A Lunar Clock

This volvelle, in conjunction with a sundial, can be used as a lunar clock to determine time at night. In use, the inner disc is set to show the current phase of the moon through the window. After determining the hour angle of the moon (with a sundial used as a moondial) and setting the lunar dial of the volvelle to that hour angle, one can find the location of the sun, hence ascertain the time at night. This instrument is different from the nocturnal that employs the pole star and selected others to determine time at night.

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