I combined all of the corrections for the table I prepared when helping with the faithful reconstruction of the Fredericton Soldiers Barracks sundial over a decade ago <http://new-brunswick.net/new-brunswick/fredericton/page1.html>. You can barely see the interpretive plaque low down on the wall. Some day I must put some better pictures of the sundial and plaque up on the Web. -- Richard Langley
On Thu, 8 Jul 2004, John Carmichael wrote: >Hello All, > >Some of you wrote me and seemed very confused with my inquiry about a way to >get a EOT table that is longitude corrected. You ot understand what I >wanted. So I'll try to explain it here. (thanks to everybody who sent me >info on this) > >Sample letter: >"I still cannot figure out your EoT problem; EoT does not depend on >longitude, it depends on an agreed absolute (GMT) and changes so little in >any time zone you cannot see the difference on a dial. Maybe I am missing >something, I often do these days." > >Yes, you are missing something. And it's so simple you will kick yourself. >You're thinking too deeply! I'll try to explain... > >Of course you are correct in implying that EOT values are "Universal" and >are basically the same all over the world, at any longitude or latitude and >on any given date. You can use the same EOT graph with almost any sundial >anywhere in the world. But you know that already. So far so good. > >But if somebody really wants to know "clock" time, then not only does he >have to correct his sundial readings for EOT, but he also has to correct for >his longitude (and Daylight Saving Time if applicable). This additional >longitude correction is expressed as a plus or minus value in minutes and >seconds. But you know this already. So far so good. > >Now this double correction is sometimes confusing to non-dialists (it's a >triple correction if one has to correct for Daylight Savings too). So to >reduce two time reading corrections into just one correction, you can add >the EOT correction to the longitude correction since both expressed in >minutes and seconds, and you come up with a new EOT graph or table that >incorporates the longitude time correction in its values. > >Now do you get it? Neat huh! > >For my sundial customers, I usually build the longitude correction directly >into my sundial faces so the customer doesn't have to do the longitude >correction, just the EOT correction. This combined EOT+Longitude Correction >Table is very useful if you have an antique pre-timezone sundial or any >sundial that doesn't have a built-in longitude correction. The downside is >that you have to make a unique customized EOT table for each sundial, >depending on its particular longitude. > >Hope this helps =============================================================================== Richard B. Langley E-mail: [EMAIL PROTECTED] Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142 University of New Brunswick Fax: +1 506 453-4943 Fredericton, N.B., Canada E3B 5A3 Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ =============================================================================== -
