Re: GJD to HJD

[aah@nofs.navy.mil]

I am probably misunderstanding Chris.  He states:
>It would appear that
>the JD -> HJD conversion is the largest source of timing uncertainty.
>Our computer clocks are very easy to keep to <<1 second.
>I'd say that unless we can get the error introduced by the conversion
>to HJD two orders of magnitude below other sources of timing error
>we keep UTC for the archive.  I know we can live with a few seconds
>of error but it is just plain embarrassing that our data reduction
>process should be the largest source of error in a measurement.
  The largest sources of experimental error remain the computer
clock setting, the knowledge of the shutter open/close times, and
the time smearing due to the finite exposure length.
  Using SLALIB, you should be able to get the GJD->HJD conversion
to under a second.  This conversion is important, as you will get
+/- 8minutes timing error due to the size of the earth's orbit.
Again, for long-period variables, an 8-minute error might not be
important, but for a 2hr eclipser, it can mean the difference between
seeing an eclipse and missing it.
  The difference between assuming the observer is at the center of
the earth or somewhere on the surface is +/- 0.02sec, below the
sources of other error in the calculation.  Ignore it.
  Using UTC in the database is fine by me, but just remember that
you *have* to apply a heliocentric correction to the data before
using it for analysis.
Arne