circumference - the length of the closed curve of a circle

angular unit - a unit of measurement for angles

arcdegree, degree - a measure for arcs and angles; "there are 360 degrees in a circle"

astrolabe - an early form of sextant

limb - the graduated arc that is attached to an instrument for measuring angles; "the limb of the sextant"

measuring device, measuring instrument, measuring system - instrument that shows the extent or amount or quantity or degree of something