When we started surveying Megalithic sites in Britain we used a cloth tape, but after some years it was realized that it was necessary to be more precise and now we use a steel tape for the important sites. Similarly with astronomy; at first we considered it sufficiently accurate to work to ¼° but now we know that it is advisable to work always to within a minute of arc. This is because we have found out that the erectors of the stones sometimes used distant foresights, either artificial or natural, and by using these they could get an accuracy of better than a minute. The method of observing was for the operator to watch the edge of the celestial body grazing a foresight or running down the edge of a hill. He moved sideways until he got exact coincidence and then marked the position. This first came to our attention when we measured up the site at Ballochroy on the west side of Kintyre.