Rectangulus

Dissatisfied with the limitations of existing astrolabes, Richard developed the rectangulus as an instrument for spherical trigonometry and to measure the angles between planets and other astronomical bodies.

[1][2] This was one of a number of instruments he created, including the Albion, a form of equatorium, and a famously complicated and expensive horologium (astronomical clock).

His Tractus Rectanguli, describing the rectangulus, was an influential text in medieval astronomy and at least thirty copies were known to survive.

[4] This was a series of nested angular scales, so that measurements in azimuth and elevation could be made directly in polar coordinates, relative to the ecliptic.

Pinhole sights on the upper arm allowed it to be pointed accurately at the astronomical target.