Glossary of Earth Science 4: Nine Planets: Vocabulary

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the point nearest to the sun in the orbit of a planet or comet
the occasional apparent backward movement of the superior planets from east to west
retrograde motion
a planet whose orbit is outside the earth's orbit
superior planet
one of the nine most massive bodies orbiting the sun
the point at which light rays come together after being bent by a lens or curved mirror; one or two points used in drawing an ellipse
focus (plural, foci)
the dark region of separation in the ring system of Saturn
Cassini's division
the mass of an object per volume
the average distance between the earth and the sun (150 million km or 93 million mi)
A.U. (astronomical unit)
the point furthest from the sun in the orbit of a planet or comet
the ability of an object such as a planet or minor planet to reflect light
the downward pull of an astronomical body on objects located at its surface
surface gravity
a measure of the amount of material an object contains, irrespective of gravity
the eclipsing of a heavenly body by the moon or by another planet
the passage of an inferior planet across the sun's disk, or the passage of a star across the meridian
planets that are the earth's size or smaller and about the same density of the earth
terrestrial planets
Jupiter's four largest moons, named after their discoverer
Galilean moons
a planet whose orbit is inside the earth's orbit
inferior planet
a reddish colored, oval-shaped area on Jupiter, possibly due to an atmospheric disturbance or storm
Great Red Spot
one of the four large outer planets
Jovian planet
the planets Jupiter, Saturn, Uranus, and Neptune, which have large gaseous atmospheres
gas giants
an oval-shaped figure having both ends alike; the characteristic shape of the orbits of planets, satellites, and periodic comets

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