Star transit

A star transit is the passage of a star across the field of view of a telescope eyepiece.

Star transits (Reticle 2°, magnif. ~10, two stars passing the vertical thread). Timing accuracy may be ±0,5 s.

The precise observation of star transits is the basis of many methods in astronomy and in geodesy. The measurements can be done in different ways:

1. visually (mostly up to 1990): accuracy 0,1" to 2" (depending on the instrument); timing with digital clocks about 0,05–0,2 seconds
2. by CCD and other electro-optical sensors: as above, time often better
3. semi automatic instruments: Photography or "impersonal micrometer", ca. 2 times better than No.1
4. by Scanning methods: Astrometry satellites like Hipparcos about 0,01".