The Mount Wilson station (1922 – 1928)
The key to Michelson’s work was the Mount Wilson station. It’s where everything happened and the measurements were made. In essence, a light pulse was sent off to a distant mountain top where a mirror reflected it back to Mount Wilson and its travel time measured. Sounds simple, but to actually pull it off in the 1920s, with Steampunk style equipment….well that was just something beyond elegant.
Seeing as how this was well before lasers were ever imagined, sending a light beam over long distances without it expanding to a useless diameter was a challenge. Michelson accomplished this by using a Sperry arc lamp, one of the most powerful of the day. If the operators weren’t careful, they’d be burned by its ultraviolet emissions. The arc light was focused into a somewhat parallel beam by 24″ diameter concave mirrors at each end of the link. Given the other necessary optics involved, aligning the system and keeping it that way was difficult.
The actual measurement process was exceptionally clever. The primary measurement point, what Michelson called “Mount San Antonio” (today known as Lookout Mountain) was 22 miles away to the northeast. The light pulses sent from Mount Wilson to San Antonio were created by reflecting the arc light off a spinning, multifaceted mirror with either 8, 12 or 16 sides. The time it took a pulse of light to travel from Mount Wilson to Mount San Antonio and back to Mount Wilson (44 miles) was 0.00023 seconds. This happened to be the same amount of time it took a 16 faceted mirror, turning at 264 revolutions per second, to move one facet. When all was adjusted just so, the light pulse went out on one mirror facet and returned on the next one presenting what appeared to be a single image to the observer.
When using mirrors with smaller numbers of facets, the rotational speed had to be increased accordingly. This created some dangerous conditions. At one point Michelson attempted to use a larger, 8 sided mirror to obtain a brighter return image. It was designed to operate at 528 turns per second (almost 32,000 RPM). But when it reached 400 revolutions per second it burst apart, showering the observation room with shards of glass. And while it was possible to place a shield around the spinning mirror, the line between it and the observers eyepiece had to be kept clear. Not a job for the timid.
The spinning mirror was driven by compressed air, continually adjusted by the observer. Timing was performed by a rather convoluted process starting with a highly calibrated free pendulum, and feeding eventually into an electric tuning fork.
Because the mirrors had a 30 foot focal length, Michelson’s installation had two structures at each end, about 20′ apart, shown below. The 24″ diameter mirror was in the structure to the right, and the covered “tunnel” between the two was where the beam was reflected back to the observing room on the left. The line of the two structures was pointed to the installation at San Antonio, off towards the left.
Michelson used his Mount Wilson station from 1922 to 1928. However what Michelson called his “definitive measurements”, between Mount Wilson and Mount San Antonio, were made during the Summer of 1926. Michelson was originally brought to Mount Wilson by the facility’s first Director, George Ellery Hale. Hale was succeeded as Director in 1923 by Walter S. Adams, who worked closely with Michelson until Michelson’s death in 1931.
The site today
Michelson’s Mount Wilson station was located on a ridge on the southerly face of the mountain, in what is today a residential area for visiting astronomers. Due to the fact these folks need to sleep during the day to work at night, this area isn’t open to the public for visitation.
The foundations of Michelson’s huts remain, along with a commemorative plaque and the benchmark of the US Coast and Geodetic Survey from 1922, labeled “Michelson”.