Circummeridian
The circummeridian method of astronomy and astrometry is a method for determining the local sidereal time or the geographical longitude . The exact north-south direction is also obtained as a by-product. To do this, one observes a star at the same altitude before and after its highest point, i.e. in the south-south-east and south-south-west.
By averaging the two time and angle measurements, one obtains the moment of passage of the meridian and the direction of the meridian (south). By comparing the transit time with a time signal, it is possible to determine the geographical length .
procedure
The measurement can be carried out with a universal astronomical instrument, a surveying theodolite or (with less accuracy requirements ) with a sextant . You wait until the star crosses the horizontal thread of the instrument, and at that moment you stop the time. If you want the exact south direction in addition to the time , the respective horizontal angle can also be read off.
The star height must be the same on both sides of the meridian (e.g. by keeping the telescope clamped), but its exact value is not required. If, on the other hand, the elevation angle or the zenith distance is read off on a precise pitch circle , a latitude can also be determined if the declination of the star is known .