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Vesto Melvin Slipher (November 11, 1875 – November 8, 1969) was an American astronomer. His brother Earl C. Slipher was also an astronomer. Slipher was born in Mulberry, Indiana, and completed his education at Indiana University. He spent his entire career at Lowell Observatory in Flagstaff, Arizona, where he was director from 1916 to 1952. He used spectroscopy to investigate the rotation periods of planets, the composition of planetary atmospheres. In 1912, he was the first to observe the shift of spectral lines of galaxies, so he was the discoverer of galactic redshifts. He was responsible for hiring Clyde Tombaugh and supervised the work that led to the discovery of Pluto. Edwin Hubble was generally incorrectly credited with discovering the redshift of galaxies; these measurements and their significance were understood before 1917 by James Edward Keeler (Lick & Allegheny), Vesto Melvin Slipher (Lowell), and William Wallace Campbell (Lick) at other observatories. Combining his own measurements of galaxy distances with Vesto Slipher's measurements of the redshifts associated with the galaxies, Hubble and Milton Humason discovered a rough proportionality of the objects' distances with their redshifts. Though there was considerable scatter (now known to be due to peculiar velocities), Hubble and Humason were able to plot a trend line from the 46 galaxies they studied and obtained a value for the Hubble-Humeson constant of 500 km/s/Mpc, which is much higher than the currently accepted value due to errors in their distance calibrations. Such errors in determining distance continue to plague modern astronomers (See cosmic distance ladder for more details). In 1929 Hubble and Humason formulated the empirical Redshift Distance Law of galaxies, nowadays termed simply Hubble's law, which, once the redshift is interpreted as a measure of recession speed, is consistent with the solutions of Einstein’s General Relativity Equations for an homogeneous, isotropic expanding space de Sitter universe or de Sitter space. Although concepts underlying an expanding universe were well understood earlier, this statement by Hubble and Humeson lead to wider scale acceptance for this view. The law states that the greater the distance between any two galaxies, the greater their relative speed of separation. This discovery later resulted in formulation of the Big Bang theory by George Gamow, a consequence of the observed velocities of distant galaxies that when taken together with the cosmological principle imply that space is expanding according to the Friedmann-Lemaître model of general relativity. Earlier, in 1917, Albert Einstein had found that his newly developed General Theory of Relativity indicated that the universe must be either expanding or contracting. Unable to believe what his own equations were telling him, Einstein introduced a cosmological constant (a "fudge factor") to the equations to avoid this "problem". When Einstein heard of Hubble's discovery, he said that changing his equations was "the biggest blunder of my life".3 He died in Flagstaff, Arizona and is buried there in Citizens Cemetery.
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