[time-nuts] Re: Derivation of time from celestial sight

[Brent]

All -

Thank you all for the fantastic feedback.  Tons of stuff to go through now!

Brent

On Tue, Dec 28, 2021 at 1:54 PM Rob Seaman <robertlewisseaman2 at gmail.com>
wrote:

> As Poul-Henning says, Polaris would be a poor choice, as would indeed any
> stars or other objects fixed to the celestial sphere since their
> differential motion is (very) slight. The issue isn’t measuring the rate,
> but establishing a zero point, otherwise one day, indeed one year, is very
> like the ones before or after. That said, spatial coordinates are
> fundamentally tied to astronomical observations of quasars via very long
> baseline interferometry. Space and time coordinates are intertwined (
> https://www.ivoa.net/documents/latest/STC.html <
> https://www.ivoa.net/documents/latest/STC.html>), and precision
> observations of pulsars can similarly be used to establish a long term
> temporal baseline (
> https://www.iau.org/science/scientific_bodies/working_groups/304/ <
> https://www.iau.org/science/scientific_bodies/working_groups/304/>).
>
> The solar system provides innumerable cycles for establishing absolute
> time, whether sidereal (stellar) or synodic (solar) (
> http://hanksville.org/futureofutc/preprints/files/28_AAS_13-515_Seaman.pdf
> <
> http://hanksville.org/futureofutc/preprints/files/28_AAS_13-515_Seaman.pdf>).
> Folks may be interested in other papers and presentations from this
> workshop: http://hanksville.org/futureofutc/ <
> http://hanksville.org/futureofutc/> (replace “www.cacr.caltech.edu <
> http://www.cacr.caltech.edu/>” with “hanksville.org <
> http://hanksville.org/>”, if you get redirected to the retired Caltech
> server) and many of the transcribed discussions are interesting in their
> own right (
> http://hanksville.org/futureofutc/preprints/files/29_AAS%2013-515discussion.pdf
> <
> http://hanksville.org/futureofutc/preprints/files/29_AAS%2013-515discussion.pdf>).
> For many more topics pertaining to UTC and leap seconds, see
> https://ucolick.org/~sla/leapsecs/ <https://ucolick.org/~sla/leapsecs/>
>
> An astronomer might quibble over the meaning of absolute time (
> https://galison.scholar.harvard.edu/publications/einsteins-clocks-poincarés-maps
> <
> https://galison.scholar.harvard.edu/publications/einsteins-clocks-poincar%C3%A9s-maps>).
> Observations of Earth’s moon and of the Galilean moons of Jupiter were used
> in the 18th Century as described in Dava Sobel’s excellent book “Longitude”
> (http://www.davasobel.com/books-by-dava-sobel/longitude <
> http://www.davasobel.com/books-by-dava-sobel/longitude>), along with some
> more creative timekeeping attempts before Harrison #4. I presume most here
> are familiar with Dumbledore’s role in the history of clocks (
> https://www.imdb.com/title/tt0192263/?ref_=nv_sr_srsg_2 <
> https://www.imdb.com/title/tt0192263/?ref_=nv_sr_srsg_2>). Poul-Henning
> mentions latitude and I’m not sure if this is what he meant or rather the
> International Latitude Service (
> https://adsabs.harvard.edu/full/2000ASPC..208..147Y <
> https://adsabs.harvard.edu/full/2000ASPC..208..147Y>), which is indeed an
> interesting historical tale in its own right.
>
> One can use asteroid ephemerides as a clock (
> https://link.springer.com/chapter/10.1007/978-3-319-59909-0_36 <
> https://link.springer.com/chapter/10.1007/978-3-319-59909-0_36>), though
> the need for precision orbital elements fights the need for rapid motion.
> Asteroid surveys detect (and redetect) tens of thousands of asteroids
> nightly. Comparisons against their predicted positions can be summed to
> arrive at fairly precise timing, eg, “this pattern of objects can only
> correspond to one specific absolute time”.
>
> At the opposite end of the scale, the 10,000 Year Clock will use daily
> solar observations (perhaps separated by decades due to waiting for sucker
> holes in some post-apocalyptic death shroud of clouds) to keep within +/- 5
> minutes of mean solar time (https://arxiv.org/pdf/1112.3004.pdf <
> https://arxiv.org/pdf/1112.3004.pdf>) …assuming humans don’t perturb the
> Earth enough to make the Equation of Time Cam inaccurate (
> https://longnow.org/ideas/02018/12/05/the-equation-of-time-cam-keeping-good-time-for-10000-years/
> <
> https://longnow.org/ideas/02018/12/05/the-equation-of-time-cam-keeping-good-time-for-10000-years/
> >).
>
> Observations of artificial satellites offer significantly more precision
> since they are much closer to the observer, thus not only brighter and
> easier to centroid, but move rapidly across the sky (though tracking rapid
> objects is itself a challenge). The advent of satellite mega-constellations
> in low Earth orbit will engender new infrastructure, including precision
> ephemerides (
> https://www.space.com/sathub-idea-threat-satellite-megaconstellations-astonomy
> <
> https://www.space.com/sathub-idea-threat-satellite-megaconstellations-astonomy>)
> that could be used for such. But, of course, the various GNSS
> constellations already address this requirement (
> https://projectpluto.com/gps_expl.htm <
> https://projectpluto.com/gps_expl.htm>).
>
> Greenwich Observatory played a fundamental role in the long history of
> transmission of time signals (
> https://blog.sciencemuseum.org.uk/ruth-belville-the-greenwich-time-lady/ <
> https://blog.sciencemuseum.org.uk/ruth-belville-the-greenwich-time-lady/>).
> To those wondering in other threads whether such topics are appropriate for
> Time-Nuts, I can only speak for myself that the engineering of large
> systems of systems for conveying time signals most certainly does seem a
> topic for this mailing list, whether via computer networks, cell phones,
> radio signals from the ground or orbiting satellites, or indeed, carried in
> a pocket, on our wrists, or in the back of an SUV (
> http://leapsecond.com/great2016a/ <http://leapsecond.com/great2016a/>). I
> invite Tom back any time to our observatory to repeat his experiment!
>
> Rob Seaman
> Lunar and Planetary Laboratory
> University of Arizona
> --
>
> > On Dec 28, 2021, at 7:40 AM, Poul-Henning Kamp <phk at phk.freebsd.dk>
> wrote:
> >
> > --------
> > Lux, Jim writes:
> >
> >> On 12/27/21 12:18 PM, Brent wrote:
> >>> My understanding (and I could be wrong) is that one could derive
> 'stellar'
> >>> time from a start sight/fix on polaris or another well tracked
> celestial
> >>> object.  I was once told that early editions of Bowditch provided the
> >>> process (for the moon I was told) although one of the relatively old
> >>> edition's that I have doesn't provide it.
> >
> > You want a bright star as close to your latitudes Zenith as possible,
> > to get maximum apperant transit velocity.
> >
> > Polaris would be a spectacular bad choice as it barely moves at all.
> >
> >> Occultation of stars by the Moon provides a "universal" time source
> >> (assuming you can see the Moon and stars).
> >
> > Interesting history search term: "Latitude observatory".
>
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