Hi John,
I agree that in your circumstances it's eminently sensible to include the longitude correction and then have only what is a universal correction to make - that for EoT. That has to be more understandable and of course any dial designed that way is then always within (about) a quarter of an hour of being correct with respect to clock time. However where (as in the UK) the longitude correction is small the 'more understandable' solution is probably to combine the longitude correction into the overall correction graph. It is after all still (more or less) within a quarter hour of local clock time. Mind you China has to be a special case!! Regards Patrick -----Original Message----- From: John Carmichael <[email protected]> To: sundial <[email protected]> Sent: Sun, Feb 13, 2011 12:29 pm Subject: RE: part 2 of longitude correction My friend, Jim Tallman and Ihave had many discussions about longitude corrected dials. I know thatmany of the Western European dialists don’t like longitude correcteddials- especially the British dialists. I can see why they feel thisway. Their three biggest arguments against longitude corrected dials arethese: 1) They aren’t traditional, 2) They aren’t symmetrical, 3) They are so close to their Prime Meridian that the longitude correction is quitesmall and almost insignificant. But there are equally goodarguments for longitude corrected dials! In The United States, and manyother places, the sundial may be located far away from the Prime Meridian atthe edge of a Time Zone. In a perfect World, a Time Zone would only span 15degrees. But most of our Time Zones have wiggly irregular boundries thatsometimes span distances far greater than 15 degrees. Russia is anextreme example of this! There are parts of Russia that are missing entirezones! Then there’s China. The entire country only has oneTime Zone! See: http://www.travel.com.hk/region/timezone.htm For places like these, a well-builttraditional Solar Time sundial will give a time reading that can be more thanan hour off of Watch Time (Standard Mean Time). A casual non-dialistwould say that these sundials don’t work, especially if there is noEquation of Time graph available with built-in longitude correction! Correctinga dial for longitude solves this problem. For this reason, I thinklongitude corrected dials are more user-friendly. This is why Jim and I routinelymake our dials with longitude correction. That’s my two centsworth. John Carmichael From: Jack Aubert[mailto:[email protected]] Sent: Sunday, February 13, 2011 9:14 AM To: 'John Carmichael'; 'Donald Christensen'; [email protected] Subject: RE: part 2 of longitude correction This has all been very instructive. I am not a completebeginner, but have to confess that I was under the impression that rotating thedial plate with respect to the fixed gnomon was sufficient. Somebody oncetold me I could do this and I never really thought about it. If youactually think about the longitude correction, it does become obvious that justrotating the dial plate is not going to be right. For any dials I have made, I either used one of the programslike Shadows Pro or Orologi Solari to include a longitude correction, ormore frequently, now, I use a delta cad macro and keep uncorrected solar time. I agree with Willy Leenders that sundials should tell solar time. I have a watch that can tell me mean time, but it has ceded to mycelphone which is even better. Thanks for the question and thanks for the good answers. Ikeep learning things here, many of which I should already know but somehowmissed. Jack From: [email protected][mailto:[email protected]] On Behalf Of John Carmichael Sent: Wednesday, February 09, 2011 6:46 PM To: 'Donald Christensen'; [email protected] Subject: RE: part 2 of longitude correction (I hit the wrong button in my last email and it went out withoutmy letter- sorry!) Hi Donald: I understand your question, because I had this same questionyears ago when I was a beginner. The answer is no. To re-phrase your question sothat everybody understands it, you are asking this- When drawing a sundialface, should you simply rotate the normal solar time hour lines about theircenter of origin by 3 degrees to obtain a longitude corrected face? No, you can not dothis. Each hour line has to be calculated and drawn individually. This is a common error some beginning dialists make when designing longitudecorrected dials. I almost made this mistake too once! To help you out (since apicture is worth a thousand words) I drew your sundial in five minutes usingShadows Pro and sent it you off list the drawing. I just didn’twant you to make a common mistake. But verify it for yourself anddon’t take my word for it. That’s how you learn! Keep asking good questions! Thanks John From: [email protected][mailto:[email protected]] On Behalf Of Donald Christensen Sent: Wednesday, February 09, 2011 3:52 PM To: [email protected] Subject: part 2 of longitude correction I'm laying out lines for a newdial I may not have been clear. I don't intend to rotate the gnomen. The dial willstill point true north By labeling 12:12 as noon and 13:12 as 13:00, I am rotating the hour marks. Myquestion is, Is it by an even 3 deg? -- Cheers Donald 0423 102 090 This e-mail is privileged and confidential. If you are not the intendedrecipient please delete the message and notify the sender. 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