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centered on the earth

direct motion

The normal eastward movement of a planet against the background of distant stars.

retrograde motion

The apparent backward motion of a planet against the background of stars.


In Ptolemy's geocentric solar system, the small circle on which a planet moved.


In Ptolemy's geocentric solar system, the large circle on which a planet's epicycle moved around the Earth.


Centered on the Sun.

inferior planet

A planet that is closer to the Sun than the Earth.

superior planet

a planet that is farther from the sun than the earth.


When a planet lines up with the Sun in the sky.

greatest elongation

The point where an inferior planet is as far away from the Sun as it can be (as seen from the Earth).


The point where a superior planet is as far away from the Sun as it can be (as seen from the Earth).


The point in its orbit where a planet is nearest the Sun.


The point in its orbit where a planet is farthest from the Sun.

semimajor axis

Half of the longest diameter across an ellipse.


a push or a pull


A measure of how an object resists accelerating when acted upon by a force. It is proportional the amount of matter in an object.


the measure of the force of gravity on an object


the force of attraction between any two objects that have mass


any change in speed or direction of an object

occam's razor

The philosophical stand that says a simpler explanation is more likely to be correct than a complicated one.

lagrangian points

Points of gravitational stability in the orbit of a planet.

synchronous rotation

When one side of a body always faces the planet it revolves around.

solar nebula

the material from which the solar system was formed.

Jovian planets

planets that are Jupiter-like


mass divided by volume


The first rock-sized bodies that formed in the solar nebula from dust grains.


The larger bodies that formed early in the solar nebula that were chemically differentiated.


The light produced when particles from the sun collide with atmospheric molecules.


the process of acquiring material

chemical differentiation

The sinking of denser elements to the center of a young molten planet.

terrestrial planets

earth like planets

plate tectonics

The movement of the Earth's crustal plates riding on top of the mantle.

greenhouse affect

The trapping of heat by carbon dioxide or other gases in the Earth's atmosphere.

condensation temperature

The temperature at which a substance in the vacuum of space solidifies.

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