Ch02 - Chapter 2 2.1 The solar system I A quick survey The...

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Chapter 2 The solar system I 2.1 A quick survey The solar system is dominated by the central sun - a star giving out light and heat, whose mass is about 300,000 times greater than that of the earth. Planets ( 行星 ): Eight planets (Fig. 2-1) and their satellites ( 衛星 ) lie close to a common plane. Planets move in nearly circular orbits around the sun in counter-clockwise sense as seen from “above” (from North Pole of the earth). Self-rotation is also in the counter-clockwise sense as seen from “above”, except for Venus and Uranus. Orbits of planets are not evenly spaced - distances between successive planets increase with their distances from the sun. Dwarf planets ( 矮行星 ): These are “minor” planets. The first three members are Ceres ( 穀神星 ) , Pluto ( 冥王星 ) and Eric (formerly known as 2003 UB 313 or Xena 齊娜 ). More about them will be discussed in Chapter 9. Fig. 2-1 The orbits of the planets lie almost in the same plane. Small solar-system bodies: including Asteroids ( 小行星 ) : Most can be found in the Asteroid belt ( 小行星帶 ) that lies between the orbits of Mars and Jupiter. Most asteroids are rocky debris having diameters less than 1 km.
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UGB 240M Astronomy The solar system I 2-2 Comets ( 彗星 ): “Dirty snow balls” moving in highly elliptical orbits around the sun. They become bright and develop tails when close to the sun. Meteoroids ( 隕星 ): Small interplanetary debris which hit the earth. They appear as sudden bright streaks in the sky called meteors ( 流星 ). Solar wind ( 太陽風 ): High energy charged particles blown off from the sun. Radioactive dating shows that meteorites are about 4.6 billion years old, which is about the same as the age for the entire solar system. 2.2 Universal gravitation ( 萬有引力 ) In hammer throw ( 投鏈球 ), the tension in the chain keeps the ball moving around a centre (Fig. 2-2a). Without the tension, the ball will fly straight away. The gravitational force from the sun is just like the tension in an invisible chain that keeps a planet in its orbit around the sun (Fig. 2-2b). More details will be discussed in Chapter 7. Fig. 2-2 (a) In hammer throw, the tension in the chain keeps the ball moving around a centre. (b) The gravitational force from the sun is just like the tension in an invisible chain. 2.3 Two kinds of planets A. Terrestrial planets (Mercury, Venus, Earth and Mars) ( 類地行星 ) All lie in the inner solar system (Fig. 2-3). Relatively dense (~3-5 g cm -3 , c.f. density of water = 1 g cm -3 ), with cores of iron ( ) and nickel ( ) surrounded by a mantle of dense rocks.
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UGB 240M Astronomy The solar system I 2-3 Small in size and mass weak gravity have a few satellites (e.g., one for Earth, two for Mars) and thin atmospheres, no ring systems. Their surfaces are scarred with craters.
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Ch02 - Chapter 2 2.1 The solar system I A quick survey The...

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