Space & Earth Science - Chp. 2

Front 1

Geocentric Theory

Back 1

Ancient description of the universe that says the earth is at the center.

Front 2

Ptolemaic theory

Back 2

An early geocentric theory, that the earth was the fixed center of the universe and all celestial objects revolved around it.

Front 3

eccentric

Back 3

off-center circle

Front 4

retrograde motion

Back 4

When certain planets are observed to sometimes slow down in their motion among the stars, stop, and then back up. After tracing out a backward loop, they resume their forward motion.

Front 5

epicycle

Back 5

small circles

Front 6

deferent

Back 6

In the Ptolemaic geocentric model of the solar system, a crystal sphere surrounding the earth for each of the visible planets, the moon, and the sun, all of which revolved around the earth.

Front 7

heliocentric theory

Back 7

Any theory of the arrangement of the solar system that places the sun at the center with the earth and the other planets in orbit around it.

Front 8

Copernican theory

Back 8

A heliocentric theory or sun-centered model of the solar system, published by Copernicus in 1543, in which the earth rotates on its axis, and the earth and other planets revolve around the sun in perfect circles.

Front 9

law of universal gravitation

Back 9

The principle that states any two objects attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.

Front 10

rotate

Back 10

To spin on an axis that passes through the center of an object.

Front 11

axis

Back 11

An imaginary line passing through the center of an object about which the object spins or rotates.

Front 12

revolve

Back 12

To circle around a point that does not lie within the object, as a planet around the sun.

Front 13

inertia

Back 13

The tendency of all objects in the universe to remain at rest if initially at rest or, if moving, to continue moving at the same speed and in the same direction unless acted upon by an outside force.

Front 14

Coriolis effect

Back 14

The tendency of objects initially moving in a straight path over the earth's surface to be deflected due to the earth's rotation. This effect causes the deflection of wind and water currents to the right in the North Hemisphere and to the left in the South Hemisphere.

Front 15

Foucault pendulum

Back 15

A pendulum first used by Jean Foucault in 1851 to demonstrate that the earth rotates on its axis. Also any similar pendulum consisting of a large pendulum mass suspended by a long wire.

Front 16

ecliptic

Back 16

The apparent path of the sun among the stars. Also, it is a plane of the earth's orbit.

Front 17

Tropic of Cancer

Back 17

The line of latitude located at 23.5 degrees North. It is defined by the sun's northernmost overhead noon position at the Summer solstice.

Front 18

summer solstice

Back 18

The day (about June 21) when the sun's overhead noon position is the farthest north.

Front 19

Tropic of Capricorn

Back 19

The line of latitude located at 23.5 degrees South. It is defined by the sun's southernmost overhead position at the Winter solstice.

Front 20

winter solstice

Back 20

The day (about December 21) when the sun's overhead noon position is southernmost.

Front 21

equinox

Back 21

Either of two days during a year when the sun's noon position is directly above the equator, making day and night approximately equal in all places on the earth. Astronomically, the two locations in the sky where the ecliptic and the celestial equator intersect.

Front 22

vernal equinox

Back 22

The day that the sun's overhead noon position crosses the equator (about March 21). At the equinox, day and night are of equal length in all parts of the earth.

Front 23

parallax

Back 23

An apparent shift in position of an observed object caused by a change in the point of observation. It is used to measure the distances to nearby stars and is irrefutable (absolute) proof of the earth's revolution around the sun.

Front 24

lunisolar calendar

Back 24

A calendar that takes into account both the solar year and the lunar month, making the necessary adjustments to keep the calendar in agreement with the seasons.

Front 25

lunar calendar

Back 25

A calendar with months that correspond to the length of the moon's cycle.

Front 26

solar calendar

Back 26

A calendar that is based on the solar year and ignores the lunar cycle.

Front 27

leap year

Back 27

In general, a calendar year in which one or more extra days is added to the calendar to keep it in phase with the seasons. The number of days depends on the calendar system being used.

Front 28

Julian calendar

Back 28

Modified by Caesar and used in Europe for 15 centuries.

Front 29

Gregorian calendar

Back 29

Our current calendar. Modified by Pope Gregory XIII.