 Astronomy

# Key Terms

AstronomyKey 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 × 108 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
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