Jim wrote > I used xephem 3.2.3 to solve for when solar declination = 0.0 and got > 2000 March 20 7:30:59 UTC. Xephem calculates that the declination of > the moon at that time to be 2:50:47.5 (degrees:minutes:seconds).
I take this to mean that the moon's plane of orbit is tilted too far from Earth's equatorial plane for a moon shadow to give a good East-West line. Presumably it only works when both solar and lunar declination are zero, which I believe to be the equinox and lunar eclipse combination I asked about already. How often does that happen, provided I don't mind a few hours gap between the two events? My own answer is: The moon is on our equatorial plane twice a month, but I'm only facing it at night once a month. These would be the strange "27 1/3" day months, wouldn't they? Equinoxes are twice a year, so the two must coincide every 27.33 / 2 = 13 2/3 years. If I want bright moonlight from a near full moon, then I need it to be 3 or 4 days either side or about one week in 4. So that means suitable conditions arise once every 55 years or so. Is my thinking right? > If you would give me your latitude and longitude I can compute the > azimuth of the moon at its rise and set for your location on that Is the implication of this point that the only condition required is for the moon to be due east and west at rise and set, nothing to do with equinoxes. If so, when does that happen? I'm at 45.08N 64.5W Steve
