The Henry Moore Sundial Sculpture is located at Sundial Plaza alongside the Adler Planetarium & Astronomy Museum in Chicago, Illinois USA. (41° 52.006′ N latitude, 87° 36.426′ W longitude.)
Bowstring Equatorial Sundial
This type of sundial is a variety of the classic equatorial sundial, often referred to as a “bowstring equatorial sundial” because it looks somewhat similar to the “bow” in “bow & arrow.”
Remember, an equatorial sundial is a reduced model of the Earth. (More about that in a follow-up blog post.) A bowstring equatorial sundial is a “reduced equatorial sundial” — think of it as a kind of skeleton equatorial sundial.
The large red circle shown in the following annotated photo represents the Earth.
The dial plate (representing the equatorial plane) has been reduced to a semi-circular band that represents approximately one-half of the Earth’s Equator, This part of the sundial is 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 (“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).
Setting the Sundial
Similar to setting the correct time on an analog clock or wristwatch (by moving the hands of the timepiece into proper position), properly orienting an equatorial sundial will move the gnomon (or style) shadow into position so that the time scale displays the correct time of day.
The gnomon should be parallel to the Earth’s axis, inclined at an angle equal to the latitude of the observer.
North American Sundial Society member Fritz Stumpges is shown using a “SmartTool” to measure the angle of inclination of the gnomon (“bowstring”).
The dial plate/equatorial band should be parallel to the plane of the Earth’s Equator (perpendicular to the gnomon), inclined at an angle equal to the complement of the observer’s latitude — the angle that when added to the angle of the observer’s latitude equals a right angle in measure (90°). This angle is also known as the colatitude.
Place the sundial on a horizontal surface; the top of the gnomon should point toward the Celestial North Pole (i.e., Polaris, the North Star). More simply, the sundial should be set so the 12 noon hour line is aligned with your local meridian.
Related Resources
Tech Tips
When I visited this sundial in 2005, I owned almost no photography gear. I used a one-time use film camera to shoot the photographs featured in this blog post. After I returned home, I had the film processed, digitized, and saved to a photo CD. The photo of the time scale/time band has a resolution of 3091 x 2048 pixels.
Copyright © 2023 Walter Sanford. All rights reserved.