60 PART 1^ |^ EXPLORING THE SKY
astronomers of the time knew of other stars and galaxies. Th e
planets circle the sun, not Earth, so the universe that Copernicus
knew was heliocentric.
Although astronomers throughout Europe read and admired
De Revolutionibus, they did not immediately accept the
Copernican hypothesis. Th e mathematics was elegant, and the
astronomical observations and calculations were of tremendous
value; but few astronomers believed, at fi rst, that the sun actually
was the center of the planetary system and that Earth moved.
How the Copernican hypothesis was gradually recognized as cor-
rect has been called the Copernican Revolution because it wasBecause Copernicus imposed uniform circular motion on
his model, it could not accurately predict the motions of the
planets. Th e Prutenic Tables (1551) were based on the Copernican
model, and they were not signifi cantly more accurate than the
13th-century Alfonsine Tables that were based on Ptolemy’s
model. Both could be in error by as much as 2°, which is four
times the angular diameter of the full moon.
Th e Copernican model is inaccurate. It includes uniform
circular motion and consequently does not precisely describe the
motions of the planets. But the Copernican hypothesis that the
universe is heliocentric is correct, considering how little
■ Figure 4-10
(a) The Copernican universe as reproduced in De Revolutionibus. Earth and all the known planets revolve in separate circular
orbits about the sun (Sol) at the center. The outermost sphere carries the immobile stars of the celestial sphere. Notice the
orbit of the moon around Earth (Terra). (Yerkes Observatory) (b) The model was elegant not only in its arrangement of the planets
but also in their motions. Orbital speed (blue arrows) decreased from Mercury, the fastest, to Saturn, the slowest. Compare the
elegance of this model with the complexity of the Ptolemaic model on page 57.
SunMercurya b Not to scaleVenusMarsJupiterSaturnEarthMoon