Angeles National Forest in Los Angeles County, California — The American West (Pacific Coastal)

Michelson Pier on Mount Wilson

Michelson Pier Marker image. Click for full size.

Photographed by Craig Baker, July 2, 2022

1. Michelson Pier Marker

Inscription.
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 in Angeles National Forest, California, in Los Angeles County. It can be reached from Mount Wilson Road. Marker is within the grounds of Mount Wilson Observatory, 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.

Regionally, this marker is in Greater Los Angeles

and in the Transverse Ranges.

It is also on the American Pacific Coast.

Globally, it is in North America,

on the Ring of Fire,

in the Pacific Rim,

in the Western Hemisphere,

in the Western World,

and in the Anglosphere.

Historically, it

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finds itself in what was once New Spain

and also Mexico’s Alta California.

Other nearby markers. At least 8 other markers are within 3 miles of this marker, measured as the crow flies: The 100-Inch Hooker Telescope (approx. ¼ mile away); The Mount Wilson 100 Inch Telescope (approx. ¼ mile away); Benjamin Davis Wilson (approx. ¼ mile away); Harvard’s Mount Wilson Station (approx. 0.3 miles away); Eaton Saddle (approx. 2.3 miles away); Mount Lowe Scenic Railway (approx. 2.3 miles away); Henninger Flats (approx. 2.7 miles away); Mount Lowe Summit (approx. 2.7 miles away). Touch for a list and map of all markers in Angeles National Forest.

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

Photographed by Craig Baker, July 2, 2022

2. Michelson Pier Marker

Through the hole in the trees you can see Mount San Antonio, popularly called Mt Baldy, 22 miles from 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 quite favorably with today’s adopted value of 299,792.458 km/sec.

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.)

Related marker. Click here for another marker that is related to this marker. Speed-of-Light Experiment -- An earlier attempt.

Also see . . . Albert A. Michelson - The Nobel Prize in Physics 1907. Life

Telephoto View of Mt Baldy image. Click for full size.

Photographed by Roger W. Sinnott, June 4, 2012

3. Telephoto View of Mt Baldy

and work of A. A. Michelson. (Submitted on July 8, 2012, by Roger W. Sinnott of Norwell, Massachusetts.)

Photographed by Craig Baker, July 2, 2022

4. Michelson Pier Marker

In the background is one of the smallest observatory domes on Mount Wilson.

A.A. Michelson image. Click for full size.

from Wikimedia Commons, 1897

5. A.A. Michelson

Historical Photograph image. Click for full size.

6. Historical Photograph

In this structure, located where the marker sits today, Michelson set up the apparatus he used to measure the speed of light. This image is part of a display in the museum/auditorium building on Mount Wilson.

Credits. This page was last revised on May 28, 2026. It was originally submitted on July 8, 2012, by Roger W. Sinnott of Norwell, Massachusetts. This page has been viewed 1,589 times since then and 69 times this year. Photos: 1, 2. submitted on January 3, 2023, by Craig Baker of Sylmar, California. 3. submitted on July 8, 2012, by Roger W. Sinnott of Norwell, Massachusetts. 4. submitted on January 3, 2023, by Craig Baker of Sylmar, California. 5. submitted on July 8, 2012, by Roger W. Sinnott of Norwell, Massachusetts. 6. submitted on October 14, 2017, by Roger W. Sinnott of Norwell, Massachusetts. • Syd Whittle was the editor who published this page.