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2.5 Stellar Aberration
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It was chiefly
therefore Curiosity that tempted me (being then at Kew, where the Instrument
was fixed) to prepare for observing the Star on December 17th, when having
adjusted the Instrument as usual, I perceived that it passed a little more
Southerly this Day than when it was observed before.
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James Bradley, 1727
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The
aberration of starlight was discovered in 1727 by the astronomer James
Bradley while he was searching for evidence of stellar parallax, which in
principle ought to be observable if the Copernican theory of the solar system
is correct. He succeeded in detecting an annual variation in the apparent
positions of stars, but the variation was not consistent with parallax. The
observed displacement was greatest for stars in the direction perpendicular
to the orbital plane of the Earth, and most puzzling was the fact that the
displacement was exactly three months (i.e., 90 degrees) out of phase with
the effect that would result from parallax due to the annual change in the
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