How it began
Pedro Nunes, known in Latin as Petrus Nonius, was born in Alcácer do Sal in 1502 and became royal cosmographer of Portugal in 1529. He taught mathematics at Lisbon and Coimbra. From his astronomical research he invented the nonius, a way of taking finer measurements on circular instruments such as the astrolabe. It dates to 1542, the year he published De crepusculis, a book on the length of twilight written in answer to a question from his pupil Prince Henrique.
Before it existed
A navigator found latitude by measuring the height of the sun or a star with an astrolabe or quadrant. The scale on such an instrument was marked in whole degrees, and a reading that fell between two marks had to be judged by eye. Nunes had shown Prince Henrique, and in 1533 the king, how to establish the latitude of a point from the height of the sun.
How it works
The nonius is a set of concentric arcs drawn on the instrument. The outermost covers 90 degrees in 90 equal divisions, the next has 89 divisions, the next 88, and so on for some 44 or 45 arcs. When an angle is measured, the observer notes on which arc the sighting rule falls exactly on a division line, and which line it is. A table then gives the precise value of the angle, including the fraction of a degree.
How it spread
The astronomer Tycho Brahe applied the nonius to the astronomical quadrant. A study published by Brill notes that the nonius scale was never widely adopted. In 1631 the French mathematician Pierre Vernier adapted the idea into the vernier scale, which did come into general use.
Where it stands today
In some languages the vernier scale is still called a nonius, after the Latin name of Pedro Nunes. The principle also survives in machinery: some absolute encoders in numerically controlled machines use nonius interpolation to measure linear or rotary movement.
