Astronomy Ch 4

Front 1

center of mass

Back 1

The balance point of a body or system of masses. The point about which a body or system of masses rotates in the absence of external forces. (p. 69)

Front 2

circular velocity

Back 2

The velocity an object needs to stay in orbit around another object. (p. 68)

Front 3

closed orbit

Back 3

An orbit that returns to the same starting point over and over. Either a circular orbit or an elliptical orbit. (p. 69)

Front 4

deferent

Back 4

In the Ptolemaic theory, the large circle around Earth along which the center of the epicycle was thought to move. (p. 51)

Front 5

eccentricity, e

Back 5

A number between 1 and 0 that describes the shape of an ellipse. The distance from one focus to the center of the ellipse divided by the semimajor axis. (p. 59)

Front 6

ellipse

Back 6

A closed curve around two points called the foci such that the total distance from one focus to the curve and back to the other focus remains constant. (p. 58)

Front 7

epicycle

Back 7

The small circle followed by a planet in the Ptolemaic theory. The center of the epicycle follows a larger circle (the deferent) around Earth. (p. 51)

Front 8

equant

Back 8

In the Ptolemaic theory, the point off center in the deferent from which the center of the epicycle appears to move uniformly. (p. 51)

Front 9

escape velocity

Back 9

The initial velocity an object needs to escape from the surface of a celestial body. (p. 69)

Front 10

geocentric universe

Back 10

A model universe with Earth at the center, such as the Ptolemaic universe. (p. 50)

Front 11

geosynchronous satellite

Back 11

A satellite that orbits eastward around Earth with a period of 24 hours and remains above the same spot on Earth's surface. (p. 68)

Front 12

heliocentric universe

Back 12

A model of the universe with the sun at the center, such as the Copernican universe. (p. 49)

Front 13

hypothesis

Back 13

A conjecture, subject to further tests, that accounts for a set of facts. (p. 59)

Front 14

inverse square relation

Back 14

A rule that the strength of an effect (such as gravity) decreases in proportion as the distance squared increases. (p. 66)

Front 15

mass

Back 15

A measure of the amount of matter making up an object. (p. 66)

Front 16

natural law

Back 16

A theory that is almost universally accepted as true. (p. 59)

Front 17

neap tide

Back 17

Ocean tide of low amplitude occurring at first- and third-quarter moon. (p. 71)

Front 18

open orbit

Back 18

An orbit that carries an object away, never to return to its starting point. (p. 69)

Front 19

paradigm

Back 19

A commonly accepted set of scientific ideas and assumptions. (p. 54)

Front 20

parallax

Back 20

The apparent change in position of an object due to a change in the location of the observer. Astronomical parallax is measured in seconds of arc. (p. 50)

Front 21

retrograde motion

Back 21

The apparent backward (westward) motion of planets as seen against the background of stars. (p. 50)

Front 22

semimajor axis, a

Back 22

Half of the longest diameter of an ellipse. (p. 58)

Front 23

spring tide

Back 23

Ocean tide of high amplitude that occurs at full and new moon. (p. 71)

Front 24

theory

Back 24

A system of assumptions and principles applicable to a wide range of phenomena that have been repeatedly verified. (p. 59)

Front 25

uniform circular motion

Back 25

The classical belief that the perfect heavens could only move by the combination of uniform motion along circular orbits. (p. 50)