During the 17th century the idea of an orbiting and rotating Earth became increasingly popular, but opponents of this view continued to point out that the theory had observable consequences that had never, in fact, been observed.1 Why, for instance, had astronomers failed to detect the annual parallax of the stars that must occur if Earth orbits the Sun? To address this problem, astronomers of the 17th and 18th centuries sought to measure the annual parallax of stars using telescopes. None of them succeeded. Annual stellar parallax was not successfully measured until 1838, when Friedrich Bessel detected the parallax of the star 61 Cygni.2 But the early failures to detect annual stellar parallax led to the discovery of a new (and entirely unexpected) phenomenon: the aberration of starlight. This paper recounts the story of the discovery of stellar aberration. It is accompanied by a set of activities and computer simulations that allow students to explore this fascinating historical episode and learn important lessons about the nature of science.3 

1.
Christopher M.
Graney
,
“Teaching Galileo? Get to know Riccioli! What a forgotten Italian astronomer can teach students about how science works,”
Phys. Teach.
50
,
18
21
(
Jan. 2012
).
2.
Alan
Hirshfeld
,
Parallax: The Race to Measure the Cosmos
(
W. H. Freeman
,
2001
).
3.
A self-contained Java program containing all of the simulations is available at www.opensourcephysics.org/items/detail.cfm?ID=12029. The activity handouts are available as supplementary documents on the same page.
4.
Hooke observed Gamma Draconis in daylight, as well as at night. He may have been the first person to observe a star through a telescope during the day.
5.
Robert
Hooke
,
An Attempt to Prove the Motions of Earth by Observations
(
London
,
1674
). Available online at www.roberthooke.com/motion_of_the_earth_001.htm.
6.
M. E. W.
Williams
,
“Flamsteed's alleged measurement of annual parallax for the pole star,”
J. Hist. Astron.
10
,
102
116
(
1979
).
7.
The telescope also moves due to Earth's rotation, but the rotational velocity is about 64 times smaller than the orbital velocity, so to a good approximation we can ignore the effect of rotation. In any case the effect Bradley was trying to explain had an annual period.
8.
Note that a fixed aberration angle would be undetectable. For a discussion of this point, and how aberration relates to special relativity, see
Thomas E.
Phipps
 Jr.
“Relativity and aberration,”
Am. J. Phys.
57
,
549
551
(
June 1989
).
9.
James
Bradley
,
“A letter to Dr. Edmond Halley Astronom. Reg. &c. giving an account of a new discovered motion of the fix'd stars,”
Phil. Trans.
35
,
637
661
(
1729
).
10.
I'd like to thank Charlie Holbrow of Colgate University for this insight.
11.
Michael
Hoskin
,
Stellar Astronomy: Historical Studies
(
Science History Publications
,
1986
), pp.
29
36
.
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