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

[Bob Holmstrom]

Ben Bradley stated > "Perhaps closer to your question: I recall in my
readings about clockmaker John Harrison (likely either in "The Quest
for Longitude” or Dava Sobel's "Longitude") that he would look from
the edge of his window at a particular star each night and note (while
counting the ticks he heard from his clock) the exact moment it would
disappear behind a nearby chimney, and knowing the Earth's rotation
takes four minutes and some (I forget) seconds off from a day, he used
this to calibrate and test the precision and accuracy of his long
clocks. It was suggested he could get within less than second with
this method."

>From Sobel - Chapter 7 > "The Harrison brothers tested the accuracy of
their gridiron-grasshopper clocks against the regular motions of the
stars. The crosshairs of their homemade astronomical tracking
instrument, with which they pinpointed the stars' positions, consisted
of the border of a windowpane and the silhouette of the neighbor's
chimney stack. Night after night, they marked the clock hour when
given stars exited their field of view behind the chimney. From one
night to the next, because of the Earth's rotation, a star should
transit exactly 3 minutes, 56 seconds (of solar time) earlier than the
previous night. Any clock that can track this sidereal schedule proves
itself as perfect as God's magnificent clockwork.”

This would be an excellent project for time-nuts to verify.  First, a
better explanation of John Harrison’s 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.
Harrison would watch a single star (obviously the same star for
several nights) in the gap between the right vertical edge of a window
mullion and the left edge of his neighbors chimney.  He would move his
eye so as to always keep the star in the gap.  Eventually, the gap
closes to zero and the star ‘winks out’. At that point he would
verbally signal his assistant watching Harrison's clock pendulum tip
swinging against a degree scale below.  Harrison’s grasshopper
escapement clocks had a very large amplitude (+/- several degrees)
compared to that used by precision clocks today, so it is said that
the assistant could record the results to a fraction of a second.

Jonathan Betts has a description of the method in his “Harrison”
published by the National Maritime Museum in 2007 - see attachment.

A pendulum clock is not required to verify the method - all that is
needed is a similar star sighting arrangement and a means to record
the time of the ‘wink out’ - preferably to a fraction of a second.
Subsequent night ‘wink out’ times should be 3 minutes, 56 seconds
apart.  (Is that single value valid over a 400 years period?)

Bob Holmström
Editor
Horological Science Newsletter
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Screen Shot 2019-03-26 at 3.11.43 PM3.png
Type: image/png
Size: 630713 bytes
Desc: not available
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20190326/9b1146e5/attachment.png>