On 7/9/2015 9:14 PM, Jeff Liebermann wrote:
On Fri, 10 Jul 2015 01:37:30 +1000, Clifford Heath
wrote:

The earth's slowing is also somewhat chaotic, inasmuch as equatorial
weather affects the sea-level heights, which introduces noise into the
earth's angular moment of inertia, and hence its rate of rotation. That
has nothing however to do with how we know we're measuring time accurately.

It might help to mention that we have two types of time accuracy. One
is sidereal time, where 12AM on Jan 1 is astronomically correct and is
used to aim telescopes on earth.

Nope. Sidereal time is different from civil (solar) time. The Earth
rotates 365 and change times per year with respect to the Sun, but 366
and (the same) change with respect to the fixed stars. So the two get
out of phase pretty fast.

This is where we says "at the tone,
the time will be... (beep). The other is the length of 1 second,
minute, hour, day... year which is a numerical count of how many
wavelengths of light or cycles of atomic gigahertz
vibrations pass during these intervals also known as atomic time.



The problem is that the two systems don't quite coincide. The current
difference between UTC and International Atomic Time (UTC-TAI) is now
36 sec and growing. The recent leap second just made things worse.

Well, worse if you don't think that the Gregorian reform was an advance.
Pretty soon the vernal equinox would have been in February.
Personally I think that civil time is more important than atomic time.
Folks who need to know the difference, do.

If we knuckle under to atomic time in civil life, our version of the
Julian problem is that midnight by the clock will soon start occurring
at sundown. The leap second inconvenience principally affects software
developers (and those who trust them).


The fun starts when tracking spacecraft in otter space. Not only does
one have to deal with relativistic effects, but one also has to use a
time system that is independent of how the earth spins, wobbles, and
thrashes around. It would be a major disaster if a leap second were
thrown into the timing if you're tracking a spacecraft such as Voyager
1 moving at 17 km/sec (38,000 mph).

You'd be off by 17 km. Is Voyager 1's position known to that accuracy?
Didn't think so.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net