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Sundial Functions
The dial face is most easily read if you stand on its south side and face north. You read the time just like an ordinary clock, with the gnomon's shadow serving as the clock hand, rotating clockwise about the center. The shadow should be read from a mentally drawn line that runs down the center of the shadow. Then estimate the number of minutes between this line and the adjacent minute line on the face. "Guesstimating" the exact time with precision requires some practice. A well-done time reading should give you correct Standard Mean Time to within 2 to 4 minutes on my 25" dial, or from 30 seconds to one minute on a large 40" dial. (Correcting for the Equation of Time)
All my sundials do much more than just tell time! All sundial functions use the sun's position in the sky and the shadow of some attached object to point to markings on the dial face. Some dials, like the heliochronometers, perform a great number of functions. Most types though, indicate the times and the directions of sunrises and sunsets on the equinoxes and solstices.
Some dials have a vertical brass pointer in the center of a compass rosette. In addition to holding the gnomon cable in place, the rosette also indicates the sun's direction (azimuth) and height (altitude).
Often I engrave a 360-degree horizon (azimuth) scale around the circumference of the dial for use as an accurate, giant compass. Your dial's face will be much more accurate because it is so much larger. Using it, one can take precise compass bearings of any object on or near the horizon.
If a sundial has a nodus located on its cable gnomon, then its shadow can tell the height of the sun above or below the celestial equator (declination). This varies between a value of 0° on the equinoxes and +/- 23.5° on the solstices. Since the solar declination changes with the date, the declination lines on the face can also show the date!
A heliochronometer is a sundial that shows Standard Time accurately and directly. The shadow of its nodus can show Standard (watch) Time without corrections since it has the curious, figure-eight analemmas drawn into its face. I place little round circles (beads) on the analemmas so that they look rather like a twisted strand of pearls. These beads represent the dates every five days, which means that the analemmas can also show the date.
My 25-inch and 40-inch sundials usually don't have a nodus, declination and date lines, or analemmas, but my heliochronometers do. However, you can still estimate the date on other sundials by the position of the vertical brass finial's shadow at the moment of sunrise and sunset.
To see detailed drawings showing sundial functions, click on the sundial type:
• Analemmatic Sundial Functions
• Polar Sundial Functions
• Horizontal Sundial Functions
• Round Heliochronomter Functions
• Rectangular Heliochronometer Functions
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