1718

Proper Motion of Stars

Edmond Halley (1656–1742)

The British astronomer Edmond Halley is most famous, of course, for discovering the periodic return of Halley’s Comet. But he also studied other aspects of astronomy, including comparing the positions of stars relative to their positions recorded by earlier astronomers, in order to identify the ones closest to the Sun and to potentially determine their absolute distances.

For most of the history of astronomy, the stars were assumed to be fixed denizens of a crystalline or otherwise solid celestial sphere that rotated around the Earth (to some) or that appeared to rotate as the Earth spun underneath (to others). The occasional appearance of supernovae or comets (“guest stars”) cast doubt on the concept of a fixed celestial sphere but did not disprove it.

Halley provided that proof, by carefully comparing the positions of bright stars in 1718 to the positions recorded by Hipparchus in the second century BCE. Three bright stars—Sirius, Arcturus, and Aldebaran—had moved significantly over 1,850 years relative to background stars. Halley calculated what he dubbed the “proper motion” of these stars—that is, the motion belonging to the star proper, rather than perceived motion that would be due to parallax. Stars with the largest proper motion are relatively closer to the Sun; indeed, the three stars above are only about 9, 37, and 65 light-years (ly) away. Stars closer to the Sun, such as Proxima Centauri (4.3 ly) and Barnard’s star (6 ly; discovered in 1916 by the American astronomer E. E. Barnard), have even larger proper motions—Barnard’s star has the largest known, moving more than 10 arc seconds (0.003°) per year.

Halley and other pioneers of stellar astrometry (positional measurements) have helped us understand that the sphere of stars that we see overhead at night is just the projection of what is actually an incomprehensibly enormous three-dimensional volume of space. Everything is in motion relative to everything else, and, as Edwin Hubble would discover in the twentieth century, the entire volume is actually increasing over time. The universe is dynamic, if we only watch patiently enough.

Taken sixty years apart, these two telescopic photos show the large proper motion of the nearby star known as Barnard’s star, which moved about one-third the width of the Full Moon against the background of other more distant and relatively fixed stars in the Milky Way.