What do you call an old instrument which was used to measure altitude of stars and planets?

The sextant basically consists of a telescope, a half silvered horizontal mirror which the telescope "looks" through and a moving arm on which the index mirror is fixed. By manipulating this arm a star or other celestial body can be made to appear on the horizon. Accurate adjustments are made by means of a micrometer knob. The angle can then be read off the arc and micrometer. The shades are to use when the object being looked at is bright - such as the sun.

The ancient Greeks learned to make precise instruments, crowned by the ingenious astrolabe. This hand-held device had a moveable arm to measure the angle of a bright star above the horizon — the star's "altitude." Rotating a metal map of stars to match engraved curves, the user could determine time and direction, locate stars in the sky, determine when the sun would rise or set, and make other calculations. Astrolabes reached their standard configuration by the fourth century, having been first developed in the first or second century near the Egyptian city of Alexandria.

Astrolabes are used to show how the sky looks at a specific place at a given time. This is done by drawing the sky on the face of the astrolabe and marking it so positions in the sky are easy to find.