Sandburg Center for Sky Awareness
A Fairfax County Public Schools Planetarium

The Henry Moore Sundial Sculpture
Sundial Plaza, Adler Planetarium & Astronomy Museum
Chicago, Illinois
41.8°N latitude, 87.6°W longitude

This type of sundial, a variety of the classic equatorial sundial, is often referred to as a "bowstring" equatorial sundial because parts of the sundial resemble a "bow and arrow." The dial plate (representing the equatorial plane) has been reduced to a semi-circular band representing approximately one-half of the Earth's Equator, referred to as the "equatorial band." The equatorial band serves as the "time scale." The gnomon (or style) is a thin rod (the "bowstring") representing the Earth's axis of rotation. The vertically-oriented "bow" represents a single meridian (line of longitude) and is referred to as the "meridian band." The bowstring is connected to the bow at the equivalent of the Earth's North and South Poles. The bow is attached to a pedestal or "foot" (ped- is Latin for foot).

The hour lines for the "bowstring" equatorial sundial are laid out on the inner surface of the equatorial band. Every hour is 1/24 of a day, or exactly 15 degrees wide. The shadow of the gnomon (or style), cast among the hour lines on the equatorial band, shows the time. The meridian band (the "bow") bisects the equatorial band ("time scale"); the 12 o'clock noon hour line is located on the time scale where the two bands intersect (since solar noon occurs when the Sun crosses the observer's meridian). Facing north, morning times are located on the left side of the equatorial band; afternoon times are on the right.

In contrast to the classic equatorial sundial, the primary advantage of the "bowstring" equatorial sundial is that the style shadow is cast upon a single time scale all year long (regardless of whether the Sun is north or south of the Celestial Equator). Unlike the classic equatorial sundial, the "bowstring" equatorial sundial should work on the day of the equinoxes (although the author has not witnessed this firsthand).

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© Copyright 2002-2012 Walter Sanford. All rights reserved.

Photographs courtesy Phillip S. Wherry.
Thumbnail images prepared using photoweb v1.3 by Phil Wherry.
Additional photographs courtesy Susan K. Henke, AMS AERA, Chicago, IL.


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