This seemed necessary for ether-wave theories to explain stellar aberration. Discovered in the eighteenth century by James Bradley (1693-1762), aberration is the apparent displacement of a starfrom its actual position due to the combined velocity of Earth and starlight. It was thought that if Earth's motion disturbed the ether, then starlight would be deflected in a manner inconsistent with this well-known effect.
Dominique François Arago's 1810 failure to measure Earth's velocity relative to the ether challenged Young's conclusions. Arago (1786-1853) accepted that the velocity of light, c, was constant in the ether and could only be measured to be c if one were at rest relative to the ether. He reasoned that motion through theether with velocity v in the same or opposite direction as a beam of light, such as Earth's motion away from certain stars and toward others in its solar orbit, should yield light velocity measurements smaller, c-v, or greater, c+v, respectively. Knowing that light beams with different velocities refract differently, Arago designed an experiment to observe this difference. However, no such effect revealed itself. This null-result suggested the existence of a stagnant layer of ether near Earth's surface. If true, this would have undermined Young's explanation of aberration.
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