The movements of planets and other nearby
objects are visible from Earth.
A jet plane travels at a greater speed and altitude than a bird. Yet if a
bird and a plane flew overhead at the same time, you might think that
the bird was faster. You would have this impression because the farther
away a moving object is from you, the less it seems to move.
Stars are always moving, but they are so far away
that you cannot see their movements. Observers have
seen the same constellation patterns for thousands
of years. Only over a much longer period does the
motion of stars gradually change constellation patterns.
The Motion of Mars
Sept. 20th
Oct. 4th
Mars
Mars
By contrast, the Moon moves across the star
background a distance equal to its width every hour
as it orbits Earth. The Moon is our closest neighbor.
The planets are farther away, but you can see their
gradual movements among the constellations over
a period of weeks or months.
These illustrations show how Mars changes
positions in the constellation Gemini over a
period of two weeks.
Planet comes from a Greek word that means
“wanderer.” Ancient Greek astronomers used this
term because they noticed that planets move among
the constellations. It is easiest to see the movements of Venus and
Mars, the two planets closest to Earth. They change their positions
in the sky from night to night.
The apparent movement of the sky led early astronomers to
believe that Earth was at the center of the universe. Later astronomers
discovered that Earth and the other planets orbit the Sun. The timeline on pages 648–651 introduces some of the astronomers who
helped discover how planets really move in the solar system.
KEY CONCEPTS
CRITICAL THINKING
1. What are the basic structures
in which objects are grouped
together in space?
4. Compare and Contrast
How is the grouping of stars
in a constellation different
from the grouping of planets
in the solar system?
2. What is a constellation?
3. How does Earth’s rotation
affect our view of stars?
590 Unit 5: Space Science
5. Apply The planet Jupiter is
farther than Mars from Earth.
Which planet seems to move
faster when viewed from
Earth? Explain.
CHALLENGE
6. Predict Suppose that you
are standing at the North Pole
on a dark night. If you keep
turning clockwise at the same
speed as Earth’s rotation, how
would your movement affect
your view of the stars?