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Re: speed of light





On Sun, 17 Nov 1996, fred brace wrote:

We were discussing the Michelson experiment to measure the speed of light
using a rotating octagonal mirror and a mirror 35 km away. I would like t=
o
find out more details on the experiment. What was the light source, how
big
were the rotating mirrors, how did he achieve the high rotational speed,
and measure the rotational speed? Is there a source for this information?
=09Also the book talks about Roemer's experiment using the moons of
Jupiter for the speed of light. How did they know the diameter of the
Earth's orbit in the 1600's?
=20
fredb@teleport.COM Public Access User --- Not affiliated with Teleport
Public Access UNIX and Internet at (503) 220-1016 (2400-14400, N81)
=20
=20

The mirror was air-driven. It's speed was measured by counting the beats
of its audible whine against a precision tuning fork. Michelson had some
talent at music. In the Mt. Wilson experiments the mirror speed was 1060
Hz. We find the beat method still the best with the modern equipment
(Leybold) we use in our lab. One nice thing about a tuning fork is that id
doesn't get out of calibration, like an oscilloscope can.

I'll refrain from an off-the-top-of-the-head response on the basis of
Roemer's knowledge of the diameter of the earth, without digging into the
historical references. His major error was not that, but a transit time
error in his observations of the Jovian satellites.

There's a wealth of resources on this subject. In addition to those below,
I'd recommend the book _The Aetherial Ether_.

Gillispe's _Dictionary of Scientific Biography_ has entries on Roemer and
MIchelson.

Also I've included some data, mostly from Jaffe's excellent book. Notice
how well it makes the point that quoted limits of error are not the same
as experimental discrepancy. Michelson's best value compares well with the
best modern value, being within the limits of error of that modern value,
which of course Michelson could not know. The modern value's limits of
error are ten times better than those quoted by Michelson. He was an
excellent experimentor.

EARLY MEASUREMENTS OF THE SPEED OF LIGHT
=20
Date Person Baseline Speed of light Discrepancy
km/s km/s %
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1676 Roemer 182x10^6mi*222,000 -78,000 26
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1729 Bradley 305,710 +5,917 2
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1849 Fizeau 867 mi 312,000 +12,000 4
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1862 Focault 20 m 298,000 -2,000 0.67
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1872 Cornu 300,200 +408 0.14
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ALBERT MICHELSON'S MEASUREMENTS OF THE SPEED OF LIGHT
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Date Location Baseline Speed of light Discrepancy
km/s km/s %
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1877-8 Naval Academy 152m x 2 300,140 =F1 480 +348 0.116
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1879 Naval Academy 610m x 2 299,910 =F1 50 +118 0.039
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1882 Case Institute 610m x 2 299,853 =F1 60 +61 0.02
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1926 Mt. Wilson 35km x 2 299,796 =F1 4 +4 0.0013
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1929-33 Irvine ranch 1.6 km 299,774 =F1 11 -18 0.0037
x 8 or 10=20
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Best modern value (1983) 299,792.5 =F1 0.1
(NBS: =F1 0.3)
=20
___________________
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* The diameter of the earth's orbit, approximately.


BIBLIOGRAPHY ON MICHELSON AND THE SPEED OF LIGHT
=20
Jaffe, Bernard. _Michelson and the Speed of Light._ Doubleday
Science Study Series, Doubleday Anchor Books, 1960.
=20
Wilson, Mitchell. _American Science and Invention, a Pictorial
History._ Bonanza Books, 1954.
=20
Dorothy Michelson Livingstone. _The Master of Light_ Date ? (1970s)

<=3D=3D=3D=3D=3D>
=20
A. A. Michelson, _Studies in Optics_ University of Chicago, 1927.
(Still in print.)
=20
A. A. Michelson, Amer. J. Sci. _15_, 394-5 (1878). "On a Method
of Measuring the Velocity of Light."
=20
A. A. Michelson, Amer J. of Sci. _22_, 120 (1881). "The
relative motion of the Earth and the Luminiferous ether."
=20
A. A. Michelson and E. W. Morley, Amer. J. Sci. _31_, 377 (1886).=20
"Influence of Motion of the Medium on the Velocity of LIght"
=20
A. A. Michelson and E. W. Morley, Amer. J. Sci. _34_, 333 (1887).=20
"On the Relative Motion of the Earth and the Luminiferous Ether."
=20
A. A. Michelson and E. W. Morley, J. Assoc. Eng. Societies
(Cleveland) May 1888.
=20
A. A. Michelson and E. W. Morley, Amer. J. Sci. _38_, 181 (1889).
=20
A. A. Michelson, Amer J. Sci. _3_, 475 (1897).
=20
A. A. Michelson, Astrophys. J. _37_, 190 (1913).
=20
A. A. Michelson, F. G. Pease, and F. Pearson, Nature _123_, 88
(1929). "Repetition of the Michelson-Morley Experiment."

=09-- Donald

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++=20
Dr. Donald E. Simanek Office: 717-893-2079
Prof. of Physics Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745 CIS: 73147,2166=20
Home page: http://www.lhup.edu/~dsimanek FAX: 717-893-2047
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