Retrograde Motion

• Planets usually move eastward but can reverse direction temporarily.

  • Prograde motion: Eastward movement of planets.

  • Retrograde motion: Westward movement.

  • Duration: Weeks to months.

Motion of Mercury and Venus

• Always close to the Sun, appearing as:

  • "Evening stars" after sunset

  • "Morning stars" before sunrise

Astronomical Models

• Geocentric Model (Aristotle/Ptolemy):

  • Earth is the center.

  • Celestial bodies move in circular orbits and epicycles.

  • Can explain retrograde motion and the closeness of Mercury and Venus to the Sun.

• Heliocentric Model (Copernicus):

  • Sun near the center.

  • Celestial bodies move in circular orbits.

  • Earth rotates and orbits the Sun.

  • Simplifies the explanation of retrograde motion as apparent due to relative motion.

Problems with the Models

1. Copernicus used many epicycles; predictions weren't more accurate than Ptolemaic.

2. Apparent star separations were not observable; fixed stars are far away.

3. Religious beliefs conflicted with the heliocentric model.

Galileo's Discoveries

• Used a telescope, identified key observations:

  1. Moon has a rough surface.

  2. Sunspots exist, indicating Sun's rotation.

  3. Moons orbit Jupiter, suggesting Earth is not the centre.

  4. Phases of Venus consistent with heliocentric model..