astro 100 unit11.pdf
astro 100 unit11.pdf astro100-02 Exploring the Universe
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astro100-02 Exploring the Universe
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This 5 page Class Notes was uploaded by Coco Chen on Sunday February 22, 2015. The Class Notes belongs to astro100-02 Exploring the Universe at University of Massachusetts taught by Daniel Wang in Winter2015. Since its upload, it has received 124 views. For similar materials see Exploring the Universe in Astronomy at University of Massachusetts.
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Date Created: 02/22/15
Unit 11 Planets The Wandering Stars Planetai from which the word planet come from means wanders 5 of the brightest stars that gradually shift from night to night instead of remaining in the same position on the celestial sphere Until telescope was invented planets were thought to be special stars and were free to roam in the sky And from their freedom to roam we can tell that they had special powers And as astronomers studied these motions and patterns of the motions they realized that Earth itself was also a planet A combination of the Earth s and the stars own orbital motions around the Sun causes planets to move against the background of stars Feature of this orbital motion planets always remain within a narrow band on either side of the ecliptic within the constellation of the zodiac a planets remain close to the ecliptic amp nearly all lie in the same plane see below planets usually move from west to east pass the stars but they don t always rise in the east and set in the west though seeing from Earth they rise in the east and set in the west because their movements on the celestial sphere were much slower than Earth s rotation NOTE Pluto Eris and a number of other small solar system bodies sometimes move outside the zodiac The motion of planets E o l St West Star chart Retrograde background moving motions a planet will move west with respect to the stars When a planet is in this motion its path through the stars turns or loops backward for a month or more Planets still rise in the east and set in the west because this motion is slow compared to Earth s rotation All planets undergo this motion when they lie in the same direction as Earth from Sun Geocentric modelsGreek astronomer Eudoxus Earth is the center of the universe and planets and stars move around it at different speed because everything rise in the east and set in the west The bodies that are closest to the Earth seem to move at a fastest speed To explain the Retrograde motion Eudoxus requires that each planet s crystalline sphere should itself be attached to another crystalline sphere that rotated at a different angle By combining the steady rotation of both spheres a zigzag path has roughly resembled retrograde motion Over the next 5 centuries Claudius Ptolmey developed a model that could predict planets positions with good accuracy Ptolmey each planet move in a small circle epicycle which in be carried along on a larger circle like a frisbee spinning on the rim of a bicycle wheel the general eastward motions of planets relative to stars is caused by a slow rotation of the larger cycle relative to the celestial sphere retrograde motion occurs when the epicycle carries the planet in a reverse direction relative to the celestial sphere device to support the model the larger circle move around the Earth and not at a constant speed around Earth but instead around another point in space but the model only lasts till 1500s because it is very complex and requires too much parameters that seemed arbitrary simplicity is important in scientific theory Nicolaus Copernicus began the demolition of the geocentric model and started a revolution in astronomical thinking Heliocentric model Sun is the center of a system of planets and Earth is just one of them he showed that such model offer a far more simple explanation of retrograde motion a simple consequence of one planet on a smaller faster orbit overtaking and passing another on a larger amp slower orbit retrograde motion occurs to Earth around time of opposition of Mars conjunction when a planet lies in the same direction as the Sun on the far side of the Sun Superior conjunction Venus and Mars undergo retrograde motion when they pass Earth in inferior conjunction bewtween Earth and Sun largest angular separation from the Sungreatest elongation is determined by the sizes of the planets orbits relative to the Earth s orbit quadrature900 from the Sun he could also use Heliocentric model to calculate the relative size of all of the planets orbits and derive the distance of the planets by using geometry
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