The Callanish Stones, on the west coast of Lewis in the Outer Hebrides of Scotland, were constructed between 2900 and 2600 BC. The site itself, however, has been occupied earlier and later than this. After the construction of the stones, a tomb was built into the site, in use between 2000 BC and 1700 BC. Some archaeologists believe that the stones were a prehistoric lunar observatory. 

The 13 primary stones form a circle about 13 meters in diameter, with a long approach avenue of stones to the north, and shorter stone rows to the east, south, and west. The layout recalls a distorted Celtic Cross. The individual stones vary from around 1 meter to 5 meters in height, with an average of 4 meters, and are of the local Lewisian gneiss, a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. It is often foliated.

Some archaeologists - not all - believe that the stones were a prehistoric lunar observatory. And the Callanish Stones site continues to provide new observations and clues of the prehistoric past.

In 2013 Hebridean archaeologist Ian McHardy made a discovery at Cnoc an Tursa - a rock formation beside the Callanish Stones site - which may be evidence of an ancient sun-powered calendar. Located just outside the main Callanish Stones circle and consisting of five large stones that create a small cave, Cnoc an Tursa was the subject of an excavation by Gerald Coles, Edinburgh University, in the 1990's. 

His research uncovered a number of pits and post holes stretching out in straight lines from the cave; yet no real explanation was found as to what they might have been. But recently, McHardy witnessed an interesting interaction between the sun and Cnoc an Tursa: at noon a shaft of light emanating out from the base of the cave, moving round as the sun travelled across the sky. Further studies revealed that the length of the light beam changed as the year progressed, getting shorter to longer from summer to winter; a large annual sundial.

Such a device would have been required for farming as well as for celebrating annual religious festivals. This idea ties in with the solar and lunar alignments in the main monument.

The Callanish site, which is seeking World Heritage Status, promises to reveal more insights into the prehistory of Scotland as archaeological investigations continue.

http://www.bradshawfoundation.com/british_isles_prehistory_archive/megaliths/index.php