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Coralline Algae is still intrigued by the sidereal Great Clock at Greenwich,
one of a pair, the other keeping Greenwich mean time.

She asks-

| Wondering further about the clock keeping sidereal time.
| Since the clocks would drift relative to the stars would the clocks be
| adjusted regularly - to show the correct sidereal time or would the error
| rate simply be recorded and a correction applied to that showing on the
| clock face.
| Also given the need for accuracy of the master clocks, I imagine that just
| moving the clock hands was not an option  to bring the stars and the
| timepieces into synchronization.

Well, now Corraline is taking me beyond what I can claim to know about,
which is mostly derived from "Greenwich Time and the Longitude", 1997, by
the late Derek Howse.

If the sidereal clock was keeping perfect sidereal time, then it wouldn't
drift, with respect to the stars (except by the tiny change resulting from
precession). Sirius would pass through the crosswires on the meridian at the
same Greenwich Sidereal Time each day, that time being the Right Ascension
of Sirius. In theory, it should have been set so that the First Point of
Aries, that spot in the sky where the equator meets the ecliptic at the
vernal equinox, passes the Greenwich meridian at zero hours sidereal time,
every day. In practice, that was done indirectly.

But those two clocks were the product of their age (1676). They were before
the days of the temperature-compensated pendulum, so inevitably, they would
run a bit slower in Summer, faster in Winter; no better than does the
gradfather clock in my hallway. And that's why it was so important to keep a
regular check on their going, by observing the passage of Sirius, whenever
the sky was clear at the right moment. I'm pretty sure that they would never
have been readjusted to read the right time when that was done, and even
more sure that the rate-of-going would not have been tinkered with, then, by
fiddling with the length of the pendulum. Instead, the clock would be
allowed to run its course, unadjusted, and the discrepancy recorded in a
notebook; exactly like the way a mariner treats his chronometer. But this is
my surmise, not based on any knowledge.

These clocks were driven by heavy weights with a long drop, so rewinding was
needed only once per year. Tompion kept them locked, and not even Flamsteed,
Astronomer Royal, had a key. If any resetting or readjustment was ever done,
presumably it would have been done then. But remember, there were several
series of measurements going on at Greenwich at any time, all relying on the
time, and the continuity in that time, so I expect that any readjustment
would be done with great reluctance, and considerable care.

For further information, Coralline will have to seek out better sources than
I have here. Hope she has access to some sort of academic library. Howse
refers to his paper, with B. Hutchinson,, in "Antiquarian Horology", "The
Tompion clocks at Geenwich and the dead-beat escapement" (from which a
reprint was made) in 1970-71,. and to his contribution to the series of
volumes about the history , "Greenwich Observatory, vol iii, its buildings
and instruments." (1975). I haven't seen these. Howse knew what he was
about, and wrote clearly.

And if she wishes to go deeper still, all the old Royal Greenwich
Observatory papers now have their own archive at Cambridge University
Library, for which an on-line index exists at-
http://janus.lib.cam.ac.uk/
There, they are under the care of Mr. Adam Perkins (+44 1223 333056;
ajp21@cam.ac.uk), who is a helpful character.

Coralline's interest in this matter could well start her off on a little
research project, if she is minded that way.

George,

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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