sphere which contained all the stars. The planets appear to move much faster than the stars
and so the Greeks placed them closer to Earth.
Today, powerful telescopes can actually see the surfaces of planets in our solar system. Even
though the closest stars have diameters that are hundreds of times larger than the Earth,
the distant stars appear as tiny dots that cannot be resolved.
Figure 25.1: Model of a geocentric universe. This diagram of the universe from the Middle
Ages shows Earth at the center, with the Moon, the Sun, and the planets orbiting Earth.
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The geocentric model may seem strange to us now, but at the time, it worked quite well. It
explained why all the stars appear to rotate around Earth once per day. It also explained
why the planets move differently from the stars, and from each other. One problem with
the geocentric model was resolved around 150 A.D. by the astronomer Ptolemy. At times,
some planets seemed to move backwards (in retrograde) instead of in their usual forward
motion around the Earth. Ptolemy resolved this problem by using a system of circles to
describe the motion of planets (Figure25.2). In Ptolemy’s system, a planet moved in a
small circle, called anepicycle. This circle in turn moved around Earth in a larger circle,
called adeferent. Ptolemy’s version of the geocentric model worked so well that it remained
the accepted model of the universe for more than a thousand years.