  Astrometric calibration is based on patterns of cataloged stars and therefore
effectively assumes a particular epoch, which can be substantially incorrect
for historical images. With the known proper motions of stars we can "run back
the clock" to an approximation of the night sky in any given year, and in
principle the year that best fits stellar patterns in any given image is an
estimate of the year in which that image was taken. In this paper we use 47
scanned photographic images of M44 spanning years 1910-1975 to demonstrate this
technique. We use only the pixel information in the images; we use no prior
information or meta-data about image pointing, scale, orientation, or date.
Blind Date returns date meta-data for the input images. It also improves the
astrometric calibration of the image because the final astrometric calibration
is performed at the appropriate epoch. The accuracy and reliability of Blind
Date are functions of image size, pointing, angular resolution, and depth;
performance is related to the sum of proper-motion signal-to-noise ratios for
catalog stars measured in the input image. All of the science-quality images
and 85 percent of the low-quality images in our sample of photographic plate
images of M44 have their dates reliably determined to within a decade, many to
within months.

Astrophysics