The Quadrant was the first altitude-measuring instrument developed for use in celestial navigation, dating back to the 15th century. Its first recorded use at sea was by Diego Gomes in 1461.
How it works
Diagram of a Quadrant
It was a very simple device constructed of wood or metal in the form of a quarter circle with degree graduations along the arc. It had sights along one of the radial arms and a plumb-bob suspended from the right angle. It makes use of gravitational principles. The observer held the Quadrant with the arc straight down and looked up through the sights at the sun or star. When his sights were aligned, he simply held the plumb-line fast against the face of the Quadrant between his finger and thumb and read off the altitude from the scale. This instrument can only be used to find the altitude of a celestial body.
The Quadrant does not require the view of the horizon to find altitude unlike most other instruments used to find altitude.
On the other hand, in order to find the altitude of the Sun, a user must look at the Sun for quite some time, which can be quite damaging to the eyes. However, it did not prevent this simple instrument from being popular among the seafarers.