Sandburg Center for Sky Awareness
A Fairfax County Public Schools Planetarium

Equatorial Sundial Gnomon Length Calculator

An equatorial sundial consists of a dial plate, and a gnomon (or style) that is perpendicular to the dial plate. The dial plate has an upper dial face and a lower dial face, both of which are marked off in hours. The gnomon is a pole (or rod) that passes through the center of the dial plate, extending above the upper face of the dial plate, and extending below the lower dial face.

Use the following rule-of-thumb to determine the length of the upper segment of the gnomon: "If the shadow of the gnomon is to reach the circle of hour numbers at the times of the solstices, the gnomon must be about half as long as the radius of that circle. (The theoretical value is 0.44 times the radius.)" Quote courtesy Sundials: Their Theory and Construction, Albert E. Waugh, Dover Publications, Inc., ©1973, p. 32.

Input the radius of the dial plate (one-half the diameter) and your latitude to calculate the length of the lower segment of the gnomon required to properly orient your equatorial sundial.

Note: Input fractions as decimals. For example, input 4 ½ as 4.50.
Radius of dial plate (including height of "foot"): (unit-independent)
Latitude: (0-90 degrees)

The gnomon should extend units from the center of the lower dial face to a horizontal surface.

For 0° latitude (the Equator), the gnomon lower segment is theoretically infinite in length.
Note that the computer is unable to display infinity (¥) properly.

© Copyright 2002-2009 Walter Sanford. All rights reserved.

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