Hi Bruce:
Would the David White 60 Degree Pendulum Astrolabe also work?
https://prc68.com/I/PendulumAstrolabe.shtml
--
Have Fun,
Brooke Clarke
https://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
axioms:
1. The extent to which you can fix or improve something will be limited by how
well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.
-------- Original Message --------
The Danjon impersonal astrolabe is perhaps better suited to accurate
measurements:
https://www.nzmuseums.co.nz/collections/3267/objects/3380/astrolabe
Bruce
On 27 March 2019 at 15:48 Tom Van Baak <[email protected]> wrote:
BobH wrote:
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.
Agreed! The project is the perfect intersection of amateur astronomy and
amateur timekeeping. Surely, a couple of people on the list could 1) attempt to
verify the Harrison method, and 2) determine what the limits of its accuracy
are, say, with little effort vs. with hard work vs. with extreme dedication.
JimL wrote:
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.
Keep in mind too that one can take more than one star reading per night. Any
identifiable star that crosses your edge is a recordable timing event that
evening. So, in theory, if you measure N stars you get sqrt(N) improvement in
accuracy per day.
I want to encourage anyone to study the problem and help solve the riddle,
either by uncovering existing professional or amateur literature or by actually
trying this at home. It boils down to how accurately can you measure earth
rotation using the Harrison method.
To put this in time nuts context, precision timekeeping prior to the middle of the 20th
century was always a form of "Earth Disciplined Oscillator". Not unlike a
GPSDO, your observatory's pendulum clock kept accurate time short-term and star tracking
(earth rotation) kept accurate time long-term. The ADEV's crossed just like a GPSDO.
The short-term ADEV of a really good pendulum clock is here:
http://leapsecond.com/pend/shortt/
The long-term ADEV of earth rotation is here:
http://leapsecond.com/museum/earth/
So the performance of a DIY earth disciplined oscillator would be a combination
of the two.
/tvb
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