Calculating the date of Easter was part of the stormy religious politics that dominated Europe in the 16th17th centuries. To find the right date - the basis of the entire Christian calendar - it was necessary to know the date of the spring equinox, the moment in late March when day and night were of equal length. In the 16th century, Catholic mathematicians turned cathedrals into vast astronomical observatories by driving a small hole high in the south wall of the building. Then they laid a long metal line, called the “Meridian Line” pointing north-south on the cathedral floor, along which the Sun’s image at local noon, cast through the hole, would shift day after day. The moment of equinox could be marked on this “meridian line”, and the date of Easter calculated.

For meridian lines in churches: http:partners.nytimes.comlibrarynationalscience101999sci-astronomy-cat hedrals.1.GIF.html

For an introductory article: http:cis.alma.unibo.itNewsLetter090496NwHeilbron.htm