Lookout_Mountain_(Los_Angeles_County,_California) Knowpia

Lookout Mountain in Los Angeles County, California, is a 6,812-foot (2,076 m)^[1] peak on the shoulder of Mount San Antonio ("Mount Baldy").

Albert A. Michelson used the peak to measure the speed of light in 1925–1929. For him to do so, the United States Coast and Geodetic Survey in 1923 established a 21.5-mile (34.6 km) baseline with an accuracy of 1 part in 11 million from Pasadena, California, at the base of Mount Baldy roughly to Rancho Cucamonga at the base of Mount Wilson.^[2]^[3]^[4] It may have been the most accurate baseline ever determined at the time.^[5]^[6]^[7] The Coast and Geodetic Survey then used the baseline to triangulate the distance to the Mount Wilson Observatory, with a distance error of about 1 part in 5 million,^[8] and Michelson measured light travel time between the stations with a rotating mirror apparatus.^[9] The concrete piers on Lookout Mountain used for the Michelson observations still exist.^[7]

The first fire lookout in the Angeles National Forest was built on Lookout Mountain in 1914 and stood until it burned in 1927.^[10]

References edit

^ United States Geological Survey 1:24,000 topographic quadrangle O34117b6 "Mount Baldy", benchmark "Antonio"
^ Garner 1949, p. 68-69.
^ Timeline: Historical highlights from Mount Wilson Observatory's first 100 years, Mount Wilson Observatory, retrieved 2018-05-10
^ Milestones of the Survey, National Geodetic Survey, June 5, 2017
^ Dracup 1994.
^ Daniel Medina (February 3, 2014), Higher Beauties: The Discovery of the Speed of Light in the San Gabriels, KCET
^ ^a ^b "The Mount Wilson-Mount San Antonio measurements (1922 – 1926)". Theotherhand.org. Retrieved 12 December 2014.
^ Garner 1949, p. 74.
^ Michelson 1927.
^ Creighton 2009, p. 27.

Sources edit

Creighton, K.J. (2009). Mt. Baldy. Images of America. Arcadia Pub. ISBN 978-0-7385-5999-5.
Dracup, Joseph F. (1994), Geodetic Surveys in the United States, The Beginning and the Next One Hundred Years, 1807 – 1940, National Oceanographic and Atmospheric Administration, OCLC 232335152, In 1922-23, the most accurate invar taped base line ever, with a precision of 0.2ppm one sigma, was measured near Pasadena, CA. The sole purpose for the 20.9 mile base line was to provide Albert A. Michelson with the best possible distance between points on Mount Wilson and San Antonio Peak used in his experiments to determine the speed of light.
Garner, C.L. (April 1949), "A Geodetic Measurement of Unusually High Accuracy" (PDF), Coast and Geodetic Survey Journal: 68–74, archived from the original (PDF) on 2009-03-25, retrieved 2018-05-10
Michelson, A.A. (1927), "Measurement of the Velocity of Light Between Mount Wilson and Mount San Antonio", Astrophysical Journal, 65: 1, Bibcode:1927ApJ....65....1M, doi:10.1086/143021, retrieved 12 December 2014