[LEAPSECS] UT1 offset

[Warner Losh]

On Wed, Dec 27, 2023 at 6:04 AM Jim Lux <jimlux at earthlink.net> wrote:

> Let’s back of the envelope the impact of a 1 second error in a longitude
> sight.
> The Sun moves 360 degrees in 86400 seconds. A one second error is then
> about 0.004 degree.  But in equatorial km, let’s assume 40,000 km
> circumference, so 40,000 km in 86,400 seconds (yeah, it’s actually less,
> sidereal day and all that). Or an error of about 1/2 km.
>
> For the vast majority of boat driving purposes, this is sufficiently
> accurate - you need to get to within visual range of the coast, so you can
> get into the desired port.
>

This does mean, though, that there will need to be a notion of DUT1 to a
second in the future for people wishing to navigate like this. Right now
that term is 0. That implies a 100ms DUT1 error gives a navigation error of
about 50m, which is likely why it's never been broadcast better than that,
because then other errors in measurement will become a factor and all the
high-precision navigation is done by GPS or similar. The demand side for
higher precision is low, even if for some applications its need is high.


> For flying a plane by celestial navigation, you might care more, because
> the plane is flying faster than a boat moves.
>

For flying a plane, though, you have a series of different flight modes
that require either clear skies so you can do the same as above "steer
around the big mountain" or you can rely on your instruments to be exactly
where you need to be because you can't see and/or there's too much traffic.
The old VFR vs IFR divide. You care more in a plane, but there's several
layers that you can use. to mitigate the errors in the 'simpler' ones.
Still, even 50m can be a lot if you are flying over a narrow mountain pass
in a plane that can't just fly super-high above it...

Warner


> > On Dec 26, 2023, at 7:10 AM, Gary E. Miller <gem at rellim.com> wrote:
> >
> > Yo Hal!
> >
> >> On Mon, 25 Dec 2023 00:25:07 -0800
> >> Hal Murray <halmurray+leapsecond at sonic.net> wrote:
> >>
> >> Who uses DUT1 via radio?  Who will be using it 50 years from now?
> >>
> >> Is it needed for anything other than navigation and astronomy?
> >
> > I just asked my brother that did a lot of transoceanic navigation
> > by sextant.  He did not even know what UT1 is.  He certainly would
> > not know what to do with DUTs.
> >
> > RGDS
> > GARY
> >
> ---------------------------------------------------------------------------
> > Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
> >    gem at rellim.com  Tel:+1 541 382 8588
> >
> >        Veritas liberabit vos. -- Quid est veritas?
> >    "If you can't measure it, you can't improve it." - Lord Kelvin
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