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 <t...@leapsecond.com> 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 > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
