8.3: The Solar System: The Sun and the Planets pg. 313

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1 8.3: The Solar System: The Sun and the Planets pg. 313 Key Concepts: 1. Careful observation of the night sky can offer clues about the motion of celestial objects. 2. Celestial objects in the Solar System have unique properties. 3. Some celestial objects can be seen with the unaided eye and can be identified by their motion. 4. The Sun emits light and other forms of radiant energy that are necessary for life to exist on Earth. 5. Satellites have useful applications for technologies on Earth. 6. The study of the night sky has influenced the culture and lifestyles of many civilizations. - The Sun, its eight planets and moons make up our Solar System. - Planets are close to the Sun. (Mercury 58 million km, Neptune 4 billion km. - At km/h, a space craft will take 50 years to travel across the Solar System. Figure 1: This drawing shows the Solar System, but does not illustrate true relative distance or sizes.

2 Measuring Distances in the Solar System Astronomical Units: approximately 150 million kilometers; the average distance from Earth to the Sun. - The Astronomical Unit (AU) is equal to 1.5 x 10 8 km. - The Astronomical unit is equal to the distance from the Sun to Earth. All other planets are compared to Earth. - Jupiter is 5.2 AU from the Sun; therefore it is 5.2 X 1.5 X 10 8 km. Jupiter is km from the Sun. Planets Big and Small - Other then the Sun the planets are the largest objects in the Solar System. - Without the use of an optical aid, we can see Venus, Mars, and Jupiter at night. - The inner solar system consists of Mercury, Venus, Earth, and Mars, Terrestrial Planets. - The giant planets, beyond Mars are; Jupiter, Saturn, Uranus, and Neptune, make up the outer Solar System. These are gaseous planets. Dwarf Planets Dwarf Planets: a celestial object that orbits the Sun and has a spherical shape but does not dominate its orbit. - To be considered a planet a celestial object must express the following characteristics; * be in orbit around a star (such as the Sun) * have enough mass to be pulled into a stable sphere shape by gravity. * dominate its orbit (i.e., its mass must be greater than anything else that crosses its orbit)

3 Figure 2: Pluto does not meet the criteria for a planet because its tilted orbit crosses Neptune s orbit. - Pluto was once considered a planet. - After 2006, the definition of a planet changed, Pluto was no longer considered a planet. - It was newly categorized as a Dwarf Planet. - Pluto orbits the Sun and is spherical in shape, but it does not dominate its orbit. - There are now 5 Dwarf Planets: Pluto, Ceres, Haumea, Makemake, and Eris. - These five lie outside the orbit of Neptune. - Scientists estimate there are 200 to 2000 dwarf planets, most should be found within the Kuiper belt. Smaller Members of the Solar System - The solar system also contains smaller celestial objects, rock, metal, and ice composition.

4 Asteroid Belt - Objects composed of rock and metal are called Asteroids. - They orbit the Sun, yet they are too small to be planets. - Most are found in the Asteroid Belt, located between Mars and Jupiter. - Some asteroids can be as large as 950 km in diameter, and irregular in shape. Meteoroids - Objects smaller then asteroids are called meteoroids, and are also made up of metal and rock. - The size of a meteoroid ranges from dust particles up to the size of cars. - Meteoroids that enter the Earth s atmosphere are called meteors and burn up before they reach the Earth s surface. - Meteorites are meteors that actually reach the Earth s surface, crashing into the ground creating craters. Meteor Showers - On average a meteor enters our atmosphere every 15 minutes. - There are time periods where the number increases per minute. Meteor showers or shooting stars travel at a speed of 1.5 X 10 5 km/h.

5 Comets Comets: a chunk of ice and dust that travels in a very long orbit around the Sun. - Comets are large chunks of ice, dust and rock. - Comets orbit the Sun. - Comets can be classified as either short or long period comets. - Short period comets originate from a region outside Neptune and travel around the Sun within 200 years. - Hailey s comet orbits the sun every years. - Long period comets originate outside of Pluto s orbit and takes more then 200 years to orbit the Sun. - Hale-Bopp takes 2380 years to orbit the Sun. - When a comet gets closer to the Sun, the solid ice becomes a gas (sublimates), gas and dust particles are released. - These materials surround the nucleus (icy nucleus as sublimation occurs) creating a Coma, a gaseous cloud 1000 s of kilometers wide. - The Sun s radiation and solar wind acts upon the coma causing tails to form. - The gaseous tail forms, directly pointing away from the Sun. - The dust tail trails from the direction of which the comet came from. - Gravity can pull comets into a planet or the Sun.

6 Figure 8: Most comets have two tails a gaseous tail and a dust tail Check Your Learning: Questions 1 9, page 317 Summary: - The average distance between the Sun and Earth is defined as one astronomical unit (AU). - The four planets closest to the Sun are the terrestrial planets. These small, rocky planets are considered part of the inner solar system. The four planets farthest from the Sun are gas giants. These large, gaseous planets are part of the outer solar system. - Pluto has bee reclassified as a dwarf planet, based on its physical properties and motion. - Besides the planets, there are smaller objects in the solar System that orbit the Sun, such as asteroids, comets, and meteoroids.

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