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.
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