Reflecting instrument
![Representative drawing of Hooke's reflecting instrument. It does not accurately depict the fine details of the instrument but rather the basic functionality.The index with telescope mounted is shown in black, the radius arm with the mirror (grey) attached in blue and the chord in green on white. The lines of sight are represented by the red dashed line.](/Images/godic/202502/06/Hooke's_reflecting_instrument2934.jpg")
![A drawing of Halley's reflecting instrument. The telescope is represented by the blue lines (as if cut open) and the mirrors and lenses are grey. The red dashed lines represent lines of sight.](/Images/godic/202502/06/Halley's-instrument2935.png")
![A sextant that has been used for over a half century. This frame shows one standard design – that with three rings. This is one design that has been used to avoid thermal expansion problems while retaining adequate stiffness.](/Images/godic/202502/06/Experienced_sextant2935.jpg")
![Borda's reflecting circle, on display at Toulon naval museum](/Images/godic/202502/06/Cercle_de_reflexion_img_26002935.jpg")
Reflecting instruments are those that use mirrors to enhance their ability to make measurements. In particular, the use of mirrors permits one to observe two objects simultaneously while measuring the angular distance between the objects. While they are used in many professions, they are primarily associated with celestial navigation, as the need to solve navigation problems, in particular the problem of the longitude, was the primary motivation in their development.