BC Gnomonics
The crescent dial shares its mechanical identity with the equatorial ring dial, though reconfigured to the point of tenuous resemblance.
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The dial features two nodi (the tips of the crescent), each with its own partial equatorial ring. During the morning hours, the eastern ring receives the nodus' shadow which proceeds down its slope throughout the day. During the afternoon, the western ring receives the shadow, which climbs up the slope until the sun sets.
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At its core, the crescent dial utilizes a method of self-orientation identical to that of the equatorial ring dial. The crescent is moved along its date scale; by offsetting the crescent's tip from the center of the equatorial ring, matching the degree of the sun's declination above or below the equator, the dial displays the time only when it is properly oriented.
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In addition to the tabletop version, there also exists a portable crescent dial. This model is effectively half of the stationary version, with one partial equatorial ring carrying the full range of hours. Because of its condensed nature, the user must determine which of the two coinciding hours is correct when the dial is oriented.
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Produced as part of the 1987 Franklin Mint collection "Great Instruments of Discovery;" available through various resellers
How to use
1. Unfold all elements of the dial, making sure to set the latitude scale 90º to the base of the dial and the date scale 90º to partial equatorial rings.
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2. Incline the dial to the latitude of use using the latitude scale.
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3. Slide the crescent along the declination scale until it sits at the date of use (make sure the crescent is symmetrically positioned and forming a "Ï®" on this scale).
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4. Rotate the dial until the shadow of the crescent's tip falls upon the center of the equatorial ring. (Note that there are two points where this beam can be seen: one being the correct time and the other being the hour of corresponding solar altitude mirrored across noon. The user must thus be aware of either the vague direction of true north or whether it is currently before or after noon). During the morning hours, this shadow will fall on the eastern side of the dial. During the afternoon, the western side.
4b. To orient the dial without using the sun:
Using a compass, orient the dial so that the inclined surface's hinge faces 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).
5. Read the hour as indicated by the crescent's tip. 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.