Geocentric vs Heliocentric Models

• Ptolemy proposed the Geocentric Model where Earth is the center of the universe, with celestial bodies orbiting it in perfect circles.
• Copernicus introduced the Heliocentric Model, placing the Sun at the center, with planets (including Earth) orbiting it.
• Kepler's laws and Galileo's observations later supported the heliocentric view.
• Celestial spheres were thought to be invisible, crystal-like layers that held stars and planets in place, rotating around Earth in ancient times.
• People thought Earth was the center because the Sun, Moon, and stars appeared to move across the sky while Earth remained stationary.

Contributions of Scientists

• Eratosthenes measured Earth's circumference using the angle of the Sun's rays at different locations.
• Ptolemy developed the geocentric model using epicycles to explain planetary motion.
• Nicholas Copernicus proposed the heliocentric model, challenging the geocentric system.
• Tycho Brahe made precise astronomical observations without a telescope, providing data for Kepler's laws.
• Johannes Kepler developed three laws of planetary motion, proving planets orbit in ellipses, not circles.
• Galileo Galilei used a telescope to observe moons orbiting Jupiter, proving not everything orbits Earth and observed Venus's phases, supporting heliocentrism.
• Isaac Newton formulated the laws of motion and universal gravitation, explaining planetary motion.

Kepler's Three Laws of Planetary Motion

• Planets orbit the Sun in elliptical paths (Law of Ellipses), with the Sun at one focus.
• A line drawn from a planet to the Sun sweeps out equal areas in equal time intervals (Law of Equal Areas); planets move faster when closer to the Sun.
• The square of a planet's orbital period is proportional to the cube of its average distance from the Sun (Law of Harmonies).
• Planets speed up when closer to the Sun and slow down when farther, explained by the Law of Equal Areas.
• A planet's speed depends on its distance from the Sun, following an elliptical orbit.

Galileo's Observations

• Observed Jupiter's moons orbiting Jupiter, proving not everything orbits Earth.
• Observed Venus's phases, explainable only by a heliocentric model.
• Observed sunspots and Moon's craters, challenging the idea of perfect celestial bodies.
• The Catholic Church opposed heliocentrism, leading to Galileo's trial and house arrest due to contradicting religious teachings.

Stellar Parallax

• Stellar Parallax is the apparent shift of stars due to Earth's motion, not observed with the naked eye.
• Brahe proposed the Sun orbited Earth, but planets orbited the Sun as a compromise as he rejected heliocentrism because of Stellar Parallax.
• Retrograde motion is the apparent backward motion of planets as seen from Earth.
• Ptolemy's geocentric model used epicycles to explain retrograde motion, while the heliocentric model explained it as Earth overtaking outer planets.
• Kepler described planetary motion mathematically but couldn't explain why it happened.
• Newton later explained gravity causes planets to move in ellipses.

Key Contributions

• Kepler developed his laws of motion based on Tycho Brahe's planetary data.
• Newton's laws of gravity and motion explain why planets move in ellipses and are kept in orbit by gravitational force.

Three Types of Planetary Motion of Earth

• Revolution: Earth moves around the Sun in an elliptical orbit (~365.25 days per orbit).
• Rotation: Earth spins counterclockwise on its axis (~24 hours per rotation).
• Precession: Earth's axis wobbles like a spinning top over a ~26,000-year cycle, slowly changing the position of the north celestial pole.
• Due to precession, the north celestial pole shifts over time.
• In 12,000 years, Vega, not Polaris, will be the North Star.
• The Earth's axial tilt changes its orientation gradually, causing precession.
• Sidereal Day (23 hours, 56 minutes, 4 seconds): the time it takes for Earth to rotate once relative to distant stars.
• Mean Solar Day (24 hours): the time it takes for the Sun to return to the same position in the sky.
• The solar day is longer because Earth moves in its orbit, needing extra rotation for the Sun to appear in the same position.
• Aphelion: Earth's farthest point from the Sun (early July).
• Perihelion: Earth's closest point to the Sun (early January).
• Earth's seasons are caused by its axial tilt, not its distance from the Sun.
• The lunar month (synodic month) is ~29.5 days.
• The Moon is tidally locked, so the same side always faces Earth because the Moon's rotation period matches its orbital period.
• The Moon's orbit is tilted ~5° relative to Earth's orbit around the Sun.
• Eclipses don't happen every month because the Moon's shadow misses Earth (solar eclipses), or Earth's shadow misses the Moon (lunar eclipses).
• Eclipses only happen when the Moon aligns precisely at the lunar nodes (intersection points of orbits).
• Lunar eclipses are more common than solar eclipses because they can be seen from anywhere on Earth's night side.
• Solar eclipses are rarer for any given location because they are visible only in a narrow path where the Moon's shadow falls.

Types of Solar Eclipses

• Total Solar Eclipse – The Moon fully covers the Sun.
• Annular Solar Eclipse – The Moon is farther from Earth and appears too small to cover the Sun fully, creating a ring (annulus).
• Partial Solar Eclipse – The Moon covers only part of the Sun.
• Totality lasts a few minutes during a solar eclipse (usually ~2-7 minutes) because of the Moon's motion relative to Earth.
• During totality, the solar corona (outer atmosphere of the Sun) becomes visible.
• Bailey's Beads & Diamond Ring Effect occur due to sunlight peeking through lunar valleys during totality.
• Even during a partial eclipse, exposed parts of the Sun emit intense radiation, which can damage the eyes; special eclipse glasses are required.
• Umbra: The darkest part of the shadow where a total eclipse occurs.
• Penumbra: The lighter shadow where only a partial eclipse is seen.
• The path of totality is the narrow region on Earth's surface where a total eclipse is seen.