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The Planets

The Planets. Discussing Planets. Measure distances to planets in Astronomical Units (AU). 1 AU = distance from Earth to the Sun = 93 million miles. Planets orbit elliptically around sun. Perihelion : point closest to the sun Aphelion : point farthest from the sun.

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The Planets

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  1. The Planets

  2. Discussing Planets • Measure distances to planets in Astronomical Units (AU). • 1 AU = distance from Earth to the Sun = 93 million miles. • Planets orbit elliptically around sun. • Perihelion: point closest to the sun • Aphelion: point farthest from the sun.

  3. Discussing Planets, continued • Orbit period: the amount of time it takes the planet to make ONE revolution around the sun (a planet’s “year”). • Rate of Rotation: how long it takes the planet to rotate once around its own axis (a planet’s “day”).

  4. The Inner Planets: “Terrestrial Planets” • All are mostly solid rock with a metal core. • Lighter elements have lower boiling points; can’t condense this close to sun: only rocks and metals can. • Dense elements like iron are pulled by gravity to center of planet during accretion (planet formation) • No rings • Surfaces have many impact craters caused by collisions with rocky objects (asteroids).

  5. Mercury • Closest to sun • Orbit period: 88 days • Rotates slowly on its axis: one “day” is 59 Earth days. • No moons • Very, very thin atmosphere (planet is too small to have enough gravity to hold gases) • Huge temperature range between day and night.

  6. This image of Mercury passing in front of the sun was captured Nov. 8, 2006 by the Solar Optical Telescope, one of three primary instruments on Hinode. Image credit: Hinode JAXA/NASA/PPARC

  7. Venus • 2nd planet from sun. • Orbit period: 225 days. • Also rotates slowly on axis: one “day” is 243 Earth days. • Shows retrograde rotation: rotates around its own axis opposite of other planets. • Sun rises in west and sets in east! • No moons.

  8. Venus, continued • Called Earth’s sister planet • Has almost the same size, mass, and density. • Very hot planet because of its thick atmosphere. • Atmosphere is 96% Carbon Dioxide clouds, which trap heat. • No liquid water; rains sulfuric acid instead. • Shows evidence of volcanic activity; has rocks and surface features (mountains, canyons, etc.) similar to Earth.

  9. Earth • 3rd planet from Sun. • Orbit period: 365.24 days • Rotation rate: 23 hours, 56 minutes • 5th largest planet in solar system; has only one moon. • Only planet in solar system with • oceans of liquid water • Proper combination of water, temperature, and oxygen to support life.

  10. Earth, continued • Has moderate atmosphere of Nitrogen and Oxygen gas, with small amounts of water vapor, carbon dioxide, and other gases. • Tilted 23.5 degrees from the perpendicular; North Pole is always tilted toward North star. • The tilt of Earth causes seasons: changes the angle at which sun hits Earth’s surface. • N. Hemisphere toward sun: summer, because days are longer and we get more direct sunlight. • N. Hem. away from sun: winter, longer days and less direct sunlight.

  11. Earth’s Moon • 1/80th the size of Earth, with 1/6th the surface gravity. • Shines because it reflects sunlight. • Formed when a large, Mars-sized planetary body collided with Earth early on in its formation. • Collision ejected Earth-like materials into space; orbited Earth and accreted

  12. Earth’s Moon, continued • No atmosphere; wide temperature range • Heavy cratering on Moon due to many asteroid impacts. • Evidence of volcanic activity (old lava-filled craters). • Craters and rocks are well preserved, (no wind, weather, water, or ice to erode them). • The moon is responsible for: • Influencing ocean tides on Earth • Stabilizing Earth’s rotation • Gradually slowing Earth’s rotation through friction. • In 1,000 years, the day will be 0.2 seconds longer!

  13. Eclipses • Eclipse: planetary body passes through the shadow of another. • Umbra: shadow where sunlight is completely blocked. • Penumbra: part of shadow where sunlight is partially blocked.

  14. Solar eclipse: moon passes between Earth and Sun • Moon’s shadow blocks out sunlight in a small part of the Earth. • Totally blocked (total eclipse) in the umbra. • Partial eclipse in the penumbra. • Lunar eclipse: Earth passes between moon and sun, preventing sunlight from reaching the moon.

  15. Mars • 4th planet from sun • Orbit period: 687 days • Rotation: 24 hours, 37 minutes • Similar tilt in axis like Earth; has seasons • Has 2 moons, Phobos and Deimos • Geologically active with large volcanoes (largest in solar system!) • Little water is trapped in polar ice caps • No real atmosphere

  16. Olympus Mons: largest volcano in solar system!

  17. Mars’ Moons: Phobos and Deimos • May be the remains of one moon that was split by an asteroid • Very small and irregularly shaped, not round like our moon • Dark surfaces that are heavily cratered • Craters indicated heavy bombardment by asteroids • Very old

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