Dear fellow dialists,
I am forwarding this inquiry I received privatly from Yaaqov Loewinger. It
seems right up our alley.
Regards,
Art Carlson
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>From [EMAIL PROTECTED] Fri Mar 2 10:21 MET 2001
Date: Fri, 02 Mar 2001 10:33:16 +0200
From: "Y. Loewinger" <[EMAIL PROTECTED]>
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To: [EMAIL PROTECTED]
Subject: Old equation of time:Equation d'Horloge
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Dear Mr. Carlson,
I read yr. excellent article on the Equation of Time. I posted
following questions, in the History of Astronomy Discussion= Hastro
group, and didn't get satisfactory answers. can you help me ? Thanks in
advance! ( you can write me- if you like , in German).
Subject: setting of clocks prior the introduction of modern mean time
History of Astronomy Discussion Group
<[EMAIL PROTECTED]>
Dear List members,
Prior the introduction of modern mean time into civil life (round
1780--1830), modern equation of time (= EoT) and modern distribution of
time, civilian public clocks were set with the help of sundials.
But as clock time is by definition a mean time, I wonder to which kind
of
mean time were the public clocks set generally, prior to, say 1780 ?
I see here 3 possibilities:
1. public clocks were simply set always to sundial time , without using
any equation of time tables. It seems that public clocks were set rather
often, say once a week, as their quality was rather poor and as in such
a short interval EoT changes only insignificantly, so it was simply
neglected.
2. clocks were set to "Nov. 3 mean time", as round this date modern
equation of time is maximal (~ + 16 minutes). So, beside looking up
sundial time, tabular values of "equation of clocks" running from 0 (on
~Nov 3) to ~31( on ~Febr 11) min, were added to sundial time to get
clock time.
This "equation of clock" = in French: "equation de l'horloge" (EdH)" is
the old Ptolemaic equation of nychtemeron(=day and night) of the Handy
Tables.
As this EdH was tabulated in the 18 th century French almanacs
Connaissance des Temps, I guess they were widely used all over Europe
(otherwise they would not have bothered printing it !).
Beside this EdH the French almanac of, e.g. of 1751, indicated also the
value of modern mean time at true noon, so modern mean time and modern
EoT were also in use. Which one, EoT or EdH were used for setting
civilian clocks ?
Can we assume that the EdH table was really used everywhere in civil
life in Europe, and can we assume in historical research that time
indications in documents from the 18 th century mean " Nov 3 mean time"
?
See an attached diagramm of Equation d'Horloge.
3. the third possibility to set clocks was, to set them to "Febr 11 mean
time". Round this date modern EoT is minimal, ~ 14 min. If a clock is
set on that date to sundial time, so a "Febr 11 equation of days", which
runs from 0 ( on ~Febr 11) to 31(on~ Nov 3) min, had always to be
*subtracted* from sundial time, to get clock time. Such equations of
days can be found till the 17 th century in astronomy books ( e.g. the
one of Huygens, near 1640), and it is also of Ptolemaic origin,
standartly used in the middle ages. They seem to be less popular than
EdH, perhaps because adding of EdH seemed to be easier for clock
setters.
To sum up my question: can we assume that a standard church clock, say
in
Central Europe, was set to November 3 mean time, still in the late 18 th
century?
Best regards
Yaaqov Loewinger, dipl.ing. ETHZ
--
Y. L o e w i n g e r
mail : P.O.B. 16 229 ; 61 161 Tel Aviv / Israel
tel. : 972-3- 604 61 79; ++ 523 98 33
fax : 972-3- 546 90 76
e-mail : [EMAIL PROTECTED]
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