52 ELLIPTICAL ORBITS
Kepler’s most productive years
came in Prague under the patronage
of Holy Roman Emperor Rudolf II
(r.1576–1612). Rudolf was particularly
interested in astrology and alchemy.B
efore the 17th century,
all astronomers were also
astrologers. For many,
including German astronomer
Johannes Kepler, casting horoscopes
was the main source of their income
and influence. Knowing where
the planets had been in the sky
was important, but of greater
significance for constructing
astrological charts was the ability
to predict where the planets would
be over the next few decades.
To make predictions, astrologers
assumed that the planets moved
on specific paths around a central
object. Before Copernicus, in the
16th century, this central body
was thought by most to be Earth.
Copernicus showed how the
mathematics of planetary
prediction could be simplified
by assuming that the central body
was the sun. However, Copernicus
assumed that orbits were circular,
and to provide any reasonable
predictive accuracy, his system
still required the planets toIN CONTEXT
KEY ASTRONOMER
Johannes Kepler (1571–1630)BEFORE
530–400 bce The works of
Plato and Pythagoras convince
Kepler that the cosmos can be
explained using mathematics.1543 Copernicus’s sun-
centered cosmos helps
astronomers to visualize a
physical solar system but still
gives no indication as to the
true shape of a planetary orbit.1600 Tycho Brahe convinces
Kepler of the reliability of his
planetary observations.AFTER
1687 Isaac Newton realizes
that an inverse square law of
gravitational force explains why
the planets obey Kepler’s laws.1716 Edmond Halley uses
observations of the transit of
Venus to convert Kepler’s ratios
of planetary distance from
the sun into absolute values.Kepler was never satisfied by a
moderate agreement between
theory and observation.
The theory had to fit exactly
otherwise some new
possibility had to be tried.
Fred Hoylemove around a small circle, the
center of which moved around
a larger circle. These circular
velocities were always assumed
to be constant.
Kepler supported the Copernican
system, but the planetary tables it
produced could still easily be out by
a day or two. The planets, the sun,
and the moon always appeared in
a certain band of the sky, known
as the ecliptic, but actual paths of
individual planets around the sun
were still a mystery, as was the
mechanism that made them move.Finding the paths
To improve the predictive tables,
Danish astronomer Tycho Brahe
spent more than 20 years observing
the planets. He next tried to
ascertain a path of each planet