Copernicus & Galileo

European rediscovery of ancient astronomical texts (like Ptolemy's Almagest) was facilitated by Arabic and Jewish scholars who preserved and expanded Greek scientific knowledge.
This intellectual revival set the stage for modern observational and theoretical astronomy.

Nicolaus Copernicus (1473–1543) reappraised existing planetary theories and published De Revolutionibus Orbium Coelestium in 1543.
Heliocentric System: Proposed that the Sun, rather than Earth, is at the center of the solar system. Earth is merely one of the planets orbiting the Sun, while only the Moon orbits Earth.
Planetary Order: Correctly ordered the known planets by distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, and Saturn, noting that planets closer to the Sun move faster.
Retrograde Motion: Explained complex planetary retrograde movements naturally without using clumsy epicycles.
Relative Motion: Addressed objections about Earth's movement by arguing that motion can be relative and that Earth's rotation explains the apparent daily movement of the sky.

Galileo Galilei (1564–1642) pioneered the modern scientific method relying on observation, quantitative measurement, and experiment.
Mechanics and Forces:
- Proved that rest is no more natural than motion; friction is what stops moving objects.
- Formulated the concept that forces are needed to change an object's speed or direction, showing that objects accelerate uniformly under gravity.

Built telescopes up to $30\times$ magnification and began observing the heavens late in 1609.
Moons of Jupiter: Discovered four moons orbiting Jupiter, proving that Earth is not the sole center of motion in the universe.
Phases of Venus: Observed Venus undergoing a full cycle of phases, directly confirming it orbits the Sun rather than Earth and disproving the Ptolemaic model.
Lunar Features: Revealed mountains, craters, and valleys on the Moon, demonstrating that celestial bodies share physical characteristics with Earth.
Opposition: Galileo's public advocacy of heliocentrism led to condemnation by the Catholic Church in 1616 and his eventual placement under house arrest.

Five planets are visible without a telescope: Mercury, Venus, Mars, Jupiter, and Saturn.
Venus: Brightest planet, visible near sunrise or sunset, never straying far from the Sun.
Mercury: Always within $28^\circ$ of the Sun, making it challenging to view except during twilight.