# Moody Descriptive Astronomy

### 34 terms by ramseycarr

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### geocentric

centered on the earth

### direct motion

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

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

### epicycle

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

### deferent

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

### heliocentric

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.

### conjunction

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).

### opposition

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

### perihelion

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

### aphelion

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

### mass

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

### weight

the measure of the force of gravity on an object

### gravity

the force of attraction between any two objects that have mass

### acceleration

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

### density

mass divided by volume

### chondrite

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

### planetesimal

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

### aurora

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

### accretion

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.

Example: