Longitude correction I have been staring at equatorial sundials for so long that I forgot how a horizontal one works. This is why I suggested that you can rotate the face 3 deg if you are 3 deg difference from the standard meridian.
I have since had a wake up call and learned/remembered that this is wrong. However, I still wouldn't leave it alone. It turns out that it is right. (well - kinda sorta) If you layout a horizontal dial by geometry, you can rotate the circle on top that looks like an equatorial dial (see attached file) before you project the lines down. Donald On Tue, Feb 15, 2011 at 4:11 AM, Simon [illustratingshadows < [email protected]> wrote: > My two cents. > > Dials that may be moved, I make those with no longitude correction, unless > the person I make it for wants that correction. > > Dials that are too large to move I include the correction. > > I have to agree with Jogn Carmichael re "the customer is always right", > after all they pay the bills, and, it was my great grandfather who > popularized that saying, he was Harry Gordon Selfridge:) > > Simon > > Simon Wheaton-Smith > www.illustratingshadows.com > Silver City, New Mexico W108.2 N32.75 and > Phoenix, Arizona, W112.1 N33.5 > > --- On *Mon, 2/14/11, John Carmichael <[email protected]>* wrote: > > > From: John Carmichael <[email protected]> > Subject: RE: part 2 of longitude correction > To: "'Frank King'" <[email protected]> > > Cc: [email protected] > Date: Monday, February 14, 2011, 10:26 AM > > > > Hi Frank and Roger. > > > > I thought I'd respond to both of you letters since they are related... > > > > (Refer again to Time Zone Map: > http://www.travel.com.hk/region/timezone.htm ) > > > > You’re right Frank about “westward creep” of many of the time zones. Most > of our time zones in the US and Canada extend more to the west of their > Prime Meridians than to the east. So as far as land area goes, there > generally is more land area west of the Prime Meridians- (I don’t know if > this extra land to the west also has more population in each zone since our > population densities tend to thin out as you go west.) > > > > Looking at the map, check out your “westward creep” in Europe! The UK and > Portugal are correctly colored yellow in the 0 degree zone, but the rest of > Europe is all green- even countries that are west of your zone border like > France, Belgium Holland and Spain. > > > > Look at the map of Canada, Roger. Your Eastern Time zone, in theory, > should span from 67.5 deg W to 82.5 deg W. But it actually spans from about > 62 deg W to 90 deg W! A definite westward creep over a 28 degree span. Your > Mountain Time zone is even worse. Its Prime Meridian is at 105 deg W, but > it spans from 102 way over to 137- That’s a 35 degree span that’s not even > centered on the Prime Meridian! Strangely, the Prime Meridian is at the far > Eastern edge of the Zone! > > > > All this Time Zone craziness is because local governments drew the time > zone boundries. They tried to not isolate communities, states and > provinces. They didn’t want time zone boundries to slice through populated > areas. They often would zig zag them around cities and states. (The > Constitution does not stipulate time zones as it was written before time > zones were invented.) But as cities grow, their old time zone boundries > often no longer go around communites, but through them. If it gets really > bad, they have to re-draw the boundries. I don’t think this is done very > often though because it just leads to more confusion. > > > > If your sundial is located far from its Prime Meridian in one of these > crazy Time Zones, and you want it to give a time reading that is close to > watch time, then designing it with built-in longitude correction is a must. > If you forget about Daylight Saving periods, at worst, a longitude corrected > dial will only be about 16 minutes off (because of the Equation of Time). On > the average, it is only off by about seven minutes- good enough to keep most > appointments! I know that I can glance at one of my sundials from a > distance, and without using an EOT graph, that it is giving me a time that > is very close to watch time. As Roger pointed out, a Solar Time dial doesn’t > even come close. > > > > For public wall dials that can be seen from a great distance, the person > reading the dial might be a half mile away from it- too far to read a little > EOT plaque. Doesn’t it make sense to use a longitude corrected dial for > public wall dials since the EOT graph is not availble to the far away users? > > > > > John Carmichael > > > > > > John > > > > > > -----Original Message----- > From: Frank King [mailto:[email protected]] > Sent: Monday, February 14, 2011 3:44 AM > To: John Carmichael > Cc: [email protected]; [email protected] > Subject: Re: part 2 of longitude correction > > > > Dear John, > > > > Your inspiring message about longitude correction > > prompts more thoughts from me. You say: > > > > > ... most of our [US] Time Zones have wiggly > > > irregular boundaries that sometimes span > > > distances far greater than 15 degrees. > > > > Everyone necessarily lives within 7.5 degrees of > > a multiple of 15 degrees but, as you correctly > > imply, time zone boundaries don't follow lines of > > longitude. You can live far more than 7.5 degrees > > from the multiple of 15 that goes with your clock > > time. > > > > Here I have a conjecture that applies specifically > > to the U.S. and, maybe, U.S. readers can tell me > > that I am wrong: > > > > A greater proportion of the population of > > the U.S. lives to the WEST of the multiple > > of 15 degrees that governs their clock time > > than lives to the EAST of that meridian. > > > > This is certainly true in the U.K. because only a > > tiny proportion of the British land mass is to the > > east of 0 degrees longitude. I just happen to live > > in that small bit. > > > > The conjecture is certainly true in summer when > > Daylight Saving Time shifts the relevant meridian > > 15 degrees further east but, I assert, it is true > > in winter too. > > > > Am I right? > > > > My conjecture is part of a bigger hypothesis that > > "time zones creep westwards". > > > > I assume that the wiggles are not hard-wired into > > the U.S. Constitution? > > > > My guess is that, every so often, a town close to > > a time-zone border asks to go to the other side. > > Is this right? If so, what is the procedure? > > > > By my hypothesis, towns on the west of a border > > more often ask for the border to be moved to > > their western side than towns on the east of a > > border ask for the border to be moved to their > > eastern side. > > > > There are villains in the U.K. who want the country > > to be in the Central European Time Zone. If this > > happened, it would be a spectacular example of a > > Time-Zone Creeping Westwards. > > > > All the best > > > > Frank > -----Inline Attachment Follows----- > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > > -- Cheers Donald 0423 102 090 This e-mail is privileged and confidential. If you are not the intended recipient please delete the message and notify the sender. Un-authorized use of this email is subject to penalty of law. So there!
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