>Dear Philip, > You have posed some interesting questions about setting up a >dial. You should bear in mind a few points before deciding. > 1. If there is any error in the gnomon angle, this will >show worse at 6am and 6pm and will be totally invisible at 12. > 2. If the dial plate is not exactly horizontal, then an >error will show due to that. I would think that in this case smaller >errors would be seen at noon too. The greatest errors due to its level >would probably be at 9 and 3.
Dear Mike: 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). > 3. If it is set up at 12, then the only correction >necessary is EOT. At other times it could take more brain power to work >it out - hence more possibility of error. Since an error in the gnomon angle is undetectable at high noon, this would be another reason NOT to set the dial around noontime. Right? > 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. Interesting concept. I bet Fred Sawyer or Ross McCluney are the ones who could answer this! > > Regards, > > Mike Cowham. > > Technical Director > > Thanks again Mike. By the way, my name is John Carmichael, not Phillip Pappas. I just use his e-mail address. John
