Re: The Housewife's Trick
Chris, I did a fairly extensive analysis of The Housewife's Trick in "Herbert's Correction", The Compendium, June 1998, 5(2):24-27. Fred Sawyer - Original Message - From: Chris Lusby Taylor To: Sundial List Sent: Sunday, January 21, 2007 8:52 PM Subject: The Housewife's Trick In his wonderful book on sundials, AP Herbert referred to "The Housewife's Trick" of turning a sundial to correct for the Equation of Time. He seemed to be suggesting that it was a well-known trick - take your horizontal sundial, turn it to agree with a watch and it will continue to tell better time than it would if aligned due north-south. He was, of course, writing in England. This trick would not work well much nearer the equator. Has anyone any evidence that housewives do this? Has anyone investigated whether it works? I have in the back of my mind that I've seen it analysed somewhere but I forget where. Anyway, I've just looked into it, as I've been trying to invent a universal sundial base that would allow any sundial mounted on it to be adjusted for the Equation of Time. Before making something theoretically correct, I thought it might be worth seeing if the "housewife's trick" is actually good enough. (Those of you who know me are probably horrified but, hey, I'm an engineer.) I examined two versions of the trick: turn the dial and gnomon together or turn just the dial, leaving the gnomon unmoved. If you turn just the dial the effect is to leave the shadow unmoved, but to move the hour lines. Apart from the minor problem that the hour lines normally radiate from two different points on either side of the gnomon, the effect of turning the dial is independent of the sun's declination, so easy to calculate. For my latitude (51 degrees) it isn't at all bad! It can reduce the difference between sun time and clock time to less than 3 minutes, and, typically, less than 1 minute throughout the day. If you turn the dial and gnomon together, the effect on the dial's timekeeping is a complicated function of the sun's declination and time of day. But the result seems to be even better, typically less than a minute error except when the sun is very low in the sky. (These are my preliminary, unchecked, theoretical results. I may be wrong. It wouldn't be the first time.) Luckily for us in the northern hemisphere, the trick works better here than in the southern hemisphere as it works poorly in summer, but the EoT is small then. So I'm inclined to forget about making a polar axis EoT base for sundials and just use the housewife's trick. It should be easy to make a turntable to mount a sundial and to mark on it the rotation needed for any date. Thank you APH. Chris Lusby Taylor 51.4N 1.3W .. -- --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
The Sundial Primer - The "Inclinometer"
Sunny Day! Okay, so there's this spot that calls out to you every time you go by: "PLEASE.I want a sundial!". But you keep ignoring it. Why? Because it's a sloped surface and you just don't feel inclined to find its inclination. Well then, come and check out the latest "SDU Cool Tool". It's the "Inclinometer" and it's easy to make and use. It has a resolution of one tenth of a degree and its accuracy is determined by how well it is constructed. Once you build it all your dialling buddies will want to borrow it. Come for a visit and get your kit. http://www.mysundial.ca/sdu/sdu_inclinometer.html Sunny Day U now has a number of devices to make your sundialling experience just a little more enjoyable. In addition to the "Inclinometer", these include the: "Correct-A-Dial": This calculator will help you determine the correction for the Equation of Time (EoT) and/or longitude. "Correct-A-Dial II": This device is based on the EoT curve and will also help you determine the correction for the EoT and longitude. A fill in the blanks model allows you to determine the correction for any longitude. There are an additional 61 models already completed for longitude intervals of 1/4°. "Dialling Buddy": This device will visually illustrate the relationship between the sun and the earth at any location as well as give you all kinds of dialling data. "Dialling Guides": There are hundreds of templates available for ten different types of sundials including two "human-sized" interactive sundials. "Wall Declinometer": This window mounted device will help you find the declination of a wall at local solar noon. "Check-A-Dial": Don't get ripped off when you buy a sundial! This device and detailed instructions will give you a good idea of what to look for when buying a horizontal, vertical or equatorial ring sundial. "SNOURTH-ometer": This device does just what it says...it helps you find the SNOURTH line! That's a total of 8 devices available to you. And those devices that require it are designed for both the Northern and Southern Hemispheres. If you haven't already got your kits, please come and visit: http://www.mysundial.ca/sdu/sdu_index.html Happy Dialling! Carl Sabanski www.mysundial.ca "Get Hooked on Gnomonics!" <> --- https://lists.uni-koeln.de/mailman/listinfo/sundial
The Housewife's Trick
In his wonderful book on sundials, AP Herbert referred to "The Housewife's Trick" of turning a sundial to correct for the Equation of Time. He seemed to be suggesting that it was a well-known trick - take your horizontal sundial, turn it to agree with a watch and it will continue to tell better time than it would if aligned due north-south. He was, of course, writing in England. This trick would not work well much nearer the equator. Has anyone any evidence that housewives do this? Has anyone investigated whether it works? I have in the back of my mind that I've seen it analysed somewhere but I forget where. Anyway, I've just looked into it, as I've been trying to invent a universal sundial base that would allow any sundial mounted on it to be adjusted for the Equation of Time. Before making something theoretically correct, I thought it might be worth seeing if the "housewife's trick" is actually good enough. (Those of you who know me are probably horrified but, hey, I'm an engineer.) I examined two versions of the trick: turn the dial and gnomon together or turn just the dial, leaving the gnomon unmoved. If you turn just the dial the effect is to leave the shadow unmoved, but to move the hour lines. Apart from the minor problem that the hour lines normally radiate from two different points on either side of the gnomon, the effect of turning the dial is independent of the sun's declination, so easy to calculate. For my latitude (51 degrees) it isn't at all bad! It can reduce the difference between sun time and clock time to less than 3 minutes, and, typically, less than 1 minute throughout the day. If you turn the dial and gnomon together, the effect on the dial's timekeeping is a complicated function of the sun's declination and time of day. But the result seems to be even better, typically less than a minute error except when the sun is very low in the sky. (These are my preliminary, unchecked, theoretical results. I may be wrong. It wouldn't be the first time.) Luckily for us in the northern hemisphere, the trick works better here than in the southern hemisphere as it works poorly in summer, but the EoT is small then. So I'm inclined to forget about making a polar axis EoT base for sundials and just use the housewife's trick. It should be easy to make a turntable to mount a sundial and to mark on it the rotation needed for any date. Thank you APH. Chris Lusby Taylor 51.4N 1.3W ..--- https://lists.uni-koeln.de/mailman/listinfo/sundial
