Mark IV data off by one day

[Stupendous Man <richmond@stupendous.cis.rit.edu>]

  The affair of GSC 3493-1324 suggested that at least one
night's worth of Mark IV data has a date off by exactly one day.
I checked this in a quick way by looking at Mark IV measurements
of another variable star, AR Her, which is an RR Lyr variable
with period of 0.47 days, close to GSC 3493-1324 in the sky.
I used SIMBAD to look up the ephemeris of AR Her, phased
the Mark IV data with that ephemeris, and plotted the points.
The Mark IV data show a periodic curve, but it isn't as
tight as I'd like.  

  The two measurements made on the same night as the disputed
GSC 3493-1324 measurements -- JD 2453109.8 -- did indeed lie
off the main locus by about 0.10 in phase.  When I changed
the date to JD 2453110.8, they lined up better with
the main locus.  This adds weight to the hypothesis that
the computer's clock was off by one day on that night.

  Hmmm.  This brings up the unpleasant possibility that there
could be more occasions on which the Mark IV computer's date
was off by one day.  I'm not sure how best to look for other
such errors.  It might be as easy as checking the markings
on various disks or files which Tom created against the dates
stored in the Mark IV database -- that would take an hour or
two, I would guess, if the information were gathered in one
place.  Another way to do it, much harder, would be to look
at a sample of 10 or 20 good variable stars and examine their
phased Mark IV light curves for points which fall far from
the locus.  If several stars share similar bad points on the
same dates, and if changing the date improves the light
curves, then that might indicate another error.

                                              Michael