Near Pasadena in Los Angeles County, California — The American West (Pacific Coastal)
Michelson Pier on Mount Wilson
On this pier in 1926 Albert Abraham Michelson measured the velocity of light by means of a beam of light transmitted to Mount San Antonio and reflected back to this station.
Topics. This historical marker is listed in this topic list: Science & Medicine. A significant historical year for this entry is 1926.
Location. 34° 13.353′ N, 118° 3.475′ W. Marker is near Pasadena, California, in Los Angeles County. Marker can be reached from Mount Wilson Road. Marker is within the grounds of Mount Wilson Observatory, on the left when traveling south on an unnamed service road. The marker is about 125 yards NNW of the astronomers’ residence (called the Monastery), and 25 yards north of one of the smallest observatory domes on the mountain. Touch for map. Marker is in this post office area: Pasadena CA 91107, United States of America. Touch for directions.
Other nearby markers. At least 8 other markers are within 5 miles of this marker, measured as the crow flies. The 100-Inch Hooker Telescope (approx. ¼ mile away); Benjamin Davis Wilson (approx. ¼ mile away); Harvard’s Mount Wilson Station (approx. 0.3 miles away); Adams Pack Station (approx. 2.8 miles away); World’s Largest Blossoming PlantMount Lowe Railway (approx. 3.7 miles away); Mount Wilson Trail (approx. 3.7 miles away); MacPherson Parkway (approx. 4.3 miles away). Touch for a list and map of all markers in Pasadena.
Regarding Michelson Pier on Mount Wilson. American physicist Albert Abraham Michelson (1852-1931) made a number of measurements of the speed of light during his lifetime. The best of these, in 1924-26, used equipment set up at this spot on Mount Wilson and at a corresponding station on Mount San Antonio, also called Baldy, 22 miles away. To help in the determination, the U.S. Coast and Geodetic Survey measured the spacing between the two stations to within 1 inch.
The apparatus at Mount Wilson consisted of a rapidly spinning disk with 8, 12, or 16 flat mirrors equally spaced around the rim. Light from a powerful arc lamp bounced off this spinning rim, traveled from Mount Wilson to Baldy, was reflected back, and bounced again off another part of the same spinning rim. The deflection of the return beam was used to calculate the light-travel time for the round trip.
Michelson’s final value for the speed of light was 299,796 ± 4 kilometers per second, which compares
Michelson is even better known for the famous Michelson-Morley experiment of 1887. Although it failed to show the “ether drift” Michelson was looking for, it helped to pave the way for Einstein’s special theory of relativity (1905).
(The above details of Michelson’s apparatus are adapted from K. D. Froome and L. Essen, The Velocity of Light and Radio Waves, Academic Press, 1969.)
Also see . . . Albert A. Michelson - The Nobel Prize in Physics 1907. Life and work of A. A. Michelson. (Submitted on July 8, 2012, by Roger W. Sinnott of Chelmsford, Massachusetts.)
Credits. This page was last revised on June 5, 2022. It was originally submitted on July 8, 2012, by Roger W. Sinnott of Chelmsford, Massachusetts. This page has been viewed 712 times since then and 25 times this year. Photos: 1, 2, 3, 4, 5. submitted on July 8, 2012, by Roger W. Sinnott of Chelmsford, Massachusetts. 6. submitted on October 14, 2017, by Roger W. Sinnott of Chelmsford, Massachusetts. • Syd Whittle was the editor who published this page.