Chapter 4: Origins of Modern Astronomy

Geocentric Model

An ancient belief that Earth is the center of the universe with celestial bodies orbiting around it, popularized by Ptolemy in the 2nd century.

Retrograde Motion

Apparent backward movement of planets in the sky due to differences in orbital speeds. It's an optical illusion from Earth's perspective.

Heliocentric Model

Theory proposing the Sun as the center of the solar system, with planets orbiting around it, developed by Copernicus in the 16th century.

Kepler’s 1st Law of Planetary Motion

planets move in ellipses with the Sun at one focus. It describes the shape of a planet's orbit around the Sun.

Kepler’s 2nd Law of Planetary Motion

A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.

Kepler’s 3rd Law of Planetary Motion

The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.