Sextant (astronomy)

They are of significant historical importance, but have been replaced over time by transit telescopes, other astrometry techniques, and satellites such as Hipparcos.

[1] Ulugh Beg constructed a Fakhri Sextant that had a radius of 40.4 meters, the largest instrument of its type in the 15th century.

Housed in the Ulugh Beg Observatory, the sextant had a finely constructed arc with a staircase on either side to provide access for the assistants who performed the measurements.

The navigator's sextant uses mirrors to bring the image of the sun, moon or a star to the horizon and measure the altitude of the object.

The frame is heavy enough to be stiff and provide reliable measures without flexural changes in the instrument compromising the quality of the observation.

In that case, the observer moved the alidade until the object of interest is centered in the sights and then reads the graduations marked on the arc.

This could be a challenge for a moving star observed with a very large instrument as a single person might not be able to confirm both sights with ease; an assistant was a great benefit.