Kepler, Newton, and Gravity: Vocabulary for Planetary Motion

Key vocabulary terms related to Kepler’s laws, Newtonian gravity, orbital motion, and major historical discoveries in planetary motion.

ellipse

A closed curve with two focal points; planets orbit the Sun in ellipses with the Sun at one focus.

semi-major axis (a)

The long radius of an ellipse; a is the distance from the center to the end of the major axis.

semi-minor axis (b)

The short radius of an ellipse; b is the distance from the center to the end of the minor axis.

Kepler’s First Law

Planets move around the Sun in ellipses, with the Sun at one focus.

Kepler’s Second Law

The line joining a planet and the Sun sweeps out equal areas in equal intervals of time.

Kepler’s Third Law (P^2 = a^3)

The square of a planet’s orbital period (P) is proportional to the cube of its semi-major axis (a).

Sun at a focus

In a Keplerian ellipse, the Sun is located at one of the ellipse’s two foci.

Two foci

Ellipse has two focal points; a circle is a special case where the foci coincide.

Universal law of gravitation

Every two masses attract with a force F = G m1 m2 / r^2.

Gravitational constant (G)

The proportionality constant in Newton’s law of gravitation that sets the strength of gravity.

Mass

A property of matter that allows gravity to act and gives objects inertia.

Distance (r)

The separation between the centers of two masses in gravity calculations.

Acceleration due to gravity

The rate at which objects accelerate under gravity; the acceleration is independent of the falling object’s mass.

Orbit

A closed path around a larger body achieved when an object travels at the right speed; too slow and it falls, too fast and it escapes.

Apollo 15 hammer and feather demonstration

Moon experiment showing gravity acts equally on all masses in the absence of air resistance.

Edmund Halley

Predicted the return of Halley’s Comet using Newton’s laws.

Uranus (discovered by William Herschel, 1781)

Planet found by Herschel; its motion suggested perturbations by unseen bodies.

Neptune predicted (1845)

Predicted to exist from Uranus’s perturbations; later observed near the predicted position.

Copernicus

Proposed a heliocentric model of the solar system, laying groundwork for later work by Kepler and Galileo.