Sanford Center for Sky Awareness
Observing Solar Cycles Using Sundials
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.
Equatorial Sundial Gnomon Length Calculator
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.
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 2002-2012 Walter Sanford. All rights
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