were many variations of this practice. The
English astrologer William Lilly practiced
horary astrology: the prediction of events
based on the time when a person poses a
question, not when he or she is born.
Lilly’s accurate predictions gained him the
support of King Charles II and a salary
from the government of Oliver Cromwell.
His predictions also got him in serious
trouble with the authorities, who accused
him of setting the 1666 Great Fire of Lon-
don after predicting this event.
Astronomical observation brought
about new theories of the true nature of
the universe during the Renaissance; the
Polish astronomer Nicolaus Copernicus
theorized that the earth revolves around
the sun, not the other way around. This
heliocentric theory was a drastic change
from traditional beliefs and initiated a
gradual decline in the acceptance of as-
trology among scientists and philosophers.
The philosopher Giovanni Pico della Mi-
randola was skeptical of astrology, but
many astronomers, including Johannes Ke-
pler and Tycho Brahe, offered their ser-
vices as astrologers to nobles and princes.
The influence of astrology among scien-
tists waned at the end of the Renaissance
and disappeared altogether in the Enlight-
enment, which followed.
SEEALSO: astronomy; Brahe, Tycho;
Melanchthon, Philipp; Pico della Miran-
dola, Giovanni
astronomy .........................................
Astronomy of the Middle Ages was
grounded in the work of Ptolemy, a scien-
tist of ancient Alexandria, whose workThe
Almagestset out the Ptolemaic system of
an earth-centered universe. The Ptolemaic
view was accepted by philosophers and
sanctioned by the church, and his work
was the foundation of studies and com-
mentaries by the medieval scholars Georg
von Peuerbach, Johann Müller (known as
Regiomontanus), and Georg Joachim
Rheticus. To verify their observations as-
tronomers drew on the Alfonsine Tables
that were set down in Toledo, Spain, in
1252, and that were based on the Ptole-
maic system. The earth was the center of
the universe around which the sun, plan-
ets, and stars revolved; the heavens were
permanent and unchanging; a perfect har-
mony and balance existed in which, ac-
cording to astrologers, celestial phenom-
ena had their effect on events and people
on the terrestrial globe.
As the skill of observers improved,
however, the Ptolemaic system came under
question. The Polish astronomer Nicolaus
Copernicus, while using the Alfonsine
Tables in 1504 in observing a conjunction
of Mars and Saturn, found them inaccu-
rate. Trusting in his own calculations, Co-
pernicus began questioning the Ptolemaic
system and concluded that a heliocentric
(sun-centered) structure accounted more
accurately for the motions of the planets.
This theory was revolutionary and, in
Copernicus’s view, dangerous, as it ques-
tioned the accepted wisdom supported by
the church for centuries. He did not allow
his theory to circulate in print until the
end of his life, although heliocentrism be-
came a common topic of debate among
clergy and scientists in the early sixteenth
century. The Alfonsine Tables would be re-
placed by the Prutenic Tables of Erasmus
Reinhold, which were based on the Coper-
nican heliocentric universe.
In 1572, the Danish astronomer Tycho
Brahe discovered a new star and, in 1577,
a comet. Through these observations
Brahe showed that the heavens were ever-
changing, producing new objects and phe-
nomena that were not accounted for inastronomy