Nearly 5,000 years ago, during the Late Stone Age, people began building Stonehenge. It became a long-term project. People added to the site for more than 1,500 years. Most likely Stonehenge was first built as a place to hold religious rituals (仪式) and bury the dead. But experts (家) are not sure because there are no written records telling us what took place at Stonehenge. A "trilithon" (line 6, para. 3 )described in the text would most probably look like

At first the site was just a circular ditch (沟渠). Within the circle were two burial hills, and a smaller circle of 56 holes. Scientists have found ashes in the holes. They believed these are from the burning of the dead bodies.

But Stonehenge may have been more than a ritual site. Some experts suggest it was also used as a huge calendar (日历) for watching stars. Outside the entrance of the construction stands the Heel Stone. On the summer solstice, the longest day of the year, a person standing at the center of the circle can look out through the entrance and see the first rays of the rising sun hit the top of the Heel Stone. At the winter solstice, the year’s shortest day, a person standing at the entrance can look into the circle and see the setting sun between the stones of a trilithon, formed by two standing stone posts topped by a stone in flat position. In this way Stonehenge may have marked the passing seasons.

The position of the stones and holes shows that Stonehenge may also have been used for watching the moon and stars. The construction may even have been a huge prehistoric" computer" used to study other natural objects in space. The Stone Age people in the area were farmers. Perhaps they used Stonehenge to figure out changes in the weather and to time the planting of their crops.