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34 terms

Moody Descriptive Astronomy

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geocentric
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.
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.
force
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.