In 1718, after comparing contemporary records of stellar positions with those of ancient times, Edmond Halley determined that the bright stars Palilicium (Aldebaran), Sirius and Arcturus had undergone a greater displacement on the two-dimensional sky than could be accounted for by precession alone. He postulated that these stars possess a ‘particular Motion of their own … which in so long a time as 1800 Years may shew it self by the alteration of their places, though it be utterly imperceptible in the space of a single Century of Years’. Halley's ‘particular’ stellar motion is what today's astronomers call ‘proper’ motion. It constitutes one component of a star's ‘space velocity’, or motion in space relative to the Sun. The other is its ‘radial velocity’, or motion in the line of sight.

Since Halley's day, astronomers have measured the proper motion of many stars. But even the nearest of our Sun's stellar neighbours is too distant to exhibit any of the visual cues (e.g. changes in apparent brightness or size) we normally rely on as evidence of motion in the line of sight. Indeed, the ability to detect, let alone measure, a star's radial velocity eluded earthbound observers until the late 1860s when William Huggins brought the new instruments and methods of celestial spectroscopy to bear on the matter. It proved to be the most influential of his contributions to modern day astronomical practice.