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29 Cards in this Set

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Geocentric Theory
Ancient description of the universe that says the earth is at the center.
Ptolemaic theory
An early geocentric theory, that the earth was the fixed center of the universe and all celestial objects revolved around it.
off-center circle
retrograde motion
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.
small circles
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.
heliocentric theory
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.
Copernican theory
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.
law of universal gravitation
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.
To spin on an axis that passes through the center of an object.
An imaginary line passing through the center of an object about which the object spins or rotates.
To circle around a point that does not lie within the object, as a planet around the sun.
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.
Coriolis effect
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.
Foucault pendulum
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.
The apparent path of the sun among the stars. Also, it is a plane of the earth's orbit.
Tropic of Cancer
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.
summer solstice
The day (about June 21) when the sun's overhead noon position is the farthest north.
Tropic of Capricorn
The line of latitude located at 23.5 degrees South. It is defined by the sun's southernmost overhead position at the Winter solstice.
winter solstice
The day (about December 21) when the sun's overhead noon position is southernmost.
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.
vernal equinox
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.
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.
lunisolar calendar
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.
lunar calendar
A calendar with months that correspond to the length of the moon's cycle.
solar calendar
A calendar that is based on the solar year and ignores the lunar cycle.
leap year
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.
Julian calendar
Modified by Caesar and used in Europe for 15 centuries.
Gregorian calendar
Our current calendar. Modified by Pope Gregory XIII.