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Can I use a gnomon (similar to that used in a sundial) to cast a shadow using the light of a full moon to create a calendar? My idea was to build a gnomon and take measurements of the shadow cast by each full moon over a course of time. I understand that the moon has elliptical an orbit and apsidal precession. My thought was to place stones on the end point of the shadow cast by the gnomon at each full moon, at the same lunar altitude, at a single site with known coordinates. My question is, would the points measured from the shadow cast repeat in a way useful for this particular calendar to function, e.g., in the next lunar year, or perhaps apsidal precession? This calendar would only be used as functional art and not to relate to the traditional calendars. I am in the first year of measurements and am wondering if this will work.

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Unlike the sun the moon is on an approximately 24 hour and 50 minute day. The moon takes, on average, about 29.5 days to repeat a visual phase. Furthermore, the moon's orbit around the earth takes about 27 1/3 days. This means the phases of the moon do not happen on the same days of the year, and moonrise and moonset happen at different times of a given day from year to year.

I believe there is too much variation and difference in the moon's phases and location to make any meaningful moondial. One issue not mentioned above, is that it would be inoperable during the day, but that might be precisely when the moon is over your marker.

As for a lunar calendar based on a lunar year, those exist in many different cultures, but they have issues with standardization of dates. Easter, for example, jumps around because it is calculated off of a lunar calendar.

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  • $\begingroup$ Thanks Jake. I edited my question a bit to address my real intent. $\endgroup$ Feb 11 '17 at 20:17

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