An astrolabe is an astronomical instrument which measures the height of the stars above the horizon, which then enables us to roughly tell the time.
It is done by drawing the sky on the face of the astrolabe and marking it, making it easy to see the positions in the sky.
To use an astrolabe, you adjust the moveable components to a specific date and time. Then once it is set, much of the sky, both visible and invisible, is represented on the face of the instrument.
It is not known when it was invented but it widely used in the 6th and 7th century by Arab pilots at sea and also used by European navigators.
A typicalyl old astrolabe was made of brass. It measured about 6 inches in diameter, although much larger and smaller ones were made.