Chapter 5 Gravity and Motion

[ click for answer ] term: angular momentum 1 of 23

An intrinsic property of a system corresponding to the amount of its revolution or spin. The amount of angular momentum of a body orbiting around a point is the mass of the orbiting body times its (linear) velocity of revolution times its distance from the point. The amount of angular momentum of a spinning object is the amount of inertia, an intrinsic property of the distribution of mass, times the angular velocity of spin. The conservation of angular momentum is a law that states that the total amount of angular momentum remains constant in a system that is undisturbed from outside itself. [ click for next term ] definition: 1 of 23

[ click for answer ] term: Astronomical Unit 2 of 23

The average distance from the Earth to the Sun. [ click for next term ] definition: 2 of 23

[ click for answer ] term: Deferent 3 of 23

In the Ptolemaic system of the Universe, the larger circle, centered at the Earth, on which the centers of the epicycles revolve. [ click for next term ] definition: 3 of 23

[ click for answer ] term: Eccentricity 4 of 23

A measure of the flatness of an ellipse, defined as the distance between the foci divided by the major axis. [ click for next term ] definition: 4 of 23

[ click for answer ] term: Ellipse 5 of 23

A curve with the property that the sum of the distances from any point on the curve to two given points, called the foci, is constant. [ click for next term ] definition: 5 of 23

[ click for answer ] term: Epicycle 6 of 23

In the Ptolemaic theory, a small circle, riding on a larger circle called the deferent, on which a planet moves. The epicycle is used to account for retrograde motion. [ click for next term ] definition: 6 of 23

[ click for answer ] term: Equant 7 of 23

In Ptolemaic theory, the point equally distant from the center of the deferent as the Earth but on the opposite side, around which the epicycle moves at a uniform angular rate. [ click for next term ] definition: 7 of 23

[ click for answer ] term: Focus 8 of 23

(a) A point to which radiation is made to converge; (b) of an ellipse, one of the two points the sum of the distances to which remains constant. [ click for next term ] definition: 8 of 23

[ click for answer ] term: Geocentric 9 of 23

Earth centered. [ click for next term ] definition: 9 of 23

[ click for answer ] term: Heliocentric 10 of 23

Sun-centered; using the Sun rather than the Earth as the point to which we refer. A heliocentric measurement, for example, omits the effect of the Doppler shift caused by the Earth's orbital motion. [ click for next term ] definition: 10 of 23

[ click for answer ] term: Inclined 11 of 23

Tilted with respect to some other body, usually describing the axis of rotation or the plane of an orbit. [ click for next term ] definition: 11 of 23

[ click for answer ] term: major axis 12 of 23

The longest diameter of an ellipse; the line from one side of an ellipse to the other that passes through the foci. Also, the length of that line. [ click for next term ] definition: 12 of 23

[ click for answer ] term: minor axis 13 of 23

The shortest diameter of an ellipse; the line from one side of an ellipse to the other that passes midway between the foci and is perpendicular to the major axis. Also, the length of that line. [ click for next term ] definition: 13 of 23

[ click for answer ] term: Model 14 of 23

A physical or mathematical equivalent to a situation; the former is typified by a model airplane that scales down a real airplane, while the latter is typified by a set of equations or of tables that describes, for example, the interior of Jupiter or processes that work there. [ click for next term ] definition: 14 of 23

[ click for answer ] term: Perihelion 15 of 23

The near point to the Sun of the orbit of a body orbiting the Sun. [ click for next term ] definition: 15 of 23

[ click for answer ] term: Period 16 of 23

The interval over which something repeats. [ click for next term ] definition: 16 of 23

[ click for answer ] term: prograde motion 17 of 23

The apparent motion of the planets when they appear to move forward (from west to east) with respect to the stars; see also retrograde motion. [ click for next term ] definition: 17 of 23

[ click for answer ] term: retrograde motion 18 of 23

The apparent motion of the planets when they appear to move backward (from east to west) from the direction that they move ordinarily with respect to the stars. [ click for next term ] definition: 18 of 23

[ click for answer ] term: Revolve 19 of 23

To move in an orbit around another body. [ click for next term ] definition: 19 of 23

[ click for answer ] term: Rotate 20 of 23

To spin on one’s own axis. [ click for next term ] definition: 20 of 23

[ click for answer ] term: semimajor axis 21 of 23

Half the major axis, that is, for an ellipse, half the longest diameter. [ click for next term ] definition: 21 of 23

[ click for answer ] term: semiminor axis 22 of 23

Half the minor axis, that is, for an ellipse, half the longest diameter. [ click for next term ] definition: 22 of 23

[ click for answer ] term: year 23 of 23

The period of revolution of a planet around its central star; more particularly, the Earth's period of revolution around the Sun. [ end of Chapter 5 ] definition: 23 of 23