BC Gnomonics
Equatorial sundials can take many shapes, but always feature an equatorial disc or band inclined to a degree matching the latitude of use, with a gnomon or nodus assembly perpendicular to the equator, paralleling the celestial axis.
Portable models (the Augsburg type being the most common) typically share the commonalities of a circular, ring-shaped hour plate, and a hinge with which to set the latitude. The latitude scale typically ranges from 0-90 degrees, allowing the dial to be used anywhere in its intended hemisphere. Some dials feature alternate forms of latitude selection, though all result in the equatorial ring being set parallel to the celestial equator.
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The form of the dial itself is often circular or rectangular, typically able to fold relatively flat while not in use.
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As opposed to stationary or monumental equatorial dials, the hour plates of portable variants are almost exclusively rings rather than solid disks. This ensures that the hour can be read on the top face regardless of the sun's declination. While always rings, the surface upon which the hour is read may vary. Some dials feature an inner hour scale with a surface parallel to the gnomon, while others use a flat ring with hours marked on the top.
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Occasionally, dials of this type will feature gazetteers–lists of major cities and their associated latitudes.
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These dials feature imbedded magnetic compasses to aid in orientation. To achieve truly precise orientation, the dial must face true north, accounting for magnetic declination. In addition to a compass, some portable equatorial dials feature a plumb bob assembly to aid in keeping the dial level.
British Sundial Society
The Sundial Primer
Helmut Sonderegger
Nicolas Bion
How to use
1. Set or unfold the gnomon so that it is perpendicular to the plane of the equator (often also the plane of the hour plate).
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2. Set the dial for the latitude of use. Often, the latitude scales on equatorial sundials take the form of curved plates which the hour plate is slid against. Sometimes, the dial will include a list of cities with corresponding latitudes.
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3. Ensure that the dial is on a level surface. If included, a plumb bob can aid in this step.
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4. Use the included compass to orient the dial true north. If magnetic declination is not known, it can be found here (the given value is the difference between magnetic and true north. If magnetic declination is 15ºW, rotate 15ºE of magnetic north to point true north).
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5. Read the position of the shadow on the hour plate. This measures local apparent time. To determine civil time, add the appropriate longitude correction and equation of time correction (more information here). If applicable, add one hour during daylight saving time.
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