[EMAIL PROTECTED] (Philip P. Pappas, II) writes: > Thank you for your thoughtful comments. I make the statement that the "time > method is the prefered method for setting a sundial if and only if the > sundial is properly designed, constructed and leveled (correcting for the > EOT and longitude of course).
I would say that it is the preferred method *especially* if you suspect you have a poorly made dial. If you set it up by the time method, then at least you know it is accurate at one time for two days of the year. This is not guaranteed to, but is likely to reduce the errors on average. > > 4. This has just occurred to me and is probably not > >relevant but it has got my mind wondering. As we know, the earth is a > >flattened sphere. Gravity, from which we derive a vertical (and > >subsequent horizontal) reference comes from the centre of the earth's > >mass. This is presumably right in its centre, assuming that differences > >in local density do not move it by much. But as we move towards the > >flattened poles the angle to the centre of gravity will no longer be a > >true vertical. But even so, it is this centre of gravity which is the > >true reference point for the earth in its orbit around the sun. > > Then there is the centrifugal force due to its rotation. Will > >this effect a true vertical? At the equator - no, but imagine a point > >at 45 degrees latitude, where the centrifugal force must have some > >effect on any plumb line/spirit level. I guess that all of these > >effects are so tiny as to be irrelevant, but I would like to know how > >much they modify the results. These effects are one and the same. The Earth is flattened at the poles *because* centrifugal force pulls it out around the equator. At the Equator and at the poles the vertical passes through the center of the Earth, inbetween it doesn't, but that doesn't affect the accuracy of a properly designed dial. Just for fun, the radius of the Earth is 6,378 km and the difference between the the equatorial and the polar semiaxis is 21.4 km. This makes the maximum discrepancy in the angle about (2*21.4/6378) = 0.4 degree. Art Carlson
