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| The described theory of the arrangements of the solar system that places the earth at the center with the sun and the planets orbiting the earth. |
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| An early, widely held geocentric theory, that the earth was the fixed center of the universe and that all celestial objects revolved around it. |
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| In cicular motion, an off-center rotation. In orbital motion, the fact that the central body is at the focus of an elliptical orbit rather than the center of the ellipse. |
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| In the Ptolemaic geocentric model of the solar system, a small crystal sphere centered on the surface of the deferent of certain planets to account for that planet's retrograde motion at certain points in its orbit. |
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| 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. |
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| 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. |
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| To spin on an axis that passes through the center of an object. |
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| An imaginary line passing through the center of an object about which the object spins or rotates. |
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| To circle around a point that does not lie within the object, as a planet around the sun. |
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| 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. |
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| A pendulum first used by Jean Foucalt in 1851 to demonstrate that the earth rotates on its axis. Any similar pendulum consisting of a large pendulum mass suspended by a long wire. |
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| The apparent path of the sun among the stars. Also, it is the plane of the earth's orbit. |
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| The line of latitude located at 23 1/2 deg. N. It is defined by the sun's southernmost overhead noon position at the winter solstice. |
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| The line of latitude located at 23 1/2 deg. S. It is defined by the sun's southernmost overhead position at the winter solstice. |
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| The day (about June 21) when the sun's overhead noon position is the farthest north. (See Tropic of Cancer) |
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| The day (about December 21) when the sun's overhead noon position is southernmost. (See Tropic of Capricorn) |
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| 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 celestrial equator intersect. |
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| 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 proof of the earth's revolution around the sun. |
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| Law of Universal Gravitation |
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| Any two bodies attract each other with a force proportional to the product of their masses and inversly proportional to the square of the distance between them. |
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| The force that holds the planets in orbit around the sun and the moons around the planets. |
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