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

[Tom Van Baak]

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