• Old Belief: Earth is stationary & everything moves around it (Ptolemaic System).
• New Belief: Earth moves around the Sun (post-Copernican System).
• Interesting Fact: There are still proponents of the flat-earth theory from Ancient Greece!

• Kepler's First Law: Planets move in elliptical orbits with the Sun at one focus.
  • Fun Fact: A circle is a special case of an ellipse.
  • Real-World Example: Think of an oval track in a sports stadium; if you place the Sun at one end, that’s roughly how a planet orbits around it.
• Kepler's Second Law: The line connecting a planet to the Sun sweeps out equal areas in equal times.
  • Real-World Example: When a car speeds up on a straight road (near the Sun), it covers more ground. On curves (far from the Sun), it's slower but covers the same area due to the curve's breadth.
• Kepler's Third Law: The square of the orbital time T² of a planet is directly proportional to the cube of its orbital radius r³.

Kepler's third law was an approximation; the precise form is T² ∝ r³ / (M + m), where M and m are the masses of the two objects. But for the Solar System, T² ∝ r³ is good enough because the Sun's mass dwarfs that of planets.

• Scientists tried to liken electrons orbiting a proton in a hydrogen atom to planets orbiting a star.
• Bohr's theory was inspired by this but ultimately failed because electrons should emit energy during motion.
• Real-World Example: Imagine a toy car (electron) on a circular track (atom). If the car always lost battery power (energy) just by moving, it wouldn’t make sense!