Lookout Mountain and the Speed of Light

The first lookout tower of the Angeles Forest was erected on Lookout Mountain No. 2 in 1913 and was active until 1927 when it was moved to Sunset Peak. Today, one will find on Lookout Mountain, in addition to a sign and a register, three in-line concrete blocks. The tallest of these, forty-two inches high, has a metal tablet marked “ANOTONIO 1922” and one of the smaller blocks, twenty-six inches high, has an unmarked survey point. If a sight is taken in a westernly direction over these two points, it will align to a spot on Mt. Wilson, marked on topo maps as “Michelson.” These blocks supported a mirror system for an exacting experiment by America’s first Nobel Prize winner, A.A. Michelson, in the years 1922 to 1926 to determine the speed of light.

Mt Wilson Observatory (MW13-27b)
Mt. Wilson Observatory (Main Photo Collection, MW13-27b). This photograph was published with the original article.

The speed of light had been measured before, but never on such a spectacular scale or with as much accuracy. At station “MICHELSON” on Mt. Wilson; an octagonal mirror was mounted on a rotor that reflected a light beam to the station “ANTONIO” on Lookout Mountain, nearly twenty-two miles distant; then reflected back to another facet of the octagonal mirror, where it was reflected in the observer’s eye. If the octagonal mirror rotated so as to make one-eighth of a turn during the time of passage of light to Lookout Mountain and return, it would present the succeeding face of the octagonal mirror to the returning beam. By this technique, Michelson was able to measure the speed of light with great accuracy. The required rotational speed was 528 revolutions per second and this was accomplished by an air blast issuing through two nozzles and impinging on the vanes of a paddle wheel. In one of the earlier tests, a glass octagonal mirror burst at a rotational speed of 400 revolutions per second. Other mirrors were made using twelve and sixteen facets, the largest being three inches in diameter. On Lookout Mountain, Michelson installed a two-foot diameter concave mirror of thirty-foot focal length to receive the light beam from Mt. Wilson. This mirror was mounted on the survey point “ANTONIO.” After reflection from the plane mirror, light returned to the concave reflector and back to Mt. Wilson. It was important for the test to know accurately the distance from the survey point “MICHELSON” on Mt. Wilson to the point “ANTONIO” on Lookout Mountain. The distance was measured by the U.S. Coast and Geodetic Survey (U.S.C.G.S.) by triangulation from a forty-kilometer surveyed base line in the valley below and was determined with remarkable accuracy. To quote Wm. Bowie, Chief, Division of Geodesy U.S.C.G.S., “It is believed that the length of this line has been determined with greater accuracy than that of any line of triangulation in this or any other country.” From his report, the probable error in the nearly twenty-two mile distance was less that one-fourth inch!

The importance of Michelson’s work was aptly expressed in a speech by Albert Einstein at Caltech’s Athenaeum before the Caltech Associates in January 1931. He gave credit to those who had helped with his work and turning to the measurer of light, he said, “You, my honored Dr. Michelson began with this work when I was only a little youngster, hardly three feet high. It was you who led the physicists into new paths, and through your marvelous experimental work paved the way for the development of the Theory of Relativity. Without your work this theory would today be scarcely more than interesting speculation.”

So, when you climb Lookout Mountain, sight over the blocks to Mt. Wilson and think of the famous experiment that took place all those years ago, when Michelson measured the speed of light.


- John Bascom

This article was originally published in the September 1980 issue of the Pasadena Museum of History (then Pasadena Historical Society) newsletter.