BC Gnomonics
Mechanical Equatorial Dial / Mechanical Equinoctial Dial / Equatorial Minute Dial
Universal, Hour-Angle
Mechanical equatorial dials (occasionally called equatorial minute dials or mechanical equinoctial dials) are designated by the unique way they display the time.
They are stationary dials, yet the hour cannot be passively read; first, the user must manually rotate an alidade or other sighting element to face the sun. This rotation engages gearing that results in the time being indicated (most commonly by two scales: the hour by an arm and the minute by a hand on a subdial. On some dials, there is no hour arm and a single clock dial displays both the hour and minute).
How to use
1. Referencing the included plumb bob, ensure that the dial sits upon a level surface.
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2. 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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3. Using the latitude scale, incline the hour plate to the latitude of use and lock it in place.
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4. If the dial's gnomon is adjustable for solar declination, make the appropriate daily adjustment.
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5. For dials with a spot-and-target gnomon:
Turn the arm until the beam of light cast by the frontmost sight lands on the line (or other target) of the rear sight. This will ensure that the arm points directly at the sun.
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5b. For dials with shadow plane alignment:
Turn the arm until the shadow cast by the gnomon either falls on its target line or is at its smallest. This will ensure that the arm points directly at the sun.
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6. Read the position of the hour marker along the edge of the equatorial dial plate. This will indicate the current solar hour.
7. Reference the position of the minute hand on its isolated dial. This will indicate the current solar minute.
8. The current solar time can be read by consulting both the hour and minute indicators. 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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