Orbital Motion, Kepler's Laws, and Eccentricity

Vocabulary practice flashcards covering orbital shapes, eccentricity, Kepler's laws, gravitational interactions, and orbital geometry based on Earth and Space Sciences Reference Table concepts.

Ellipse

An oval-shaped curve that describes the path of many orbiting objects.

Focus (plural: foci)

One of two fixed points inside an ellipse; for an object orbiting a star, the star is located at one focus of the elliptical orbit.

Primary body

The central object that is being orbited by a satellite.

Satellite

An object that orbits another object, which can be natural (such as a moon) or artificial (such as a spacecraft).

Periapsis

The point in an orbit where an orbiting object is closest to the primary body.

Apoapsis

The point in an orbit where an orbiting object is farthest from the primary body.

Perihelion and Aphelion

The specific terms for an orbit around the Sun: perihelion is the closest point to the Sun, while aphelion is the farthest point.

Perigee and Apogee

The specific terms for an orbit around Earth: perigee is the closest point to Earth, while apogee is the farthest point.

Eccentricity

A unitless value rounded to the nearest thousandth that describes how stretched an ellipse is, calculated as: $\text{Eccentricity} = \frac{\text{distance between the two foci}}{\text{length of the major axis}}$

Eccentricity Scale ($0.000$ to $1.000$)

An eccentricity of $0.000$ represents a perfect circle. Values closer to $0.000$ indicate a more circular orbit with foci closer together, while values closer to $1.000$ indicate a more stretched, elliptical orbit with foci farther apart.

Major axis

The full length of the longest axis of an ellipse, which passes through both focal points.

Gravity

The attractive force between objects with mass that pulls an orbiting body toward the primary body it orbits.

Inertia

The tendency of an object to resist a change in its motion, allowing an orbiting object to continue moving forward.

Orbital speed variation

An orbiting object travels fastest near periapsis (closest distance to the primary body) and slowest near apoapsis (farthest distance).

Gravitational Attraction vs. Distance

An inverse relationship where gravitational force increases as the distance between two objects decreases, and decreases as the distance increases.

Gravitational Attraction vs. Mass

A direct relationship where gravitational force increases as the mass of one or both objects increases at a constant distance, and decreases as mass decreases.

Kepler's First Law (The Law of Ellipses)

Planets orbit the Sun in elliptical paths, with the Sun located at one focus of each ellipse.

Kepler's Second Law (The Law of Equal Areas)

A line connecting an orbiting object to the primary body sweeps out equal areas in equal amounts of time, causing the object to travel faster near the primary body and slower when farther away.

Kepler's Third Law (The Law of Periods)

For objects orbiting the same central body, objects with greater average orbital distances have longer orbital periods.

Period of revolution

The time required for an object to complete one full orbit around its central body.

Period of rotation

The time required for a celestial object to complete one spin on its own internal axis.

Angular diameter

The angle an object's diameter appears to make from an observer's perspective; it appears larger when the object is closer and smaller when it is farther away.

Axial tilt

The angle of inclination of a celestial body's rotational axis relative to a perpendicular to its orbital plane.