Los Angeles County, California
Los Angeles County, officially the County of Los Angeles, and sometimes abbreviated as L.A. County, is the List of the most populous counties in the United States, most populous county in the United States and in the U.S. state of California, ...
is a peak on the shoulder of
Mount San Antonio
Mount San Antonio, commonly referred to as Mount Baldy or Old Baldy, is a summit in the San Gabriel Mountains on the border of Los Angeles and San Bernardino counties of California. Lying within the San Gabriel Mountains National Monument and An ...
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
in 1925–1929. To do so, a 21.5 mile baseline was established in 1923 by
U.S. Coast and Geodetic Survey
The National Geodetic Survey (NGS) is a United States federal agency that defines and manages a national coordinate system, providing the foundation for transportation and communication; mapping and charting; and a large number of applications ...
, with accuracy of 1 part in 11 million, in Pasadena at the base of Mount Baldy, roughly to Rancho Cucamonga at the base of Mount Wilson. It may have been the most accurate baseline ever determined, at the time. Coast and Geodetic Survey then used the baseline to
triangulate
In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points.
Applications
In surveying
Specifically in surveying, triangulation involves only angle me ...
the distance to the Mount Wilson Observatory, with distance error about 1 part in 5 million, and Michelson measured light travel time between the stations with a rotating mirror apparatus. The concrete piers on Lookout Mountain used for the Michelson observations still exist.
The first fire lookout in the
Angeles National Forest
The Angeles National Forest (ANF) of the United States Forest Service, U.S. Forest Service is located in the San Gabriel Mountains and Sierra Pelona Mountains, primarily within Los Angeles County, California, Los Angeles County in southern Calif ...
was built there in 1914 and stood until it burned in 1927.