John Carmichael writes:
> >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? Roger Bailey <[EMAIL PROTECTED]> writes: > Hello John, > > My advice is "Don't go there. There be monsters!" * Good advice. > 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. I wouldn't agree the Equation of Time does not apply, just that other corrections are much larger. John does, after all, want to correct for the phase of the moon, so the position of the sun is relevant. > 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. This makes it sound like these are two separate orbital parameters. They are simply connected by the length of the year: 1/27.32 - 1/29.53 = 1/365 In fact, the time between any two particular adjacent lunations will have a correction closely related to the Equation of Time. > 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. Even easier than correcting a sundial for the Equation of Time. I have been interested for some time in the related problem of finding directions from the moon, possibly given watch time. I haven't formulated the mathematics yet. To quantify the error of various methods I will need some more information on the distribution of the relative positions of the sun and moon. This is certainly known. Is it also readily available in a comprehensible form? Art Carlson
