# Key Terms

- angular momentum
- the measure of the motion of a rotating object in terms of its speed and how widely the object’s mass is distributed around its axis

- aphelion
- the point in its orbit where a planet (or other orbiting object) is farthest from the Sun

- apogee
- the point in its orbit where an Earth satellite is farthest from Earth

- asteroid belt
- the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun

- astronomical unit (AU)
- the unit of length defined as the average distance between Earth and the Sun; this distance is about 1.5 × 10
^{8}kilometers

- density
- the ratio of the mass of an object to its volume

- eccentricity
- in an ellipse, the ratio of the distance between the foci to the major axis

- ellipse
- a closed curve for which the sum of the distances from any point on the ellipse to two points inside (called the foci) is always the same

- escape speed
- the speed a body must achieve to break away from the gravity of another body

- focus
- (plural: foci) one of two fixed points inside an ellipse from which the sum of the distances to any point on the ellipse is constant

- gravity
- the mutual attraction of material bodies or particles

- Kepler’s first law
- each planet moves around the Sun in an orbit that is an ellipse, with the Sun at one focus of the ellipse

- Kepler’s second law
- the straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time

- Kepler’s third law
- the square of a planet’s orbital period is directly proportional to the cube of the semimajor axis of its orbit

- major axis
- the maximum diameter of an ellipse

- momentum
- the measure of the amount of motion of a body; the momentum of a body is the product of its mass and velocity; in the absence of an unbalanced force, momentum is conserved

- Newton’s first law
- every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force

- Newton’s second law
- the change of motion of a body is proportional to and in the direction of the force acting on it

- Newton’s third law
- for every action there is an equal and opposite reaction (
*or:*the mutual actions of two bodies upon each other are always equal and act in opposite directions)

- orbit
- the path of an object that is in revolution about another object or point

- orbital period (
*P*) - the time it takes an object to travel once around the Sun

- orbital speed
- the speed at which an object (usually a planet) orbits around the mass of another object; in the case of a planet, the speed at which each planet moves along its ellipse

- perigee
- the point in its orbit where an Earth satellite is closest to Earth

- perihelion
- the point in its orbit where a planet (or other orbiting object) is nearest to the Sun

- perturbation
- a small disturbing effect on the motion or orbit of a body produced by a third body

- satellite
- an object that revolves around a planet

- semimajor axis
- half of the major axis of a conic section, such as an ellipse

- velocity
- the speed and direction a body is moving—for example, 44 kilometers per second toward the north galactic pole

This book was adapted from the following: Fraknoi, A., Morrison, D., & Wolff, S. C. (2016). Key Terms. In

*Astronomy*. OpenStax. https://openstax.org/books/astronomy/pages/3-key-terms under a Creative Commons Attribution License 4.0Access the entire book for free at https://openstax.org/books/astronomy/pages/1-introduction