[LEAPSECS] Question about UT1 and the IERS Reference Meridian

[Michael Deckers]

   On 2015-04-30 01:01, Mike Lawson wrote:

> So, what I take from this is, 1) UT1 (and hence UTC) is based on the
> International Reference Meridian, not the Prime Meridian (Greenwich).

   No and yes.

   In 2000, the IAU has decided that UT1 (since 2003) measures the angle
   between the "Terrestrial Intermediate Origin" TIO and the "Celestial
   Intermediate Origin" CIO. Both points lie on the equator of the intermediate
   pole of rotation (CIP), whose position moves both relative to the Earth's
   surface and the celestial sphere. The TIO also moves on the Earth, and
   even though this method of determination of UT1 is called the "method of
   non-rotating origins", the points TIO and CIO _do_ rotate secularly.
   (The reason for this choice of the TIO is the decision that rotations
   of the CIP on the Earth should not affect the Earth rotation angle.
   The disadvantage is that the value of UT1 at any time depends on
   all prior movements of the CIP.)

   So, conceptually, UT1 is _not_ based on the International Reference
   Meridian, but on the point TIO that differs from it by the
   "TIO locator" s' which increases over time.

   On the other hand, the rotation of the TIO on the Earth is very slow,
   about 15 µm/a on the equator. Hence the IAU could state that
     "UT1 is considered to be nominally equivalent
      to mean solar time reckoned from midnight on the
      meridian of Greenwich".
   Thus, numerically, you are right.

   While the IERS Conventions describe all this at length and in all
   numerical detail, the concepts involved and the underlying geometry
   are better explained in other sources. An excellent short account by
   Nicole Capitaine and Patrick Wallace is at
   [iau-c31.nict.go.jp/pdf/2009_IAUGA_JD6/JD06_capitaine_wallace.pdf]

   Michael Deckers.