Nonius (device)

Nonius is a measuring tool used in navigation and astronomy named in honour of its inventor, Pedro Nunes (Latin: Petrus Nonius),[1] a Portuguese author, mathematician and navigator.

The nonius was created in 1542 as a system for taking finer measurements on circular instruments such as the astrolabe.

On a standard scale of 90 degrees, there are an additional 44–45 concentric circles, with each divided into a specific unit size such that a scale unit on position

When an angle was measured, the circle and the division on which the alidade fell was noted.

[5][3] In numerically controlled machines, the nonius is part of several absolute encoders, that measure linear or rotational displacements.