BC Gnomonics
Lunar Volvelle
Astronomical volvelles are most often found bound into the pages of books, made up of a series of rotating ​disks. These devices have been made to suit a wide variety of specialized purposes, and even the lunar volvelle in particular varies greatly between iterations.
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All lunar volvelles feature at least a calanderical scale and solar and lunar indexes, allowing them to accomplish their most basic function of locating the moon in the ecliptic. Many also feature a lunar phase indicator.
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With the ecliptic degree of the moon known, the moon's daily motion can be modeled using an astrolabe.
Other potential scales may indicate the sun's culminating degree, the sun's rising hour, the number of daylight hours for a given date, the angular declination of the sun, or the astrological aspects.
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At times, lunar volvelles are built into portable sundials to fulfill the particularly useful function of telling time with the shadow cast by the moon. This calculation can be made with seperate volvelles, though only if an hour scale is present.
How to use
(Telling time with moonlight)
1. Set the lunar index to the time indicated by your sundial
(this method only works with hour angle dials which to not depend on declination. I.e. equatorial ring dials will not function, but diptychs will).
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2. Ensure that the lunar index also indicates the current lunar age (days since new moon).
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3. Reference the solar index, which points to the approximate hour.
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4. 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.