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In 1871 the Astronomer Royal of England, George Airy did a variation of Arago which again was designed in such a manner it could measure absolute earth motion. Looking at stars, if the earth is moving, or if the light source (the star) is moving and the earth staionary, the relative motion of the telescope and the light traveling down the telescope would be sideways relative to the telescope and Airy would have to tilt the telescope to keep the light from hitting the side. This is exactly what all astronomers seem to have to do. The effect is called abberation. Airy then filled his telescope with water which is supposed to slow light to 77% of c. Only earth motion, and not star motion, would affect the amount of additional tilt required on the telescope. Airy was supposed to have to tilt his telescope more to see a star that with no water in the telescope. He did not, the experiment is known as "Airy's failure."

I studied it in college, but nothing was made of it. In some respects it may be the most important experiment in the history of physics. A variety of "theories" were invoked to "explain" the failure: Fresnell Drag, shrinking of the tube in the direction of earth motion, expansion of the tube perpendicular to earth motion, and various combinations. Remember, it was only a failure because it did not agree with Copernicus, Gallileo, et.al.

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Stellar Aberration
In 1725, James Bradley (Professor of Astronomy at Oxford) sent Halley (then Astronomer Royal) an "Account of a New Discovered Motion of the Fix'd Stars" in which he describes an effect which he termed aberration. During the course of a year, each star describes an elliptical motion when viewed carefully through a telescope which is motor-driven so as to compensate for the earth's rotation about its axis. This phenomenon can be explained on the basis of the ether hypothesis, as follows.
Suppose light from a particular star arrives vertically and that the earth is travelling horizontally with a speed u relative to the ether. During the time interval t which light takes to travel down the telescope, its base moves horizontally by an amount ut (C&N Fig.12-9). Therefore the telescope must be tilted through an angle alpha to enable the light to travel down its axis; a vector triangle of displacements gives: alpha~ tan alpha = u t/(c t) = u/c

If the ether is stationary relative to the sun, u varies between +30km/s and -30km/s, depending on the time of year, giving alpha ~ 10-4 rad (20 arc sec). This is a small tilt: the end of a 10m-long telescope would need to be moved only 1 mm. The observed effect is close to this prediction.

(Note that if the ether is actually moving at some speed V relative to the sun, the earth's velocity relative to the ether would be V +/- 30 km/s, so the prediction is unaltered)

If the telescope is not tilted, light will move down the telescope slightly off-axis; combined with the parallax effect, aberration produces an elliptical apparent motion of the star. [For a star observed at an elevation theta, the angular tilt becomes alpha =(u/c) sintheta, so a smaller effect is observed for stars which are not directly overhead.]

In 1871, Sir George Airy repeated the experiment with a telescope filled with water, refractive index n = 1.33, expecting that the angular tilt might increase to alpha = u/(c/n) = n (u/c). But he observed no difference, compared to the case of a telescope in air.

In fact, this null result had been predicted by Fizeau, assuming that light travelling in water would be dragged horizontally at a velocity f u , where f = 1-1/n^2 is the Fresnel coefficient of drag. The refractive index of the water slows down the light but its coefficient of drag increases the velocity and (because f = 1-1/n^2) the two effects exactly cancel.

So Bradley's original observation and Airy's experiment are both consistent with the earth moving through an ether which is stationary (or moving at constant velocity) with respect to the sun. However, the role of the ether was soon to be challenged.

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