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

[Attila Kinali]

On Mon, 27 Dec 2021 15:18:56 -0500
Brent <brent.evers at gmail.com> 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