[time-nuts] reply re Harrison's timing method - #13 in Vol 176, Issue 44 digest

[Kevin Birth]

There is a large period literature on ³dialing² which not only included
sundials, but all sorts of ways to measure time from  celestial objects
using angles. Discussions of trigonometry, surveying, navigation, and
³dyaling² in relationship were also quite common during the period. These
could be found in popular books, and sometimes in almanacs (some of which
had huge circulations).

The use of building features to reckon sidereal time is quite old. In the
6th century, Gregory of Tours wrote DE CURSU STELLARUM about sidereal
timekeeping, and the techniques were later simplified to use buildings
within a monastery in HOROLOGIUM STELLARE MONASTICUM.

The secondary sources on these two medieval works are:

Constable, Giles. 1975.  Horologium Stellare Monasticum.  Corpus
Consuetudinum Monasticarum, volume 6, pages 1-18.
McCluskey, Stephen. 1990.  Gregory of Tours, Monastic Timekeeping and
Early Christian Attitudes to Astronomy.  Isis, volume 81, pages 8-22.

Sobel gives the false impression that such knowledge was held by only a
few and that Harrison was a bit of a country bumpkin, but in fact, when
one considers that the inventor of the deadbeat escapement was from
Carlisle, and that gear-cutting machines for clocks were improved by
Hindley in York, this suggests that there was widespread interest in
clock-making in addition to the widespread interest in astronomy in the
North of England, and a look at all the books and pamplets published on
the topic suggests that the interest extended into Ireland and Scotland,
as well.

Basically, the sort of expertise to do what Harrison did using his window
and a neighbor¹s chimney was extremely widespread, and the literature
giving instructions is overwhelming in its size.  I did a quick search on
astronomy and dialing in one of my go-to databases (18th Century
Collections online) and got 383 hits.

If you know French, you can go to gallica at
https://gallica.bnf.fr/accueil/en/content/accueil-en?mode=desktop and
search for works on astronomy and dialing, and find similar sources,
although my impression always has been that the English were more obsessed
with the topic in the 17th and 18th century than the French.

Best,

Kevin



-- 
Kevin K. Birth, Professor
Department of Anthropology
Queens College, City University of New York
65-30 Kissena Boulevard
Flushing, NY 11367
telephone: 718/997-5518

"Tempus est mundi instabilis motus, rerumque labentium cursus." --Hrabanus
Maurus

"We may live longer but we may be subject to peculiar contagion and
spiritual torpor or illiteracies of the imagination" --Wilson Harris




On 3/26/19, 10:48 PM, "time-nuts on behalf of Tom Van Baak"
<time-nuts-bounces at lists.febo.com on behalf of tvb at leapsecond.com> wrote:

>EXTERNAL EMAIL: please report suspicious content to the ITS Help Desk.
>
>
>BobH wrote:
>>> This would be an excellent project for time-nuts to verify.  First, a
>>> better explanation of John Harrison¹s method is in order.  A vertical
>>> window edge is not sufficient - a second vertical reference at a
>>> distance is required - Harrison used a chimney on a neighbor's house.
>
>Agreed! The project is the perfect intersection of amateur astronomy and
>amateur timekeeping. Surely, a couple of people on the list could 1)
>attempt to verify the Harrison method, and 2) determine what the limits
>of its accuracy are, say, with little effort vs. with hard work vs. with
>extreme dedication.
>
>JimL wrote:
>> To get 1 second accuracy, you need 360/86400 = 0.004 degree
>> measurements. That's 0.073 milliradian - 1 cm  at 140 meter distance.
>>
>> I'm not sure an "edge" is sharp enough (diffraction, etc.), although
>> your eye is pretty good at "deconvolving" the linear equivalent of an
>> Airy disk/rings.
>
>Keep in mind too that one can take more than one star reading per night.
>Any identifiable star that crosses your edge is a recordable timing event
>that evening. So, in theory, if you measure N stars you get sqrt(N)
>improvement in accuracy per day.
>
>I want to encourage anyone to study the problem and help solve the
>riddle, either by uncovering existing professional or amateur literature
>or by actually trying this at home. It boils down to how accurately can
>you measure earth rotation using the Harrison method.
>
>To put this in time nuts context, precision timekeeping prior to the
>middle of the 20th century was always a form of "Earth Disciplined
>Oscillator". Not unlike a GPSDO, your observatory's pendulum clock kept
>accurate time short-term and star tracking (earth rotation) kept accurate
>time long-term. The ADEV's crossed just like a GPSDO.
>
>The short-term ADEV of a really good pendulum clock is here:
>
>http://leapsecond.com/pend/shortt/
>
>The long-term ADEV of earth rotation is here:
>
>http://leapsecond.com/museum/earth/
>
>So the performance of a DIY earth disciplined oscillator would be a
>combination of the two.
>
>/tvb
>
>
>_______________________________________________
>time-nuts mailing list -- time-nuts at lists.febo.com
>To unsubscribe, go to
>http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>and follow the instructions there.