Relative to Harrison's method of star position measurement and timing, and the possible effects of diffraction thereof:
Hanbury Brown's book "The Intensity Interferometer" may provide some useful insight about various error sources, even though in a different context, that of measurement of a star's angular size. The opening chapter discusses some early attempts at angular size. Galileo apparently made the first really serious attempt, based on how far behind a cord of known diameter one must stand so that the cord *barely* blockes the star's image in the eye. He found a size of 5 arcseconds for Vega, for example. Evidently previous estimates were in the two arc minute range. Hanbury Brown mentions that a pretty detailed description of Galilieo's methods and attention to detail were given in Galileo's "Dialogue Concerning the Two Chief World Systems". It is interesting to note that Galileo's method still yielded a figure some 3 orders of magnitude larger than Vega's currently accepted (as of 1974) angular size of about 3 milli-arc seconds. Hanbury Brown attributed the discrepancy largely to angular scintillation from the Earth's atmosphere. It seems to me that this has to be a noisy factor in the Harrison method, too, and it would be interesting to see more research done on this point. It also seems to me that the diffraction issue would be complicated by the different distances to the two obstacles necessitated by Harrison's approach. Right now I'm beginning to think about whether I have the smarts and computational resources to do some simulations of this situation. Hanbury Brown's book is basically about his invention of the Intensity Interferometer, which is an attempt to go beyond the capability of Michelson's stellar interferometer. The tale spans from HB's original late-night inspiration until the eventual construction and successful operation of a special telescope in Narrabri, Australia. It is a fascinating story winding through HB's own initial self doubts, scientific criticism of the validity of the concept, technical hurdles, bureaucratic hurdles like getting funding, more technical hurdles, etc. Dana On Wed, Mar 27, 2019 at 5:00 AM Brooke Clarke <bro...@pacific.net> wrote: > 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 <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. > > > > > > > _______________________________________________ > 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.
