BC Gnomonics
Nocturnal /
Nocturlabe
Universal, Star
Nocturnals, like sundials or astrolabes, can be made of different materials to suit multiple price points, including paper, wood, or one of several metals.
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These instruments utilize the predictable movement of the night sky in order to determine the solar hour.
Because every star appears to rotate around Polaris, with the date known the sky can be read as if it were the face of a clock. The nocturnal sights Polaris and one other reference star to accomplish this. Depending on the model, the reference star may be from the constellation Ursa Minor, Ursa Major, or Cassiopeia. Some models feature all three, in which case one is chosen by the operator.
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Included reference stars are always circumpolar, meaning that they are close to the axis of the celestial sphere and are therefore visible regardless of date.
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The most common reference star featured on nocturnals is Kochab of Ursa Minor.
How to use
1. Rotate the solar index to the current date. If the nocturnal has multiple indices, use the one labeled with the reference star you plan to use.
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2. Raise the nocturnal to the stars, keeping the handle level with the local horizon.
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3. Sight polaris through the center hole.
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4. Rotate the inner edge of the rotating arm into alignment with the nocturnal's reference star.
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5. Read the rotating index arm's position on the hour scale inscribed on the disk of the solar index. This will yield the current solar time.
6. 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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