[LEAPSECS] 2 meetings on UTC and the impending ITU-R RA vote

Michael Deckers michael.deckers at yahoo.com
Mon Jul 11 17:52:20 EDT 2011

On 2011-07-11 12:43, Tony Finch wrote:

> Quartz clocks can be more stable than Earth rotation: see for example

> http://www.ieee-uffc.org/main/history.asp?file=marrison

Thanks for this interesting reference!

> The first outstanding application of the quartz clock to astronomy was

> made in Germany with the installation at the Physikalisch-Technische

> Reichsanstalt. This was described by Scheibe and Adelsberger in 1932 and

> 1934, and reports of its splendid performance continued periodically. It

> was with this installation that it was possible for the first time to

> observe and measure variations in the earth's rate occurring over

> intervals as short as a few weeks. Previous measurements of such

> variations, involving studies of motion of the moon, the planets, and

> Jupiter's satellites, had required years to obtain comparable

> information which, of course, by nature, could never reveal short-term

> factors.

Yes, quartz clocks are much better than pendulum clocks in
interpolating astronomical time between observations. But they
are not good enough to establish an _independent_ time scale to
which astromnomical time could be referred. Their long term
stability is not good enough for that purpose (apparently even
today, see eg McCarthy, Seidelmann: "TIME -- From Earth Rotation
to Atomic Physics", page 151).

> I thought this innovation was one of the reasons for moving to ephemeris

> time as the basis for calibrating clocks, instead of relying on transit

> instruments.

Ephemeris time was introduced because the dynamical ephemeris (of
the Moon mostly) at the time showed that its time coordinate
differed from universal time, and this could no longer be ascribed
to defects in the theory. Quartz clocks have been instrumental
(since around 1937) in showing the variations of the length of day,
which are the cause of the difference between UT and ET. And
since the length of the second of UT in the early 19th century
is not an easily reproducible definition, the second was redefined
in 1954 using the rate of ET at 1900.0 (which is not so easy to
reproduce either).

But ephemeris time was deduced from astronomical observations of the
Moon, not from quartz clocks. (Even today, astrometric observations
of inertial time are not superfluous -- they reveal a slight
difference between the rates d(TT) of TT and d(TAI) of TAI.)

Michael Deckers.

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