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
..
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