seconds-after-J2000.0

[hal@MURREN.AI.MIT.EDU (Hal Abelson)]

   Posted-From: The MITRE Corp., Bedford, MA
   From: John D. Ramsdell <ramsdell%linus@mitre-bedford.ARPA>
   Posted-Date: Wed, 15 Jun 88 10:46:52 EDT
   Date: Wed, 15 Jun 88 10:46:52 EDT

   I propose adding the function seconds-after-J2000.0.

   (seconds-after-J2000.0)					procedure
   seconds-after-J2000.0 returns a real number giving the number of
   seconds after Noon of January 1, 2000.  J2000.0 is the Julian epoch of
   the time origin.


I object to this function on the grounds that it is woefully
underspecified: do you mean UT0 (siderial time corrected to mean solar
time), UT1 (UT0 corrected for polar movement), or UT2 (UT1 corrected
for seasonal variations)?  All are subject to variations from
deceleration and uneveness of the earth's rotation.  Or perhaps you
mean ephemeris time?  Maybe you can fix this with appropriate optional
arguments.

Even so, will be dificult to implement this in a way that admits a
complete, unambiguous specification for Scheme.  Scheme programmers
who use this procedure on high-velocity spacecraft or near black holes
may encounter different timing behavior due to relativistic effects
and therefore have difficulty sharing their programs with people on
Earth.  It would be difficult to prove any theorems about Scheme
programs that use this procedure.

How do the denotational semantics people handle calendar time, since
we all know that anything they do is obviously the right decision in
language design?

 
-- Hal