer Hipparchus described stars at almost the same locations we find them in today.