Most Greek scholars assumed that the Sun, the Moon, the stars, and
the planets revolve about a stationary
Earth. A model of this kind, in
which the Earth is at the center of
the universe, is called a geocentric
model. Similar ideas were held by
the scholars of ancient China.
Today we recognize that the stars are not merely points of
light on an immense celestial sphere. But in fact this is how the
ancient Greeks regarded the stars in their geocentric model of the
universe. To explain the diurnal motions of the stars, they assumed
that the celestial sphere was real, and that it rotated around
the stationary Earth once a day.
The Sun and Moon both participated in this daily rotation of
the sky, which explained their rising and setting motions. To explain
why the Sun and Moon both move slowly with respect to
the stars, the ancient Greeks imagined that both of these objects
orbit around the Earth.
ANALOGY Imagine a merry-go-round that rotates clockwise as
seen from above, as in Figure 4-1a. As it rotates, two children
walk slowly counterclockwise at different speeds around the
merry-go-round’s platform. Thus, the children rotate along with
the merry-go-round and also change their positions with respect
to the merry-go-round’s wooden horses. This scene is analogous
to the way the ancient Greeks pictured the motions of the stars,
Sun, and Moon. In their model, the celestial sphere rotated to
the west around a stationary Earth (Figure 4-1b). The stars rotate
along with the celestial sphere just as the wooden horses
rotate along with the merry-go-round in Figure 4-1a. The Sun
and Moon are analogous to the two children; they both turn
westward with the celestial sphere, making one complete turn
each day, and also move slowly eastward at different speeds
with respect to the stars.
The geocentric model of the heavens also had to explain the
motions of the planets. The ancient Greeks and other cultures of
that time knew of five planets: Mercury, Venus, Mars, Jupiter,
and Saturn, each of which is a bright object in the night sky. For
example, when Venus is at its maximum brilliancy, it is 16 times
brighter than the brightest star. (By contrast, Uranus and Neptune
are quite dim and were not discovered until after the invention of
the telescope.)
Like the Sun and Moon, all of the planets rise in the east and
set in the west once a day. And like the Sun and Moon, from
night to night the planets slowly move on the celestial sphere, that
is, with respect to the background of stars. However, the character
of this motion on the celestial sphere is quite different for the
planets. Both the Sun and the Moon always move from west to
east on the celestial sphere, that is, opposite the direction in which
the celestial sphere appears to rotate. The Sun follows the path
called the ecliptic (see Section 2-5), while the Moon follows a
path that is slightly inclined to the ecliptic (see Section 3-3). Furthermore,
the Sun and the Moon each move at relatively constant
speeds around the celestial sphere. (The Moon’s speed is faster
than that of the Sun: It travels all the way around the celestial
sphere in about a month while the Sun takes an entire year.) The
planets, too, appear to move along paths that are close to the
ecliptic. The difference is that each of the planets appears to wander
back and forth on the celestial sphere with varying speed. As
an example, Figure 4-2 shows the wandering motion of Mars
with respect to the background of stars during 2011 and 2012.
(This figure shows that the name planet is well deserved; it comes
from a Greek word meaning “wanderer.”)
Most of the time planets move slowly eastward relative to the
stars, just as the Sun and Moon do. This eastward progress is called
direct motion. For example, Figure 4-2 shows that Mars will be
in direct motion from October 2011 through January 2012 and
from April through August 2012. Occasionally, however, the
planet will seem to stop and then back up for several weeks or
months. This occasional westward movement is called retrograde
motion. Mars will undergo retrograde motion during February
and March 2012 (see Figure 4-2), and will do so again about
every 221⁄2 months. All the other planets go through retrograde
motion, but at different intervals. In the language of the merrygo-round analogy in Figure 4-1, the Greeks imagined the planets
as children walking around the rotating merry-go-round but who
keep changing their minds about which direction to walk!
Take care😊
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