It is the sextant or some member of the sextant family-such as the quadrant, octant, etc.
"Lectures in Navigation"
Ernest Gallaudet Draper
Latitude was easy to determine using an octant and later a sextant with mirrors and a small telescope to measure the angle between a celestial body such as the sun or north star and the horizon.
"Our Legal Heritage, 4th Ed."
S. A. Reilly
I made him a present of an Hadley's octant; and though, probably, it was the first he had ever seen, he made himself acquainted, in a very short time, with most of the uses to which that instrument can be applied.
"A General History and Collection of Voyages and Travels, Volume 16"
Robert Kerr