The 48 minutes that I also have seen online quite a bit should probably be 49 minutes. It can be calculated with the formula (Hours x Minutes / lunar month) to get (60 x 24) / 29.5. This gives 48.8. Many online examples use 30 for the synodic month. It actually takes the Moon 29.53 days to return to the same apparent position in the sky in relation to the Sun.
Relation to the Sun is probably what you need to look at, because you are probably using the fact that the Full Moon rises opposite the setting Sun and therefore reaches it's most southerly point (meridian) near midnight.
By using the Links at The Sundial Group (linked in my last message), or by using software such as Starry Night, you will find that the variations in the Moon's rise and set times are not nearly as divergent as 49 minutes near the Full Moon. In fact, for the three days before and after the Full Moon, there is little or no change at some latitudes.
Latitudes:
At my latitude of 35 N, near the Full Moon rise time is 15-22 minutes later each night. Near new moon it can be as much as 90 minutes! (Maybe more. I didn't bother to check beyond that.)
At 78 degrees latitude: I found an website that says that the Moon will actually rise EARLIER on successive nights at certain times of the year above a certain latitude. I would be willing to bet that 78 degrees latitude falls into this category.
As you can see, the 49 minute average may not be an accurate approximation for latitude 78 degrees. I suggest using an online calculator, or an astronomy program, to create tables of information, and try to determine the actual RTS (rise/transit/set) times for the latitude of the dial. It may be more appropriate to make the scale match with the smaller increments near the Full Moon, since that is when a shadow is more likely to be visible. To be complete, you might also consider putting a different, more divergent scale on the other half of the circular dial face to represent Moon time near the new moon. (I don't know how this would work, because I'm not very familiar with whether or not the Moon circles the sky, as the Sun does at high elevations. I would think no, since the Moon orbits in Earth's equatorial plane and probably at least dips below the horizon throughout the year.)
Albert
35 N
95 W
Louise Rigozzi <[EMAIL PROTECTED]> wrote:
Dear Sundial Mailing List members,
I have just joined the list and have quite a tricky question to ask already! I am working with Tony from Lindisfarne Sundials in England to create a sundial at a very high latitude (78 degrees) and I am including a correction graph for reading from the moon's shadow. This is because for three to four months of the year there will be little or no sun. I have made the graph illustrate the principle that the moon is 48 minutes 'fast' each day before the full moon and 48 minutes 'slow' each day after it. The graph hopefully gives the idea that the more precise you can be with the number of days (and even half or quarter days) you are from the full moon, the more accurate your reading will be. If anyone is interested in seeing it, I can JPEG it to you. My main question is, after adding/subtracting the hours and minutes from the reading of the moon's shadow, whether you then need to add/subtract the minutes according to the Equation of Time (!
! which
will also be shown on the dial face) for that day of the year. Thanks! Louise Rigozzi
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