MOVABLE ASTRONOMICAL QUADRANTS
In the XVIIth and XVIIIth centuries movable quadrants were part of the basic equipment observatories were expected to have. With their help and using the "method of corresponding altitudes" it was possible to work out at what moment, read on a clock with regular operation, a star had passed across the meridian.
The procedure consisted of working out the times when a given star had the same elevation on the horizon, at its rising and its setting. The mean of these times was the moment when the star passed across the meridian. It was thus possible to identify the south and adjust meridians and mural instruments.