(I)The prehistoric monument known as Stonehenge is situated on Salisbury Plain, north of Salisbury, England, and dates from the late Stone Age. (II)The monument itself consists of four concentric ranges of stones, (III) Grouped around the main structure are a number of barrows, some of which contain chips of a blue stone similar to that found in the concentric ranges. (IV) The purpose of this structure was a mystery for ages until an American astronomer determined that it could have been used to predict the positions of the sun and the moon, and thus serve as a sort of calendar. (V) The early Romans also used a calendar that was based on the moon, but the year in this calendar was 355 days long.