Founded with Carnegie Institution of Washington support secured by George Ellery Hale in 1904, the Mount Wilson Solar Observatory soon expanded from solar work to stellar telescopes, first with a 60-inch instrument and then with the 100-inch Hooker telescope after “Solar” was dropped from the observatory’s name. Named for Los Angeles businessman John D. Hooker, who donated $45,000 in 1906 for its mirror, and supported by additional Carnegie funding, the telescope was built around an optical glass mirror blank cast in France and brought to Pasadena in 1908. Weighing 4½ tons, slightly more than 100 inches in diameter, and 12 inches thick, it was the largest solid glass mirror blank ever cast. The blank was ground, figured, and tested in Pasadena, while the telescope itself was designed and constructed between 1910 and 1917. Engineers solved major problems such as supporting the mirror without stressing it, and they adapted the 60-inch telescope’s mercury flotation system to carry a 100-ton instrument and let it rotate with virtually no friction. From 1917 to 1948, the 100-inch Hooker telescope was the world’s largest telescope and became one of the 20th century’s most famous instruments in observational astronomy. Work done with it transformed scientific understanding of the universe: in 1923 Edwin Hubble showed that the Andromeda nebulae lay outside the Milky Way galaxy; in 1929 Hubble and Milton Humason found that the universe is expanding, measured the size of the known universe, and determined its expansion rate; and in the 1940s Walter Baade’s observations doubled the size of the known universe as Hubble had calculated it. Used by many leading astronomers and astrophysicists, it changed humanity’s view of the cosmos by showing that our galaxy is only one of many, and it remains in research use today.
