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

[Rob Seaman]

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".