gnomon

Let a Gnomon, being firſt ſharpened into a Point, be ſhaped, and faſtned in ſuch wiſe, that it no way hinder either the draught of the Horizontal Line, or the Point of the ſhadow from having free access to the Dial at all times of the Year. […] Upon the Superfices of the Dial, obſerve the Point of the Shadow of the Gnomon (making a mark at it) and the Sun's Altitude, both of them at the ſame inſtant of time.

This muſt be performed by help of a Spherick Gnomon, (as Blaew [Willem Janszoon Blaeu] calls it,) which is a ſmall Pin or Needle fixed perpendicularly into a ſmal Baſis with an hollow concave bottom, that it may ſtand upon the convexity of the Globe.

Index of a globe, the little style or gnomon, which being fixed on the pole of the globe, and turning round with it, points out the hours upon the hour circle.

If a Parallelogram be divided into four leſſer ones, by two Lines interſecting each other; and one of theſe Parallelograms be retrench'd, or taken away; the other three will make a Gnomon, ordinarily call'd a Square.

https://books.google.com/books?id=R0d76m-Be10C&pg=PA16 page 16 The figurate number's gnomon is therefore 1+nb in the general case. A trivial family of numbers, those for which b=0, might be called linear. […] https://books.google.com/books?id=R0d76m-Be10C&pg=PA17 page 17 Clearly, a figurate number of rank n is equal to the sum of its first n gnomons, and in particular the sum of the first n odd numbers is equal to n².