How do Planets move? (Lec. 6).docx

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Department
Astronomy & Astrophysics
Course
AST101H1
Professor
J.B.Lester
Semester
Fall

Description
How do Planets move? Planets do not move constantly in one direction like the way the moon does planets move "forward" {=prograde motion}, compared to the background stars like the  moon] plans also move reverse {=retrograde motion} compared to background stars what causes retrograde motion? Astronomy's challenge is its limit to Earth we must think up an explanation for what we see­ like a detective investigating crime we use our explanation to predict another observation to test if our idea was correct  (testing the explanation on another observation/idea) Explaiang retrograde motion explaiantion #1­developed 2200 years ago mars is orbiting the sun we observe form earth, which is also orbiting the sun ­earth is moving location ­earth is orbiting faster than mars ­earth is passing mars difference in orbit speed makes it appears as if its going faster this depends on earth orbiting around the Sun The opposite sides of earth's orbit are 2 AU= 300,000,000 km apart observing from opposite sides of Erath''s orbit, we look in different direction to ses a  nearby star the nearby star should appear to shift compared to background stars= parallax result of parallax test Greek astronomers searched but there was no parallax concluded that Earth does not orbit­ wrong! rejected the correct explanation method they were following was correct, they just reached 
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