Sections

Key Terms
# Key Terms

- aphelion
- farthest distance between a planet and the sun (called apoapsis for other celestial bodies)

- Copernican model
- the model of the solar system where the sun is at the center of the solar system and all the planets orbit around it; this is also called the heliocentric model

- eccentricity
- a measure of the separation of the foci of an ellipse

- Einstein’s theory of general relativity
- the theory that gravitational force results from the bending of spacetime by an object’s mass

- gravitational constant
- the proportionality constant in Newton’s law of universal gravitation

- Kepler’s laws of planetary motion
- three laws derived by Johannes Kepler that describe the properties of all orbiting satellites

- Newton’s universal law of gravitation
- states that gravitational force between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

- perihelion
- closest distance between a planet and the sun (called periapsis for other celestial bodies)

- Ptolemaic model
- the model of the solar system where Earth is at the center of the solar system and the sun and all the planets orbit around it; this is also called the geocentric model