The Ancient Art of Astrometry

Abstract

The best hope of finding Earth-like planets around Sun-like stars lies not in Doppler spectroscopy, stellar occultation, water maser detection, or even direct imaging via telescopic ground arrays: it’s in the ancient art of astrometry. Astrometry has long been a tedious and painstaking method of measuring a star’s position, distance, and its proper motion (essentially its movement across our line of sight). In the second century B.C., without the benefit of binoculars or a telescope, the Greek astronomer Hipparchus spent years producing a catalog of over 1,000 star positions. Almost two millennia later, in 1718, Edmund Halley (whose name would later become inextricably linked with the comet) found that three stars—including Arcturus, a northern star that Hipparchus had recorded—had changed their positions on the sky by several fractions of a degree. Halley compared his findings against measurements compiled by Ptolemy, the Egyptian astronomer, who had assembled the work of Hipparchus and others in the Almagest, an astronomical compendium published two centuries after the birth of Christ. Halley’s assumption that the stars must be moving across our line of sight marked the birth of modem astrometry.