Stonehenge is a megalithic monument on the Salisbury Plain in Southern England, composed mainly of thirty upright stones (sarsens, each over ten feet tall and weighing 26 tons), aligned in a circle, with thirty lintels (6 tons each) perched horizontally atop the sarsens in a continuous circle. There is also an inner circle composed of similar stones, also constructed in post-and-lintel fashion.Every year on the first day of summer, the Sun rises at a point that is farther north than on any other day of the year. At the ruins of Stonehenge in England, this solstice sunrise appears on the horizon in direct alignment with the massive heel stone. This is the most outstanding feature of this ancient monument, built during the same era as the Great Pyramid of Egypt. There is now a general consensus that the builders of Stonehenge used it to mark this special day as the beginning of each year. By counting the number of days between these annual alignments, they could determine the length of the year. This could serve as a practical calendar to mark holidays and seasonal festivals and to ensure the timely planting and harvesting of crops.
But to predict eclipses, knowledge of two other cycles is required. One of these - the length of the lunar month - is easily determined. It is simply the number of days between one full Moon and the next. This cycle of 29-1/2 days is marked at Stonehenge by two rings of 29 and 30 holes, which together average 29-1/2. The other cycle, however, is of an altogether different character: it is a cycle of rotation of two invisible points in space. The evidence shows that the builders of Stonehenge probably discovered this cycle and could have used it to predict eclipses.
These two invisible points in space are called the lunar nodes (from the Latin for "knot"). They are the points where the Moon's orbit, which is tilted at a slight angle, intersects the plane of the Earth's orbit. It would have taken many decades of watching countless risings and settings of the Moon to figure out the cycle of the lunar nodes. This information - which must have been passed on from generation to generation - is preserved at Stonehenge. All the Moon alignments necessary for determining this cycle are marked by massive stones.
Who were these people who observed this subtle cycle even before the first metal tools were used by humankind? Some have suggested that Stonehenge was built by Druids, but we don't really know much about the builders. We do know that the actual motions of the Sun and the Moon are reflected in the structure of Stonehenge, and we can reason how it may have been used to keep track of these cycles. The number of stones or holes in the ground in the various rings around Stonehenge each represents a certain number of days or years in the cycles. By moving markers (such as stones) around a ring in time with the cycles, the positions of the Sun and Moon -- and the two invisible points -- can be tracked.
An eclipse can occur only when the Sun is close to being aligned with a node. By using Stonehenge to keep track of the position of the Sun and the nodes, these "danger periods" for eclipses can be predicted. A new (or full) Moon appearing during one of these periods would call for a special vigil to see if the solar (or lunar) eclipse would be visible from Stonehenge. A total solar eclipse would be a rarity. But the law of averages confirms that either a partial solar eclipse or a lunar eclipse can be seen (weather permitting) from the same point on the Earth about once every year.
Why would eclipses have been so important to the ancient people of Stonehenge? Perhaps they considered the darkening of the Sun or the Moon a fearsome event - a celestial omen of doom or disaster. Many cultures have interpreted eclipses this way. But the sophistication of the astronomy of Stonehenge suggests that the builders had something different in mind. Their understanding of the solar and lunar cycles must have led to a high regard for the cosmic order. Eclipses may have been seen as affirmations of the regularity of these cycles. Or perhaps the unseen lunar nodes formed an element of their religion as invisible gods capable of eclipsing the brightest objects in the heavens.
The idea that Stonehenge may have been a center for some kind of worship has occurred to many. It is not hard to imagine Stone Age people gathering at a "sacred place" at "sacred times" (such as solstices, equinoxes, and eclipses) to reaffirm their religious beliefs through ritual practices. British antiquarian Dr. William Stukeley, who in 1740 was the first to note the summer solstice alignment at Stonehenge, advanced the notion that the monument was built by Druids to worship the serpent. He claimed that Stonehenge and similar stone circles had been serpent temples, which he called "Dracontia." Could this serpent symbolism be related to eclipses? Recall that the key to eclipses is the position of the lunar nodes. The length of time for the Moon to return to a node (about 27.2 days) astronomers call the draconic month. (Draco is the Latin word for "serpent" or "dragon.) Perhaps the mythical serpents of Stonehenge and the legendary dragon that eats the Sun are symbols of the same thing: the invisible presence in time and space that eclipses the Sun and the Moon.
