lecture23spring2009 - Astronomy100Dr.Rhodes Lecture Chapter...

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Astronomy 100 - Dr. Rhodes Lecture # 23 - 3/11/2009
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Chapter 10: Dwarf Planets and Solar System Debris Comets Comets are among the most spectacular sights available to the naked eye. Comet Orbits—Newton and Halley Newton proposed that comets orbit the Sun according to the laws of gravitation. Newton concluded that since comets were visible for only short periods of time, their orbits were very eccentric, i.e., elongated.
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Chapter 10: Dwarf Planets and Solar System Debris Comets Edmund Halley , a friend of Newton, used Newton’s methods, his own observations, and prior comet descriptions to calculate orbits for a number of comets. He correctly surmised that these prior comets were in fact the same comet. He correctly predicted the next return of the comet that was then named in his honor. Comet Halley is probably the most famous periodic comet.
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Chapter 10: Dwarf Planets and Solar System Debris Comets Comet Halley has a period of approximately 76 years. Because the comet is slightly affected by the gravity of the planets during its return from deep space, its period varies slightly one appearance to the next. Halley was the first to recognize this effect of other bodies on comets’ orbits. About 100 comets have periods of less than 200 years.
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Chapter 10: Dwarf Planets and Solar System Debris Comets The planes of revolution of comets are not limited to the ecliptic but are randomly oriented. Comets sweep past the Sun from all directions. The head of a comet can be as large as a million kilometers in diameter. The tail of a comet can be as long as 1 AU.
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Chapter 10: Dwarf Planets and Solar System Debris Comets The Parts of Comets The head of a comet consists of its coma and nucleus. The coma is the part of a comet’s head made up of a diffuse cloud of gas and dust. The nucleus of a comet is the solid chunk of a comet, located in the head. The tail of a comet is the gas and/or dust that is swept away from a comet’s head. All comets have a straight gas , or ionic , tail; some comets also have a curved dust tail .
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Changes in a Comet’s Tails Over a Few Days (The Gas Tail is Changing More Rapidly Than is the Dust Tail)
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Chapter 10: Dwarf Planets and Solar System Debris Comets U.S. astronomer Fred Whipple proposed in 1950 that the nucleus of a comet is essentially a dirty snowball made up of water ice, frozen carbon dioxide, and small solid grains.
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