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The Solar System Observing the Solar System • The first astronomers made important observations: – Star Trails – “Wandering Stars” – Models of the Solar System – Orbits • The stars appear to move across the sky in predictable patterns. • In the Northern hemisphere, the sky appears to rotate around “Polaris” – The North Star Star Trails “Wandering Stars” • These stars appeared to change position. • Planet- means wondering star in Greek • The Romans later named them: • Mercury, Venus, Mars, Jupiter, Saturn Models of the Solar System • The early Greek astronomers though that the Earth was the center of the universe: – Geocentric – the earth is the center of all revolving planets • Ptolemy (140 A.D.) explained the motions of the planets as they moved around Earth – Retrograde motion Heliocentric • In the 1500’s Copernicus created the heliocentric model – the Sun at the center – “Helios” means sun in Greek • This better explain the motions of the Planets Our solar system now has eight bona fide planets and three "dwarf planets" (Illustration: NASA/JPL) Contributions • Some astronomers who made important contributions to the study of our solar system: – – – – – – Ptolemy Copernicus Galileo Brahe Kepler Newton • Galileo – – Used a telescope to prove the heliocentric model – Observed Venus’ phases • Brahe and Kepler – Accurate observations to figure out the shapes of the planets orbits - ellipses • Newton – Concluded that two factors, inertia and gravity, combine to keep the planets in orbit Elliptical orbits • From the work of Brahe and Kepler we know that the orbits made by the planets are not circular – rather they are oval Ellipses Inertia and Gravity • Inertia- the tendency of an object in motion to remain in motion – The more mass the more inertia • Gravity is a force between two masses • A planet’s orbit is based on the balance between inertia and gravity The Planets What is a planet? • A planet is a celestial body that orbits a star and does not give off its own light. • There are two groups of planets; – Terrestrial – Gaseous Terrestrial Planets • Terrestrial planets are small, solid, and rocky • They are the first four planets from the Sun: – Mercury – Venus – Earth – Mars Mercury • Is about the size of Earth’s Moon • Has no moons of its own • Studied and photographed by the Mariner missions • Has no atmosphere Venus • Shines very brightly and can be seen in the early morning or early evening • About the same size as Earth, no moons • Has a highly pressurized atmosphere that experiences a green house effect • Rotates (very slowly) opposite from other plants – east to west • Studied and photographed by the Magellan missions Earth • Has an atmosphere that can support life • Has one moon • It over 70% water • Contains a layered interior Mars • aka “the Red Planet” • CO2 atmosphere • Has ‘frozen’ polar ice caps • Has a tilted axis like earth – so it has seasons • Has two small moonsPhobos and Deimos • Studied by the Viking missions, Mars Pathfinder, Sojourner, and Surveyor The Outer Planets • Gaseous Planets (or Jovian) – made almost entirely of gasses – Jupiter, Saturn, Uranus, Neptune • Dwarf Bodies – the small, rocky, icy planets that include Pluto and beyond….. – Pluto, UB313 (Xena) Ceres, Structure of Gas Giants • The first four outer planets are much larger than Earth and made almost entirely of gases. – Jupiter, Saturn, Uranus, Neptune • Have more mass, so exert higher gravitational forces • Possible solid cores…. • All have a ring system Jupiter • 300x more massive than Earth • Atmosphere is banded in color – Contains the “Great Red Spot” • Over 17 moons • Galilean Moons: Io, Callisto, Europa, Ganymede Io Callisto Europa Ganymede • Beautiful ring system • Less dense than water • 18 moons – Titan is larger than Earth’s Moon Saturn Uranus • Looks bluish due to traces of methane • Axis is tiled 90° • 21 moons – Mostly ice Neptune • Contains visible clouds • Has a storm similar to Jupiter's; The Great Dark Spot • 8 moons – Triton is the Largest Triton Dwarf Bodies • Smaller planets beyond the gas giants • Rocky, Icy worlds • Includes: – Pluto, Ceres, UB 313