BLIND DATE: USING PROPER MOTIONS TO DETERMINE THE AGES OF HISTORICAL IMAGES

Abstract

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 the years 1910–1975 to demonstrate this technique. We use only the pixel information in each image; we use no prior information or metadata about image pointing, scale, orientation, or date. Blind Date returns date metadata for each input image. 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 90% of the low-quality images in our sample of photographic plate images of M44 have their dates reliably determined to within a decade, while 75% of our science-quality images have their dates reliably determined to within 3 years. This method has the potential to insert “lost” data into incomplete sets of time-domain imaging, improving constraints from historical data on transients, variable stars, stellar motions, and minor planet orbits.