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

[Roger Tilsley]

Greetings time-nuts,

Harrison's method depends on the star having the same true azimuth at 1 Sidereal day intervals (23 hours 56 minutes 04 seconds).  It does not depend on a visible horizon since the altitude (which changes with the season) is not relevant except that one must be able to see the star and the reference markers in the field of view.  Star sights in marine navigation depend on the horizon being visible through the sextant because one is measuring the altitude of the star rather than its azimuth. Harrison's method does not depend on the time of observation being at dawn or dusk and will work in full darkness since the star "goes out" when its light is obstructed by the chimney and the light of a candle enables the window bar to be seen for fixing the position of the observer's eye.  It does, however, depend on both the chimney and the window bar being vertical!

Today one could repeat the experiment with, for example, a theodolite with two fixed positions (one for the theodolite itself and a remote one to be able to reset the theodolite to the same azimuth), or an Azimuth Circle and a Nautical Almanac, or an Observatory (for the more wealthy time-nuts!).  Any repeatable method of determining the instant when the star is at a fixed azimuth will accurately provide the instant when the time is an integral number of sidereal days after the previous observation so a few days of cloud are of little consequence.

Like most of Harrison's methods, this method is fundamentally sound and remarkably accurate.

Best regards,
Roger.

On Wed, 27 Mar 2019 10:27:44 +1100, Neville Michie <namichie at gmail.com> wrote:

> It must be a sign of the dedication that Harrison applied to his work.
> It is not as simple as the description first appears, this is England,
> and the method presupposes that there are no clouds. It might be a week 
> or two before two nights occurred, when an unclouded night was followed 
> by another night within several days that was not clouded.
> Similarly, with the longitude method, stars must be visible within a 
> short period of dawn or dusk, when the horizon is visible together with 
> the star. Sun sights are not so difficult.
> GPS makes is so easy for us!
> cheers, 
> 
> Neville Michie
> 
> > On 27 Mar 2019, at 09:48, Bob Holmstrom <holmstro at gmail.com> wrote:
> > 
> > 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
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