American astronomer (1875–1969)
Born in Mulberry, Indiana, Slipher graduated from the University of Indiana in 1901 and obtained his PhD there in 1909. He spent the whole of his career from 1901 to 1952 at Percival Lowell's observatory in Flagstaff, Arizona, being made acting director on Lowell's death in 1916 and director in 1926.
Slipher was basically a spectroscopist. One of his major achievements was to determine the rotation periods of some of the planets by spectroscopic means. Thus in 1912, in collaboration with Lowell, he found that the spectral lines at the edge of the disk of Uranus were displaced by an amount corresponding to a speed of 10.5 miles (16.8 km) per second. Knowing the circumference it was easy to work out the rotation period as 10.8 hours. Although still the accepted figure, it is now thought that this rotation period could be considerably longer. Slipher also produced comparable data for Venus, Mars, Jupiter, and Saturn and showed that Venus's period was much longer than expected.
Slipher also studied the matter lying between the stars in our Galaxy. Like Johannes Hartmann he concluded in 1908 from his spectroscopic research that there must be gaseous material lying between the stars. He also studied the spectra of the luminous nebulae in the Pleaides cluster of stars and proposed in 1912 that they were illuminated by starlight reflected off dust grains. This was an early indication of the presence of solid material in nebulae and other interstellar clouds.
Slipher's most important achievement however was his determination of the radial velocities of spiral nebulae by the measurement of the displacement of their spectral lines. Such measurement relies on the Doppler effect by which the wavelength of light from an object moving away from an observer will be lengthened, i.e., shifted toward the red end of the spectrum, while light from an object moving toward an observer will have its wavelength shortened, i.e., moved toward the blue end of the spectrum. Thus by measuring the change in wavelength, known as the Doppler shift, the velocity of the moving object can be determined easily.
Slipher's work produced two surprising results. The first was the immense speed of the Andromeda nebula (galaxy), which he first successfully measured in 1912. He found it to be moving toward the Earth with a velocity of nearly 300 kilometers per second, which was then the greatest velocity ever observed. Secondly, by 1917 he had obtained the radial velocities of 15 spiral nebulae of which it would have been thought that roughly half would be receding while the other half would be approaching. But he found that 13 out of the 15 were receding. What was equally significant was their velocity, which in many cases exceeded that of the 300 kilometers per second of the Andromeda nebula. Many astronomers questioned these findings. At that time there was considerable controversy over whether the spiral nebulae were part of our Galaxy or lay far beyond it as independent star systems. Slipher's work was, in retrospect, evidence both for the extragalactic hypothesis, since the velocities of the spiral nebulae were too great for them to be members of the Galaxy, and for the expanding universe, which was first proposed by Alexander Friedmann in 1922 and later shown to be correct by Edwin Hubble.
