In astronomy, a transit instrument is a small telescope with an extremely precisely graduated mount used for the precise observation of star positions. They were previously widely used in astronomical observatories and naval observatories to measure star positions in order to compile nautical almanacs for use by mariners for celestial navigation, and observe star transits to set extremely accurate clocks (astronomical regulators) which were used to set marine chronometers carried on ships to determine longitude, and as primary time standards before atomic clocks. The instruments can be divided into three groups: meridian, zenith, and universal instruments.

For observation of star transits in the exact direction of South or North:

Allow transit measurements in any 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 of Tycho Brahe) had no telescope and were limited to about 0.01°.