Rotation (Diurnal Motion): Earth is fixed and rotates on its axis every 24 hours; causes day/night cycle (geocentric model).
Revolution (Annual Motion): Earth orbits the Sun in a circular path; initial observations by Greeks later refined by Aristarchus and Copernicus.
Precession: Slow conical motion of Earth's axis, influenced by gravitational forces, taking ~26,000 years to complete; observed by Hipparchus.
Understanding Earth's Shape
Lunar Eclipse Observation: Circular shadow of Earth on Moon indicates curved, spherical shape.
Ships and Horizon: Ships disappear hull-first over the horizon, showcasing Earth's curvature.
Eratosthenes' Measurement: Calculated Earth's circumference based on shadow angles from Syene and Alexandria (est. 40,000 km, close to actual).
Horizon Shape: Higher altitudes show horizon dropping, supporting a spherical Earth.
Models of the Universe
Geocentric Model: Earth-centered universe proposed by Ptolemy; explained celestial motions through epicycles, remaining dominant until challenged by heliocentricity.
Observations supporting geocentrism: Sun and stars appear to revolve around Earth, and Earth seems stationary.
Ptolemy's Contributions: Developed the Ptolemaic system; worked on orbital paths and epicycles.
Aristotle's View: Positioned Earth at the center based on physical principles.
Medieval Support: Christian theologians harmonized geocentrism with religious beliefs.
Heliocentric Model
Introduction: Proposed Earth and planets revolve around the Sun; resurfaced by Copernicus.
Key Contributors:
Aristarchus of Samos: Proposed heliocentrism in 3rd century BCE.
Nicolaus Copernicus: Published pioneering work in 1543, challenging geocentric views with a mathematical framework.
Galileo Galilei: Used telescope to provide evidence for heliocentrism (phases of Venus, Jupiter's moons).
Johannes Kepler: Formulated three laws of planetary motion; affirmed heliocentric model with elliptical orbits.
Kepler's Laws of Planetary Motion
First Law: Orbits are elliptical, with the Sun at one focus.
Second Law: Equal areas are swept out in equal time by a planet orbiting the Sun.
Third Law: The square of a planet's orbital period relates to the cube of its semi-major axis.
Plato's Problem of Saving the Appearances
Challenge to reconcile observed celestial motions with geocentric models.
Greeks employed epicycles; ultimately surpassed by simpler heliocentric explanations in the Copernican revolution.