Illustration depicting star-rise over Stonehenge, the megalithic stone circle on Salisbury Plain, Wiltshire, England. Construction of this prehistoric temple began over 5000 years ago although the largest stones, some of which weigh over 40 tonnes & stand nearly 7 metres high, were erected around 2000 years BC. It is thought that the alignment of the stones was used to make predictions about certain astronomical events such as soltices and lunar & solar eclipses. For instance, on the longest day of the year (the summer solstice) an observer at the centre of the circle sees the sun rise directly over a distant pillar called the heel stone.