# Q & A: Michelson's speed of light measurement between Mt. Wilson and Mt San Antoni

Q:
I have read extensively about the Michelson experiments between Mt Wison and Mt San Antonio. I understand the basic mechanism of the 8-sided mirror rotating at about 528 rps. However I cannot find any details on how exactly the speed of light was visually determined by the observers at Mt. Wilson. In other words, how did they know how long it took for the light from a mirror facet to retun to its' source? I have spent days searching for an answer to this but no joy so far. Thanks!
- Ed (age 70)
Aiken SC USA
A:
It's a bit tricky to find a good description of the method, however it is possible to find the original paper published in the Astrophysical Journal in January 1927.   Go to http://adsabs.harvard.edu/full/1927ApJ....65....1M .   A diagram of the method is illustrated on page four.
The round trip distance between Mt. Wilson and Lookout Mountain (nee Mt. San Antonio) is d = 2*22 miles = 70,840 meters.   Dividing d  by by speed of light c = 3x1010 meters per second gives  ttof = 2.36x10-4 seconds.  Now if you have an octagonal mirror rotating at 528 revolutions per second the time between successive surfaces is trot  = (1/528)/8 =  2.36e-004, the same number.  So the return image should be in the same place.   Michelson devised a clever way to measure a displacement from this perfect agreement by controlling the mirror rotation rate.  When the rate was 528 the return image was perfectly aligned with the outgoing image, otherwise it was shifted a bit.

LeeH

(published on 10/05/2012)