The Heliocentric Universe
Ptolemy’s geocentric model worked pretty well, but it was complicated and occasionally
made errors in predicting the movement of planets. At the beginning of the 16th century
A.D., Nicolaus Copernicus proposed a different model in which Earth and all the other
planets orbited the Sun. Because this model put the Sun at the center, it is called the
heliocentric modelof the universe.Heliocentricmeans “sun-centered.”Figure25.3shows
the heliocentric model compared to the geocentric model. Copernicus’ model explained the
motion of the planets about as well as Ptolemy’s model, but it did not require complicated
additions like epicycles and deferents.
Figure 25.3: Unlike the geocentric model (top image), the heliocentric model (lower image),
had the Sun at the center, and did not require epicycles. ( 5 )
Although Copernicus’ model worked more simply than Ptolemy’s, it still did not perfectly
describe the motion of the planets. The problem was that, like Ptolemy, Copernicus still
thought planets moved in perfect circles. Not long after Copernicus, Johannes Kepler refined
the heliocentric model. He proposed that planets move around the Sun in ellipses (ovals),
not circles. This model matched observations perfectly.
BecausepeopleweresousedtothinkingofEarthatthecenteroftheuniverse,theheliocentric
modelwasnotwidelyacceptedatfirst. However, whenGalileoGalileifirstturnedatelescope
to the heavens in 1610, he made several striking discoveries. He found that the planet Jupiter
has moons orbiting around it. This was the first evidence that objects could orbit something