If you want to know how the ancients did it, and how it might apply to the zodiac at Rennes!
A unique configuration of the stars and landscape: during the years around 1,380BC, the when the constellation of Crux, or the Southern Cross, crossed the meridian, the top and bottom stars, Alpha and Gamma Crucis, transited at the same moment. In other words, Crux crossed the meridian "upright". This facilitated the accurate determination of the due south bearing, with the use of a simple plumb-bob. From a practical point of view, the latitude of 43°N was the ideal location from which to exploit this unique moment in the precessional movement of the vault of the heavens. The video shows the view looking south, in 1,380BC over Pech Cardou and along the meridian of 8 peaks further to the south, (on longitude 2°19'40"E ) as Crux moves across the southern sky. Notice how it crosses the meridian upright, with the lower star about 2° above the horizon.
Video made with Stellarium.
By Simon Miles.
Regarding his fabulous as well as astonishing new book about the Affair de Rennes - The Map and the Manuscript.