Planets display both forward motionaka direct w e and

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Planets display both forward motion(a.k.a. direct W E) and backward motion(a.k.a. retrograde: E W) 2. A planet’s angular speed is not constant ( planets slow down and speed up in the sky) Eudoxus (~410-355) : Attempted to explain planetary motion by placing the planets on systems of invisible nested spheres ( crystal orbs ) CCM ( constant circulate motion ) He explained the features of planetary motion by proposing: Each planet is fixed to a pair of invisible counter-rotating spheres, which induce a figure-8 path Each pair of spheres is nested inside more spheres which spin at different rates, biging each planet its daily and sidereal motion Aristotle (384-322BC)
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Devised a geocentric ( Earth- centered ) model of the Universe ( a cosmology) that endured for nearly 2000 year Earth is fixed at the centre of the Universe Everything on earth is composed of 4 element: Earth, water, air and fire The heavens are composed of a 5 th element that is pure and eternal (quintessence= 5 th element of another = “pure air” ) All celestial bodies are imperishable, unblemished spheres. Planetary motion follows the Eudoxan model; planets are carried around Earth by invisible spheres which spin with constant, circular, eternal motion( “crystal spheres/orbs”) Since comets do not appear to have eternal motion, they cannot be celestial bodies( therefore, they are atmospheric phenomena) The outermost sphere is the domain of the Prime Mover ( the spiritual and motionless imparter of motion) 1. Ancient Greece: the Hellenistic period (323-156BC) - When Alexander the Great conquered the know world, he initiated a knowledge transfer between the Greeks, Babylonians and Egyptians. 2. Aristarchus (~310-230 BC) - Documented correct methods for measuring 1. Size of Moon relative to Earth (1/2 actual 1/4) 2. Distance and size of Sun relative to Moon (19x; actual 382x) - His measurements weren’t accurate, but he correctly deduced: sun is bigger than Earth (x10; actual 100x) - He proposed: the sun is at the center of the universe. (heliocentric model )
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3. - Parallax: the perceived shift of an object relative to its background due to the motion of the observer. - Stellar parallax can’t be seen with the naked eye , implying that earth is fixed. However, Aristarchus correctly proposed: Perhaps earth moves, but the stars are so distant that their parallax is undetectable. (animation) Applet: Stellar Parallax 4. Hipparchus (~190-120 BC) - Made accurate measurement of the coordinates of celestial bodies. With his data, he compiled the 1 st comprehensive star catalogue (~1000 stars) in the western world. 5. Hipparchus discovered: - The orbital period is 20min longer than the solar year (the time between the same solstice or equinox; 365.24 days) - The earth’s solstice/equinox positions are therefore shifting 0.014 degree of sky per year (precession of the equinoxes), making 1 cycle around the sun every 26,000
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years 6,7. Observational Effects of procession 1.The sun’s location in the sky at a given date and time shifts .014 degree East per year. (“Age of Aquarius”: when the sun on the spring Equinox reaches the
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