A Precision Multi-Band Two-Epoch Photometric Catalog of 44 Million
  Sources in the Northern Sky from Combination of the USNO-B and Sloan Digital
  Sky Survey Catalogs
  A key science driver for the next generation of wide-field optical and radio
surveys is the exploration of the time variable sky. These surveys will have
unprecedented sensitivity and areal coverage, but will be limited in their
ability to detect variability on time scales longer than the lifetime of the
surveys. We present a new precision, multi-epoch photometric catalog that spans
60 years by combining the USNO-B and SDSS Data Release 9 catalogs. We
recalibrate the photometry of the original USNO-B catalog and create a catalog
with two epochs of photometry in up to five different bands for 43,647,887
optical point sources that lie in the DR9 footprint of the northern sky. The
recalibrated objects span a magnitude range 14 < m < 20 and are accurate to
$\approx$ 0.1 mag. We minimize the presence of spurious objects and those with
inaccurate magnitudes by identifying and removing several sources of systematic
errors in the two originating catalogs, with a focus on spurious objects that
exhibit large apparent magnitude variations. After accounting for these
effects, we find $\approx$ 250,000 stars and quasars that show significant (>
4$\sigma$) changes in brightness between the USNO-B and SDSS DR9 epochs. We
discuss the historical value of the catalog and its application to the study of
long time-scale, large amplitude variable stars and quasars.
