Sky & Telescope - August 2019 - 59

or the entirety of human civilization, roughly 6,000
Sun-centered - or heliocentric - representation of the cosmos.
years, most of the stars in the nighttime sky have
This revolutionary idea countered the teachings of the then
seemingly retained the same relative positions. The
very powerful Catholic Church, which embraced Aristotle's
stars are so far from us, and in most cases one another, that
Earth-centered - or geocentric - picture, which by then had
the motions we now know they possess are almost insigniﬁprevailed with minor modiﬁcations for some 1,800 years.
cant. Coupled with the "ﬁ xed" stellar background and easily
Copernicus completed work on his model in 1532 but didn't
noticeable to the earliest watchers of the heavens are ﬁve
publish the treatise that outlined his ideas, De revolutionibright celestial dots of light - Mercury, Venus, Mars, Jupiter,
bus orbium coelestium (On the Revolutions of the Heavenly
and Saturn - which, like the Moon, move about in predictSpheres), until 1543. He built his system more on mathematiable yet at times complex ways. By 400 BC, and possibly as
cal analysis and insight than on observations, and except for
early as the 6th century BC, the ancient Greeks were articuexchanging the positions of the Sun and Earth, his model,
lating interpretations of their observations of the ﬁve objects,
though a step in the right direction, didn't differ extensively
which they named plane¯te¯s, meaning "wanderer," from which
from Ptolemy's. Both systems assumed circular orbits and
the English word planet is derived. Underlying assumptions,
both used epicycles, but Copernicus's, despite difﬁculties and
promulgated chieﬂy by Aristotle (384-322 BC), were that all
inconsistencies, opened the door to modern astronomy.
cosmic motions are circular and uniform, that the cosmos
In the late 1500s, fate paved the way for a very improbable
beyond the Moon is a ﬁnite, spherical, eternal, and changepartnership between a wealthy, self-centered Danish nobleless model of perfection, and that Earth occupies the center
man, Tycho Brahe (1546-1601), and an underprivileged
of the unchanging sphere, thereby implying that the planets
German mathematician with mystical tendencies, Johannes
revolve around us.
Contrary to the order inferred by Aristoqu MARS THE TRICKSTER When
tle's suppositions was the puzzling observed
viewed from Earth, planets appear to
"retrograde motion," occurring when the outer
travel from west to east - most of
the time. When a planet is near opplanets, Mars, Jupiter, and Saturn, are at oppoposition, it appears to move east to
sition - opposite in the sky from the Sun when
west, a phenomenon caused by the
Sun
viewed from Earth - and temporarily appear
relative motion of Earth and planet.
to reverse course. Generally the outer planets
For superior planets (those that orbit
move eastward with respect to the stars, but as
the Sun outside of Earth's orbit), this
it
of
Apparent path
E a rt h
apparent change of direction is relathe more rapidly orbiting Earth passes on the
O
of Mars
rb
tively easy to observe, as is shown
inside track, they temporarily veer westward.
it o
from Earth
f M ars
below in a chart of the position of
We now know this illusion results from the
Mars near the 2020 opposition.
combined motions of Earth's revolution around
the Sun with those of our more distant solar
system neighbors.
Continuing observations, which didn't necessarily ﬁt theory, compelled unwieldy reﬁneARIES
ments of the models. Representations of circu14
PEGASUS
+20°
lar orbits and spheres carrying the planets were
Feb 4
PISCES
25
the norm in ancient Greek astronomy. The

RE TROG R A DE M OTION: G REGG DINDER M A N / S&T

development of such models culminated with
Claudius Ptolemy (c. AD 100-170), a GrecoRoman 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 Earthbound astronomers.

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June 9
May 30, 2020

CETUS

A Sun-Centered System
There things stood until the early 1500s, when
Nicolaus Copernicus (1473-1543), a Polish
cleric, physician, and astronomer, articulated a

0°

AQUARIUS
3h

23h -20°

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Sky & Telescope - August 2019

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