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

[jimlux]

On 3/26/19 3:48 PM, Bob Holmstrom 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.
>


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.

A small telescope and a camera might work, lining up with the two edges 
as a "fixed offset knife edges".  It could also work in day time (you 
can see Polaris in the day time with a 28x telescope with a 1" objective 
- a surveyor's theodolite)

There's a collection of navigation papers from ION available on CD-ROM 
and there's a fair amount of info in there about celestial trackers and 
detectors.