In astronomy, atransit instrument is a smalltelescope with an extremely precisely graduatedmount used for the precise observation ofstar positions. They were previously widely used inastronomical observatories andnaval observatories to measure star positions in order to compilenautical almanacs for use by mariners forcelestial navigation, and observestar transits to set extremely accurate clocks (astronomical regulators) which were used to setmarine chronometers carried on ships to determinelongitude, and as primary time standards before atomic clocks. The instruments can be divided into three groups: meridian, zenith, and universal instruments.
For observation ofstar transits in the exact direction of South or North:
Allow transit measurements inany direction
Depending on the type of instrument, the measurements are carried out
The accuracy reaches from 0.2" (theodolites, small astrolabes) to 0.01" (modern meridian circles, Danjon). Early instruments (like the mural quadrants ofTycho Brahe) hadno telescope and were limited to about 0.01°.