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26.2
Observing the Solar System:
A History
KEY IDEA
When you look up and notice that the sun has moved from east to west
over the course of a few hours, doesn’t it seem as though Earth is standing
still and the sun is moving across the sky? This perception—that Earth is
stationary—was the basis of ancient Greek theories that prevailed for over
two thousand years.
The Movements of Planets and Stars
For thousands of years, the predominant model of the universe stated that
Earth stood still at the center of the universe. Such a model of the universe
is called a geocentric (JEE-oh-SEN-trihk), or Earth-centered, model.
As long as 6000 years ago, astronomers were recording the movements
of the stars. They noted that the stars appeared to move across the sky, but
they did not move in relation to each other. To explain the apparent
motions of the stars, early astronomers envisioned the stars as holes in a
solid celestial sphere that surrounded Earth. Beyond the sphere, they
imagined, was a source of intense light that shone through the holes. They
concluded that the stars moved around Earth as the sphere rotated.
Early astronomers also noticed that the same constellations, or groups
of stars, became visible at the same time every year. People in many
different cultures noticed this phenomenon and used the changing
constellations as a basis for a calendar.
Not all points of light in the sky are fixed in constellations. A few seem
to wander across the sky, changing position over the course of days, weeks,
and months. These wandering points of light are planets, and early
astronomers inferred correctly that planets are closer to Earth than the
stars are.
Early astronomers noticed that most of the time the planets moved
eastward in front of the background of constellations. Periodically,
however, the planets stopped moving eastward and moved westward for a
few weeks, then resumed their eastward paths. This pattern of apparent
backward motion is called retrograde motion.
Mar.
April
Sept.
Aug.
Jan.
Feb.
May
Throughout history scientists
have developed models to
account for their observations of
the stars and planets. The work of
past scientists contributes to
today’s sun-centered model of the
universe.
KEY VOCABULARY
• geocentric
• heliocentric
• gravitation
DEVELOP MODELS
Design a way to demonstrate
retrograde motion.
Dec.
June
July
West
East
RETROGRADE MOTION Viewed from Earth, the planet Mars seems to go backward
during its orbit. The effect is due to Earth catching up with and passing Mars.
Chapter 26 The Sun and the Solar System
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