Hello John, My advice is "Don't go there. There be monsters!" *
The motion of the moon is quite complicated and the "equation of time" shortcut will not work. You were right is concluding that the solar "equation of time" does not apply, and the eccentricity and obliquity of the ecliptic were the determinants of the equation of time. The major problem with the moon is the time between new moons (lunation) is 29.53 days, different from the orbital period of 27.32 days. This means the declination cycle, connected with the orbital period, is out of phase with the lunation cycle. If it did work the limits of the geocentric declination are +/- 29 degrees, greater that the 23.44 we are used to for the sun. The eccentricity is larger 0.0549 vs 0.0167. But I would guess that a larger effect is due to the difference between the geocentric co-ordinates compared to the actual rotation around the barycenter of the mutually orbiting bodies. It would take quite a table, equivalent to an almanac, to correct for all these effects. The usual 48 minutes per day correction can only be an approximation. I would also speculate that the largest error in such a table would be determining the start point. Half a day off on the new or full moon determination would be almost half an hour off in the time check. For night time checks, I use a "nocturnal" and determine the time based on the rotation of the big dipper around Polaris. The date / sidereal time correction is easier to build into the instrument. Thanks for the good question. I know the celestial "mechanics" will have better answers, including numbers, but I have enjoyed the challenge. Roger Bailey Walking Shadow Designs N51 W115 * The quotation is typical of old charts identifying "Terra Incognita". At 09:20 AM 1/12/99 -0700, Philip P. Pappas, II wrote: > > >Hello Dialists: > >I am currently writing the 4th edition of my Sundial Owner's Manual and want >to make sure that the information I,m giving my customers is correct. > >I have a section which tells how to tell time by using moonlight and a >sundial. I provide a table of corrections from which the time can be >estimated if one knows the age (the phase) of the moon. > >One question though: Is it nessary to correct moontime with the Equation of >Time ? >Since the Equation of time is due to the eccentricty of the earth's orbit >around the sun and the tilt of the earth's axis, it seems to me that this >has nothing to do with the moon and should not be considered in the >corrections. Am I right? > >Thanks for your help, > >John Carmichael > > >
