An instrument for measuring the zenith distances, and/or times of meridian transit, of stars close to the zenith. The telescope is mounted so that it points vertically, and so does not distort under its own weight, as it would if it were not vertical. In addition, the effects of atmospheric refraction are smallest at the zenith. Hence star positions can be measured at the zenith with maximum accuracy.
Subjects: Astronomy and Astrophysics.