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We present a study of variable faint optical sources discovered by comparingthe Sloan Digital Sky Survey (SDSS) and the Palomar Observatory Sky Survey(POSS) catalogs. We use SDSS measurements to photometrically recalibrateseveral publicly available POSS catalogs; a piecewise recalibration in 100arcmin2 patches generally results in an improvement of photometric accuracy(rms) by nearly a factor of two, compared to the original data. The POSS Imagnitudes can be improved to ~0.15 mag accuracy, and POSS II magnitudes to\~0.10 mag accuracy. We use the recalibrated catalogs for the ~2,000 deg2 ofsky in the SDSS Data Release 1 to construct a catalog of ~60,000 sourcesvariable on time scales 10-50 years. A series of statistical tests based on themorphology of SDSS color-color diagrams, as well as visual comparison of imagesand comparison with repeated SDSS observations, demonstrate the robustness ofthe selection methods. We quantify the distribution of variable sources in theSDSS color-color diagrams, and the variability characteristics of quasars. Wedetect a turn-over in quasar structure function which suggests that thecharacteristic time scale for quasar variability is of the order one year. Thelong-term (>1 year) quasar variability decreases with luminosity and rest-framewavelength similarly to the short-term (<1 year) behavior. We also demonstratethat candidate RR Lyrae stars trace the same halo structures, such as the Sgrdwarf tidal stream, that were discovered using repeated SDSS observations. Weutilize the POSS-SDSS selected candidates to constrain the halo structure inthe parts of sky for which repeated SDSS observations do not exist. (abridged)

Variable Faint Optical Sources Discovered by Comparing the POSS and SDSS Catalogs