Stonehenge

        The best-known evidence that early humankind used the sky is Stonehenge, near Salisbury, England. Built between 3100 and 1550 BC, Stonehenge consists of an impressive array of megaliths and lintels, stones weighing up to 50 tons each, arranged as portals that are precisely aligned in relationship to the sun on an ancient summer solstice, which occurred on June 24. Many like to believe that Stonehenge was built by the Druids, or Celtic priests, who lived in the area. Today the summer solstice falls on June 20 or 21 . Over the millennia, solstices shift slightly forward in the calendar. Astronomers have accounted for the shift by observing that the Earth precesses slowly as it rotates on its axis, like a top wobbles as it spins. This precession makes one complete cycle every 26,000 years. This phenomenon causes the time of the solstices and equinoxes to change and also causes, over time, changes in the apparent location of a polestar, or North Star. Some scientists believe that early peoples were able to foretell eclipses of the sun and the moon by the positions of these celestial bodies in relation to the stone monument. The site may have served as an observatory where rituals or religious ceremonies took place on specific days of the year.

        There are at least 900 other structures of a similar nature that exist in the British Isles alone. At Carnac on the western coast of France and more than 3,000 stone monuments for which astronomical alignments have been claimed.