For USNO NOMAD catalog stars, the maximum proper motion is of the

order of 2 arcseconds per millenium. Most of the times, we're running
the simulation at the current time, which is not further than a few
decades from the epoch. So the proper motion corrections for all USNO
NOMAD stars is less than an arcsecond. The error in cataloged
coordinates is probably this large anyway. By ignoring the proper
motion corrections when the proper motion distance is less than 1
arcsecond, we save a good deal of computation time, as is shown by
cache profiling.

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