Unlike the sun, planets and moons don't produce their own light. The nearby planets and moons are visible because sunlight is reflected from their surfaces. In Figure 3 you can see that the orbits of the first four planets—Mercury, Venus, Earth, and Mars—are relatively close to the sun. The orbits of the outer planets, Jupiter, Saturn, Uranus, and Neptune, are much farther from the sun.
Distances between objects in the solar system are much smaller than distances to the stars, but are much larger than distances on Earth. As a result, astronomers often use astronomical units to describe distances within the solar system. One astronomical unit (AU) equals the average distance from Earth to the sun—149,598,000 kilometers. In comparison, Neptune is an average of about 30.1 AU from the sun.
What is an astronomical unit?
Exploring the Solar System
Modern technology, including complex telescopes, piloted spacecraft, and space probes, has allowed scientists to explore the solar system. The first rockets powerful enough to escape Earth's atmosphere and enter space were developed in the 1940s and 1950s. The Soviet Union launched the first artificial satellite, Sputnik 1, into orbit in 1957. Sputnik stirred the United States to start its own space program. The socalled space race was underway. On April 12, 1961, Soviet cosmonaut Yuri Gagarin became the first human to orbit Earth. Astronaut Alan Shepard became the first American in space on May 5, 1961.
Travel to the Moon Following Shepard's flight, President John F. Kennedy directed the National Aeronautics and Space Administration (NASA) to launch a major effort to place an astronaut on the moon. The quest to reach the moon progressed through a series of increasingly complex missions. The final step in this effort was a series of space flights called the Apollo program. On July 20, 1969, Neil Armstrong, commander of the Apollo 11 spacecraft, became the first person to set foot on the moon.
Quick Lab
Modeling Orbits
Procedure 
Cut a 200-cm piece of string. Tie one end of the string securely to a whiffle ball.
Wrap the string's other end around your hand, leaving 50 cm of string between your hand and the ball.
To model a planet's orbit, swing the ball above your head with just enough force to keep the ball moving parallel to the ground. CAUTION Make sure the ball cannot hit anyone or anything. Record the number of orbits the ball makes in 10 s.
Repeat Step 3, increasing the string's length to 150 cm from the ball.
Analyze and Conclude
Calculating How long did a single orbit take in Step 3? In Step 4?
Analyzing Data How did the radius of the ball's orbit affect the time to complete one orbit?
Using Models How does this lab model the orbits of the planets?
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