[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 
), 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/ 
).

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 
). 
Folks may be interested in other papers and presentations from this workshop: 
http://hanksville.org/futureofutc/  
(replace “www.cacr.caltech.edu ” with 
“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
 
).
 For many more topics pertaining to UTC and leap seconds, see 
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
 
).
 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 
), 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 
). 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 
), 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 
), 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 
) …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-1-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 
)
 that could be used for such. But, of course, the various GNSS constellations 
already address this requirement (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/ 

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

[Steve Allen]

On Tue 2021-12-28T14:40:43+ Poul-Henning Kamp hath writ:
> You want a bright star as close to your latitudes Zenith as possible,
> to get maximum apperant transit velocity.

Most precise time determination in history was by meridian telescopes
which only had one axis of motion not counting the ability to reverse
the tube as a way of checking for systematic errors.
Near the zenith was best for the meridian telescopes.

The Danjon "Impersonal" Astrolabe with its prism and mercury bath
allowed selection of targets looking around the sky rather than
waiting for something near overhead.  The Danjon Astrolabes were
widely deployed as part of preparations for the International
Geophysical Year.  That allowed dozens of folks to take up
paid residence in funky places like Curacao for a year.
The data from the IGY then took a decade to reduce and publish.
At the point when the optical astronomy measures of the IGY were
published all of the techniques which had been used were obsolete.
(That was true of much of the rest of the IGY data because of
the advancement of technology during the late 1950s and early
1960s.)

The optical measurements suffered from the "personal equation" which
was a systematic offset of how much each observer tended to be early
or late.  Many of the optical measurements were done by pulling a
trigger.  Some has a screw to be turned to match the motion of the
star across the zenith and then measure by looking at the chart
recording of the potentiometer.

Stars pretty much at the zenith were required for Photographic Zenith
Tubes.  PZTs were unable to compare their results with anyone else
because each one had a unique list of target stars.  They were the
most accurate optical measure of earth rotation.

The evolution of precision of time measured by optical observatories
was plotted by the Stoykos as they prepared to retire from BIH.

https://www.ucolick.org/~sla/leapsecs/BHsHn05p142.html

Note that mean difference between the best observatories never fell as
low as a millisecond, and that was after the BIH had removed the
persistent systematic differences which resulted from the fact
that the observatories used conventional longitudes which were not
self consistent.

Note that the random error fell about as low as 0.4 millisecond for
the best observatories.

The longitudes and latitudes of everything changed in 1968 as the
new recommendations for terrestrial reference frame were implemented.
The USNO actually, and finally, removed the about 0.03 s offset to UT
that had been in all US measures of time since the inception of USNO.

Of course for non-stationary observatories the 18th through 20th
century measurement to determine the offset of the chronometer was
using a sextant to shoot a lunar and determine the time based on the
angle between moon and star.

--
Steve Allen  WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
1156 High Street   Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064   https://www.ucolick.org/~sla/  Hgt +250 m
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[time-nuts] Re: Derivation of time from celestial sight

[Poul-Henning Kamp]

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".


-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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[time-nuts] Re: OCXO Oven design (was: E1938A phase noise improvement)

[Magnus Danielson via time-nuts]

Hi,

On 2021-12-26 23:38, Attila Kinali wrote:

On Sun, 26 Dec 2021 15:54:06 -0500
Bob kb8tq  wrote:


The market is what dictates how fancy an OCXO gets made. Bottom
line is that there really isn’t that big a market (and willingness to pay
for super duper TC). If indeed you could do all the fancy stuff and
still keep the sell price below $10 then who knows ….

I wonder about that. I was told by an ex-manager of
Oscilloquartz, that their biggest problem with the 8607
was that the Option 08 sold too well. So much, that
they had a huge over-supply of the other, lesser
versions of the 8607, to the point that even raising
the price of the Option 08 beyond what a car cost
didn't recover its cost.

Sure, such a high stability oscillator doesn't have a
mass market. And it's definitely not a comodity item.
But there seems to be decent market even if its very
expensive.


Well, within some very small market segment it may feel unlimited. Trust 
me when I say I wish I could fit 8607s into my boxes, but they would not 
fit the price-range, size and to some degree power options. There is 
actually quite many oscillators made today that also do not fit the bill.


There isn't one market. There is actually several parallel market 
segments, each with their own quirks and logics to them. The oscillators 
is put in different environments for various reasons.


I see increased market for wider temperature range, as more devices 
needs to operate in cars, so outside of 0 to 70 C range into -40 to +85 
C. That does only benefit a small class of oscillators market and 
feature wise.


Another thing we see is that synthesizer chips take over. We use less 
odd frequency oscillators today in our designs. That also changes the 
market.


So what may be true for Oscilloquartz and their very narrow 
customer-range, does not apply to other uses, and I am also in telecom 
timing just as them.


Cheers,
Magnus

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

[Lux, Jim]

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.


Occultation of stars by the Moon provides a "universal" time source 
(assuming you can see the Moon and stars). It was the competitor to 
Harrison's clock based approach to timekeeping for the measurement of 
longitude. For stars sufficiently far away, the Moon passes in front of 
it at the same time everywhere on Earth. There are some problems - the 
Moon has a rough edge, etc.  And, of course, you need accurate 
ephemerides for the Moon and accurate celestial position of the stars.


Observations of a star can only give you local time; you need two things 
moving at different speeds to get the time at a specific longitude.





Some theodolite manufacturers provided attachments to aid the process (for
the high zenith where a theodolite experiences reduced accuracy), and those
attachments were dated and calibrated for their year of manufacture and
came with tables for use in future years.

That's about all I know or can find on the subject.  Can anyone here point
me to any published literature?  Anyone have experience trying?  Any idea
what type of accuracy can be expected?


There is a CD-ROM published by ION (Institute of Navigation) with 
hundreds of papers on such things.


https://www.ion.org/publications/upload/CelestialNavTOC.pdf

https://www.ion.org/publications/order-publications.cfm

$50 worth of reading on all manner of topics navigation and time related 
- Papers on "how Vikings navigated" "how Columbus navigated" (and papers 
refuting the theories in the previous papers, etc.)






Got some new toys coming and need something to do with them



And, of course, you can get yourself a copy of the Nautical Almanac and 
some sight reduction tables (get the airplane ones, not the ship ones) 
and do some celestial nav. You can make an artificial horizon with a pan 
of water at some distance, so you can sight your object of interest and 
the reflection at the same time.




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

[Bruce Griffiths]

The Danjon astrolabe typically achieved a timing precision of around 25millisec 
or so. 
Manually timing transits with a stopwatch will be better than a second but 
somewhat worse than the Danjon astrolabe.

Bruce
> On 28/12/2021 22:42 David Taylor via time-nuts  
> wrote:
> 
>  
> On 27/12/2021 20:18, 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.
> 
> My previous answer mentioned precision, of course, not accuracy.  And the 
> error 
> I mentioned was not the error in GPS which places the zero degree meridian 
> some 
> 102m east of Greenwich!
> 
> David
> -- 
> SatSignal Software - Quality software for you
> Web: https://www.satsignal.eu
> Email: david-tay...@blueyonder.co.uk
> Twitter: @gm8arv
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[time-nuts] Re: Derivation of time from celestial sight

[Attila Kinali]

On Mon, 27 Dec 2021 15:18:56 -0500
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.  

Yes, this has been the way how time was defined until the adoption of
the new second in 1967. And countries used it to derive their official
time way into the late 70s. Still today, our time is bound to celestial
time. The leapseconds we get every now and then, are to adjust our atomic
time to match our rotational time.

The institution that does coordinate this today is the International
Earth Rotation Service (IERS https://www.iers.org ). They use the
International Celestial Reference System (ICRS 
https://www.iers.org/IERS/EN/Science/ICRS/ICRS.html),
a set of stars that are far enough to be virtually fixed in the sky, yet bright 
enough
to be easily observed.

The observations are done by various astronomical institutions around the world 
that
maintain meridian telescopes, i.e. telescopes that are fixed on the plane of 
their
meridian and only rotate on one axis. The IERS then integrates these 
measurements and
prepares a report every half year onto whether a leap second is needed and if 
it is, what
direction it should be.

The IERS website contains quite a few documents on the whole process but you 
will
have to do some digging to find the ones that describe what you are looking for.

Attila Kinali

-- 
Science is made up of so many things that appear obvious 
after they are explained. -- Pardot Kynes
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[time-nuts] Re: Derivation of time from celestial sight

[David Taylor via time-nuts]

On 27/12/2021 20:18, 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.


My previous answer mentioned precision, of course, not accuracy.  And the error 
I mentioned was not the error in GPS which places the zero degree meridian some 
102m east of Greenwich!


David
--
SatSignal Software - Quality software for you
Web: https://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv
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[time-nuts] Re: Derivation of time from celestial sight

[David Taylor via time-nuts]

On 27/12/2021 20:18, 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.

Some theodolite manufacturers provided attachments to aid the process (for
the high zenith where a theodolite experiences reduced accuracy), and those
attachments were dated and calibrated for their year of manufacture and
came with tables for use in future years.

That's about all I know or can find on the subject.  Can anyone here point
me to any published literature?  Anyone have experience trying?  Any idea
what type of accuracy can be expected?

Got some new toys coming and need something to do with them

Brent


Brent,

You might find something like these of interest:

  http://www.thegreenwichmeridian.org/tgm/articles.php?article=6

  https://www.rmg.co.uk/stories/topics/airys-transit-circle-dawn-universal-day

  http://www.royalobservatorygreenwich.org/articles.php?article=1087

There is a possibly apocryphal tale that some students at Cambridge did this in 
the late '60s or early '70s and getting about a second or two accuracy, and 
discovering that the longitude of the Cambridge Observatory was some hundred 
metres out.  You might find a reference, and I might be mistaken!


Cheers,
David
--
SatSignal Software - Quality software for you
Web: https://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv
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[time-nuts] Re: Derivation of time from celestial sight

[Hal Murray]

brent.ev...@gmail.com said:
>  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 think Polaris is a bad example, actually worst case.  You want something 
close to the equator so its position changes as the Earth rotates.

This is the same idea as using GPS for timing.  If you know where you are 
located, you can reverse the calculations to get the time.


> Any idea what type of accuracy can be expected?

How good is your telescope?  and the mount?

The unit is arc-seconds.

The moon is 1/2 degree in dia.  So if you want 1 second of accuracy, you need 
to measure the position of your chosen star to 1/7200 the diameter of the moon.

The USNO used to be the national source of time.  Google found:
> Visual Transit Circle telescopes are being replaced by newer instruments
> capable of determining stellar positions to an accuracy of 0.01 arcseconds.
> USNO ...
(but their web server didn't give me that page)

Somewhere down at that level of detail you run into problems with the 
instability of the atmosphere.


I think there was a thread several years ago on tracking the sun with 
something as simple as a camera connected to a computer.


-- 
These are my opinions.  I hate spam.


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

[Bruce Griffiths]

Surely Polaris is a spectacularly poor choice for obtaining accurate Sidereal 
time via a meridian transit of a known star?
A theodolite suited to high altitude observations like the Wild T4 was is 
desirable.

Otherwise a prism and a mercury mirror 9or equivalent) can be used to covert a 
theodolite into a prismatic astrolabe.

Bruce.

> On 28/12/2021 09:18 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.
> 
> Some theodolite manufacturers provided attachments to aid the process (for
> the high zenith where a theodolite experiences reduced accuracy), and those
> attachments were dated and calibrated for their year of manufacture and
> came with tables for use in future years.
> 
> That's about all I know or can find on the subject.  Can anyone here point
> me to any published literature?  Anyone have experience trying?  Any idea
> what type of accuracy can be expected?
> 
> Got some new toys coming and need something to do with them
> 
> Brent
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[time-nuts] Derivation of time from celestial sight

[Brent]

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.

Some theodolite manufacturers provided attachments to aid the process (for
the high zenith where a theodolite experiences reduced accuracy), and those
attachments were dated and calibrated for their year of manufacture and
came with tables for use in future years.

That's about all I know or can find on the subject.  Can anyone here point
me to any published literature?  Anyone have experience trying?  Any idea
what type of accuracy can be expected?

Got some new toys coming and need something to do with them

Brent
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[time-nuts] Re: Where do people get the time?

[Demetrios Matsakis via time-nuts]

This line in Thomas Erb’s email prompts me to point out that the power 
companies twice tried to eliminate the requirement to keep the Time accurate to 
UTC (2011 and more recently).  According to the FERC’s summary in 2020 they 
denied the most recent petition because two people wrote letters as private 
citizens.  I was one and Jonathan Hardis of NIST was the other. See 
https://elibrary.ferc.gov/eLibrary/idmws/file_list.asp?accession_num=20200204-3048
 


I’m not trying to brag - more like I’m worried they try this again in a few 
years, and perhaps no one will notice to speak out.  These things are announced 
by the FERC, and I was on a mailing list with a key-word search, although I let 
it lapse.  Maybe one of you might want to take it up.

Demetrios

P.S.  I am attaching a PTTI paper we wrote, along with Blair Fonville.  The 
paper itself just described the situation, and did not take a stand.  I don’t 
think the USNO monitors the situation any more.

> On Dec 27, 2021, at 7:01 AM, Thomas D. Erb  wrote:
> 
> 
> The power company (still does) keeps  line frequency accurate to a time 
> standard.  Henry Warren got this standardized - I believe Tesla proposed it.
> 
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