A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Chris Post
Date: 2021 Jun 22, 14:28 -0700
Geoffrey's clever sun clock for depicting the moment of meridian passage is more than an abstract proof-of-concept experiment. John Harrison used a similar procedure to test the accuracy of his early longcase clocks by the time of meridian passage of stars. The following excerpt is from Andrew L. King's essay "John Harrison, Clockmaker at Barrow; Near Barton upon Humber; Lincolnshire": The Wooden Clocks, 1713-1730.
Throughout [his] 1730 manuscript, Harrison never reveals too much about the construction of his clocks, but, when it comes to how he actually tested them, it is another matter. He had two basic methods. In section nineteen, he describes how he checks the clocks against one of the fixed stars by taking daily sidereal sightings. With an assistant calling out the seconds as the fixed star approaches the line of his meridian, Harrison explains how he uses
...a very large sort of an Instrument, of about 25 Yards Radius, Composed of the East side of my Neighbors Chimney (wch is situated from my House towards the South) & the West side of an exact place of some one of the upright parts of my Own Window Frames; by which the Rays of a Star are taken from my sight almost in an Instant:...
With constant practice, Harrison seems to have been able to use this system with great accuracy.
King's essay appears in The Quest for Longitude, edited by William J.H. Andrewes (Cambridge, MA: Harvard University, 1996). It would appear Geoffrey's in very good company.