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skyandtelescope.com • AUGUST 2019 59
Sun-centered — or heliocentric — representation of the cosmos.
This revolutionary idea countered the teachings of the then
very powerful Catholic Church, which embraced Aristotle’s
Earth-centered — or geocentric — picture, which by then had
prevailed with minor modifi cations for some 1,800 years.
Copernicus completed work on his model in 1532 but didn’t
publish the treatise that outlined his ideas, De revolutioni-
bus orbium coelestium (On the Revolutions of the Heavenly
Spheres), until 1543. He built his system more on mathemati-
cal analysis and insight than on observations, and except for
exchanging the positions of the Sun and Earth, his model,
though a step in the right direction, didn’t differ extensively
from Ptolemy’s. Both systems assumed circular orbits and
both used epicycles, but Copernicus’s, despite diffi culties and
inconsistencies, opened the door to modern astronomy.
In the late 1500s, fate paved the way for a very improbable
partnership between a wealthy, self-centered Danish noble-
man, Tycho Brahe (1546–1601), and an underprivileged
German mathematician with mystical tendencies, JohannesMay 30, 2020June 91929July 91929(^2818)
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Or
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Sun
quMARS THE TRICKSTER When
viewed from Earth, planets appear to
travel from west to east — most of
the time. When a planet is near op-
position, it appears to move east to
west, a phenomenon caused by the
relative motion of Earth and planet.
For superior planets (those that orbit
the Sun outside of Earth’s orbit), this
apparent change of direction is rela-
tively easy to observe, as is shown
below in a chart of the position of
Mars near the 2020 opposition.
F
or the entirety of human civilization, roughly 6,000
years, most of the stars in the nighttime sky have
seemingly retained the same relative positions. The
stars are so far from us, and in most cases one another, that
the motions we now know they possess are almost insignifi -
cant. Coupled with the “fi xed” stellar background and easily
noticeable to the earliest watchers of the heavens are fi ve
bright celestial dots of light — Mercury, Venus, Mars, Jupiter,
and Saturn — which, like the Moon, move about in predict-
able yet at times complex ways. By 400 BC, and possibly as
early as the 6th century BC, the ancient Greeks were articu-
lating interpretations of their observations of the fi ve objects,
which they named plane ̄te ̄s, meaning “wanderer,” from which
the English word planet is derived. Underlying assumptions,
promulgated chiefl y by Aristotle (384–322 BC), were that all
cosmic motions are circular and uniform, that the cosmos
beyond the Moon is a fi nite, spherical, eternal, and change-
less model of perfection, and that Earth occupies the center
of the unchanging sphere, thereby implying that the planets
revolve around us.
Contrary to the order inferred by Aristo-
tle’s suppositions was the puzzling observed
“retrograde motion,” occurring when the outer
planets, Mars, Jupiter, and Saturn, are at oppo-
sition — opposite in the sky from the Sun when
viewed from Earth — and temporarily appear
to reverse course. Generally the outer planets
move eastward with respect to the stars, but as
the more rapidly orbiting Earth passes on the
inside track, they temporarily veer westward.
We now know this illusion results from the
combined motions of Earth’s revolution around
the Sun with those of our more distant solar
system neighbors.
Continuing observations, which didn’t nec-
essarily fi t theory, compelled unwieldy refi ne-
ments of the models. Representations of circu-
lar orbits and spheres carrying the planets were
the norm in ancient Greek astronomy. The
development of such models culminated with
Claudius Ptolemy (c. AD 100–170), a Greco-
Roman astronomer working in the Roman city
of Alexandria. The Ptolemaic model assumed
each planet traveled on its own circular path
called an epicycle. In turn, the epicycle orbited
the Earth along a larger circle called a deferent.
This interconnected system of circles addressed
the varying speeds and occasional changes in
direction of the planets observed by Earth-
bound astronomers.
A Sun-Centered System
There things stood until the early 1500s, when
Nicolaus Copernicus (1473–1543), a Polish
cleric, physician, and astronomer, articulated a