  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.
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 and18th 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. 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.
