GEOCENTRIC IN-DEPTH

Now for a little more detail…

Unit 2: History of Astronomy

BACK TO THE GREEKS

• Ancient Greeks like Aristotle noticed two types of celestial objects: those that were stationary, and those that changed their locations from time to time
• They called them “wanderers” 🡪 this is how we named planets 🡪 ‘planet’ comes from that Greek word
• Greeks charted stars and planets, noticed that planets seemed to stop and change direction sometimes

RETROGRADE MOTION

You’re going the wrong way!

RETROGRADE MOTION

• On Earth, planets appear to move from west to east due to revolving counter-clockwise around the sun (prograde motion)
• Once in a while, planets appear to stop and move east to west (retrograde motion)

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TIME LAPSE OF MARS

RETROGRADE MOTION

• For superior planets (further from the Sun), Earth’s orbit is faster than these planets
• Earth catches up to the planet in its orbit
• Planet appears to stop and move backwards (east to west)
• For inferior planets (closer to the Sun), their orbits are faster than Earth’s
• They circle the Sun and overtake Earth as they swing around the Sun

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THEN PTOLEMY COMES ALONG

• Ptolemy had the solution!
• He claimed that the universe was geocentric (Earth-centered), but he developed mathematical equations to accurately predict where planets would be in the sky

PTOLEMY’S �MODEL

• Ptolemy argued that planets move on two sets of circles, a deferent and an epicycle
• Deferent: the circular orbit around Earth
• Epicycle: circular orbit of the planet along a point on the deferent

PTOLEMY’S MODEL

HE WAS *SORT OF* RIGHT

• His overall idea was wrong (Earth is NOT the center), but his math was right
• His equations are used to design planetarium equipment 🡪 the gears work like the epicycles (all the circles that circle around each other) in his model

IN SUMMARY…