Sidereal Daze
About fifty years ago, astronomers determined
and defined the exact length of the sidereal year for 1900.0 (and
consequently the length of the tropical year for 1900.0) through
precise transit measurements of certain fixed stars. Evidently,
the length of the sidereal year can only be calculated by exactly
measuring and determining the mean rotation rate of the Earth on
its axis. When precession was applied to the absolute spin period
of the Earth, the so-called mean sidereal or tropical day of 86164.09054
seconds (86164.0905382 s exact) not only became a primary astrodynamic
constant [http://ssd.jpl.nasa.gov/astro_constants.html]
but also the absolute criterion for time.
Since the usage of the word 'sidereal' is not
consistent in astronomy, we shall also use the unambiguous term
"Galilean day" [http://home.att.net/~numericana/answer/units.htm#day]
when motion with respect to the fixed stars is meant. The duration
of the Galilean day is 86164.0996612 seconds (i.e. about 9.12 ms
longer than the mean sidereal day), according to the astronomical
data published at http://hpiers.obspm.fr/eop-pc/models/constants.html#rotation
(sidereal year, 365.256362268* mean solar days of 86400 s; general
precession in longitude, 5028.792" per century).
* ( 365.256362268 × 86400 ÷ (365.256362268
+ 1) = 86164.0996612 )
While the Observer's Hand Book of the Royal Astronomical
Society of Canada defines the mean rotation period of the Earth
(fixed star to fixed star) with 86164.101 seconds, the latest technological
advances in VLBI observations provide us with a somewhat different
length for the sidereal or Galilean day. In fact, the International
Earth Rotation Service declares that the conventional duration of
the sidereal day is exactly 86164.09890369732 seconds [http://hpiers.obspm.fr/eop-pc/models/constants.html#rotation].
Astronomers, who took a Galilean day of 86164.0996612
s and applied the constant of precession to determine the mean sidereal
day, might be very surprised to learn that the Galilean day is said
to increase by about 0.00164 s per century, supposedly due to the
braking effects of tides. But even more surprising is the result
that somehow over the last 50 years the rate of the general precession
in longitude would have dramatically decreased by more than 4",
although the tropical day or the primary astrodynamic constant of
86164.0905382 s has remained unchanged.
Astronomers, however, are trying to get rid of
this annoying equinoctial period once and for all [http://aa.usno.navy.mil/faq/docs/ICRS_doc.html].
But as long as the mysterious "dark matter" and "dark
energy" in the cosmos keeps puzzling
them, there is still hope: "Let there be light".
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