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Planetary Motion < Back Next > Preview Main Planetary Motion A Revolution in Astronomy • Each planet spins on its axis. The spinning of a body, such as a planet, on its axis is called rotation. • The orbit is the path that a body follows as it travels around another body in space. • A revolution is one complete trip along an orbit. < Back Next > Preview Main Planetary Motion A Revolution in Astronomy • When Earth completes one FULL rotation around it’s axis, what have we experienced? How long does this take? •What Earth completes one FULL revolution around the sun, what have we experienced? How long does this take? < Back Next > Preview Main Earth’s Rotation and Revolution < Back Next > Preview Main Planetary Motion A Revolution in Astronomy, continued • Johannes Kepler made careful observations of the planets that led to important discoveries about planetary motion in the early 1600s. • Kepler’s First Law of Motion: Kepler discovered that the planets move around the sun in elliptical orbits. What type of model is this??? Heliocentric model < Back Next > Preview Main Planetary Motion A Revolution in Astronomy, continued • Kepler’s Second Law of Motion: Kepler noted that the planets seemed to move faster when they are close to the sun and slower when they are farther away. • Kepler’s Third Law of Motion: Kepler observed that planets more distant from the sun, such as Saturn, take longer to orbit the sun. < Back Next > Preview Main Planetary Motion Newton to the Rescue! • Kepler did not understand what causes the planets farther from the sun to move slower than the closer planets. • Sir Isaac Newton’s description of gravity provides an answer. < Back Next > Preview Main Planetary Motion Newton to the Rescue! continued • The Law of Universal Gravitation: Newton’s law of universal gravitation states that the force of gravity depends on the product of the masses of the objects divided by the square of the distance between the objects. • According to this law, if two objects are moved farther apart, there will be less gravitational attraction between them. < Back Next > Preview Main Our Solar System < Back Next > Preview Main Our Solar System Our Solar System Our solar system includes the sun, the planets, and many smaller objects. < Back Next > Preview Main Our Solar System Measuring and Interplanetary Distances Scientists use the astronomical unit to measure distances in space. One astronomical unit is the average distance between the sun and Earth, or approximately 150,000,000 km (or 93,205,678.8 miles) < Back Next > Preview Main Our Solar System The Inner and Outer Solar Systems • The Inner Planets: The planets closest to the sun include Mercury, Venus, Earth, and Mars. • The Outer Planets: The outer planets include Jupiter, Saturn, Uranus, and Neptune. < Back Next > Preview Main • The Inner Planets: The planets closest to the sun include Mercury, Venus, Earth, and Mars. < Back Next > Preview Main • The Outer Planets: The outer planets include Jupiter, Saturn, Uranus, and Neptune. < Back Next > Preview Main The Inner Planets Mercury: Closest to the Sun Mercury is a very hot, small planet. It only takes Mercury 88 days to revolve around the sun. < Back Next > Preview Main The Inner Planets Venus: Earth’s Twin? • The Atmosphere of Venus: Of all the inner planets, Venus has the densest atmosphere. • Mapping Venus’s Surface: The Magellan spacecraft mapped the surface of Venus by using radar waves. < Back Next > Preview Main The Inner Planets Earth: An Oasis in Space • Water on Earth: Earth is warm enough to keep most of its water from freezing and cool enough to keep its water from boiling away. Liquid water is important to life on Earth. • The Earth from Space: Satellites are used to study the Earth from space in order to better understand global systems. < Back Next > Preview Main The Inner Planets Mars: Our Intriguing Neighbor • The Atmosphere of Mars: Mars has a thin atmosphere with low air pressure. • Water on Mars: Liquid water cannot exist on Mars’s surface today, but most likely it was there in the past. < Back Next > Preview Main The Inner Planets Mars: Our Intriguing Neighbor, continued • Where Is the Water Now?: Mars has two polar icecaps made of frozen water and carbon dioxide. Many scientists think that there is more frozen water beneath the Martian soil • Martian Volcanoes: Mars has two large volcanic systems, one of which includes the largest mountain in the solar system. < Back Next > Preview Main The Inner Planets Mars: Our Intriguing Neighbor, continued • Missions to Mars: Several recent missions to Mars were launched to gain a better understanding of the planet. < Back Next > Preview Main The Outer Planets Jupiter: A Giant Among Giants • Jumbo Sized: Jupiter is the largest planet in our solar system. • NASA Missions to Jupiter: NASA has sent five missions to Jupiter to study Jupiter’s atmosphere and moons. < Back Next > Preview Main The Outer Planets Saturn: Still Forming • The Rings of Jupiter: Saturn’s rings are the largest of all of the gas giants’ rings. • NASA Exploration of Saturn: Launched in 1997, the Cassini spacecraft is designed to study Saturn’s rings, moon, and atmosphere. < Back Next > Preview Main The Outer Planets Uranus: A Small Giant • Uranus’s Atmosphere: The atmosphere of Uranus is mainly hydrogen and methane, which makes the planet appear to be blue-green in color. < Back Next > Preview Main The Outer Planets Uranus: A Small Giant, continued • A Tilted Planet: Unlike most other planets, Uranus is tipped over on its axis. So its axis of rotation is tilted by almost 90° and lies almost in the plane of its orbit. < Back Next > Preview Main The Outer Planets Neptune: The Blue World • Discovery of Neptune: Neptune was not discovered until 1846. • The Atmosphere of Neptune: The composition of Neptune’s atmosphere is similar to that of Uranus’s atmosphere, but Neptune has belts of clouds that are much more visible. < Back Next > Preview Main The Outer Planets Pluto: A Dwarf Planet • A Small World: Pluto is less than half the size of Mercury. • Reclassified: Since its discovery in 1930, Pluto has been called the ninth planet. However, in 2006, it was reclassified as a dwarf planet. • A dwarf planet is any object that orbits the sun, is round because of its own gravity, but has not cleared its orbital path. < Back Next > Preview Main • "Hasn't cleared its orbital path" means that in Pluto's orbital space there are other objects still there. • Planets have cleared their orbital path of debris and small objects, but Pluto orbits within the Kuiper Belt, a region filled with millions of small objects. < Back Next > Preview Main