The henge surrounds a large stand-stone known as the Altar Stone. The Altar Stone sits in the middle of a horseshoe of smaller stones called bluestones, which came from Pembrokeshire in the north of Wales, about 240 miles away. Although these bluestones are in a horseshoe shape around the altar stone now, archeologists think that they were probably arranged in a full circle at first and moved later. The bluestones each weigh about six tons, and there were once 60 or more of them at Stonehenge.
Outside the horseshoe of bluestones, there is a second horseshoe of bigger stones, called sarsen stones. The sarsen stones came from 20 miles away, and weigh anywhere from 20 to 50 tons. The opening of the two horseshoes faces Stonehenge's entrance, to the northeast.
The outer-most ring of Stonehenge is a complete circle of smaller sarsen stones with another large stone, the Heel stone, marking the entrance.
At the Summer Solstice, the longest day of the year, the sunrise aligns perfectly with the entrance and the Heel Stone, and at the Winter Solstice, the shortest day of the year, the sun sets right behind the Altar Stone. This leads archeologists to believe that the henge was built for religious purposes.