1. Section 25.1

2. Models of the Solar System Most ancient Greeks though the Earth was the center of the universe. Geocentric Model- (“geo” meaning Earth). Earth is stationary while object in the sky move around it. Original model was modified about A.D. 140 by an Egyptian astronomer.

3. Models of the Solar System (cont.) Not everyone thought the Earth was the center of the universe in ancient times. Heliocentric Model- Earth and other planets revolve around the sun. Over time, other scientists observed this idea and proved this model was correct.

4. Planetary Orbits Johannes Kepler discovered that the orbit of a planet around the sun is not a circle but an ellipse (an oval). Most planets orbit is circular, while some are elliptical. Plane of this imaginary tabletop containing Earth’s orbit is called the ecliptic plane.

5. Planetary Orbits (cont.) Gravity and inertia combine to keep the planets in orbit around the sun. The sun is exerting a gravitational force on the planets to keep them in orbit. Otherwise, the planet's inertia would cause them to fly out into space.

6. Components of the Solar System Our solar system consists of the sun, the planets, their moons, and a variety of smaller objects that mostly revolve in the same plane around the sun. Except for Mercury and Venus, all the planets have moons. A moon is a relatively small natural body in space that revolves around a planet.

7. Components of the Solar System (cont.) Planets and moons do not produce their own light, the sun’s light reflects of from their surfaces. Distances between objects in the solar system are much smaller than distances to the stars, but much larger than distances on Earth. Astronomers use astronomical units (AU) to describe distances between the solar system. 1 AU = 149,598,000 km

9. Exploring the Solar System Modern technology, including complex telescopes, piloted spacecraft, and space probes, has allowed scientist to explore the solar system. First satellite: Sputnik 1, 1957 (Soviet Union) First human to orbit Earth: Yuri Gagarin – April 12, 1961 (Soviet Union) First America in space: Alan Shepard – May 5, 1961 (USA)

10. Travel to the Moon After Shepard’s flight, US president John F. Kennedy directed the National Aeronautics and Space Administration (NASA) to place an astronaut on the moon. Series of space flights were called the Apollo Program On July 20, 1969, Neil Armstrong became the first person to set foot on the moon.

11. Recent Space Missions Humans have not travelled across the solar system, but scientists have gathered much new information about moons and planets. Space probes- unpiloted vehicle that carries scientific instruments into space and transmits information back to Earth. Hubble Space Telescope (launched in 1990) has provided many new views of the solar system and beyond.

12. Recent Space Missions Today, countries cooperate in the exploration of space. The International Space Station, is a permanent laboratory designed for research in space. Astronauts from various countries live abroad the station