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aphelion the point that is farthest from the sun in the orbit of a planet or comet.
apogee the point in the orbit of the moon or of a man-made satellite that is farthest from the earth. (Cf. perigee.) [1/2 definitions]
ascendant in astrology, the point in the earth's orbit intersecting with the apparent path of the sun, or the sign of the zodiac rising in the east, at the time of an event such as one's birth. [1/4 definitions]
cislunar lying between the earth and the moon's orbit; on this side of the moon.
eclipsing binary two stars that orbit a common center of gravity and periodically move in front of each other, causing a fluctuating magnitude.
mean distance the arithmetic average of the greatest and least distances of a planet from the sun, used to describe the size of the planet's orbit.
moon the earth's natural satellite, revolving completely around the earth from east to west in about twenty-eight days and accompanying the earth in its orbit around the sun. [1/6 definitions]
orbit to send into an orbit. [2/7 definitions]
perigee in astronomy, the point in the orbit of a moon or other satellite at which it is closest to the earth. (Cf. apogee.)
perihelion the point in the orbit of a planet or other body at which it comes closest to the sun. (Cf. aphelion.)
Pluto a dwarf planet in the Earth's solar system with a highly elliptical orbit and a diameter of 2,370 kilometers, or approximately 1,475 miles. Pluto was formerly considered a planet in the solar system, the smallest and farthest from the sun, but it was reclassified in 2006 as a dwarf. However, the debate about the classification continues in the scientific community. [1/2 definitions]
Roche limit the lowest altitude at which a natural satellite can form and orbit a planet or the like without being disturbed by gravitational forces.
satellite a man-made object that is sent into orbit around a planet or other heavenly body, usu. to gather or relay information. [1/5 definitions]
solar system the Sun, its eight planets and their satellites, and all other bodies that orbit the Sun (prec. by "the"). [1/2 definitions]
space station a large artificial satellite in long-term orbit, designed to be used as a military outpost, scientific research laboratory, or site for the assembly and launching of other spacecraft.
suborbital of a rocket or other spacecraft, not reaching a high enough altitude or speed to complete one full orbit around the earth. [1/2 definitions]
superior in astronomy, designating a planet whose orbit is farther from the sun than is the orbit of earth. [1/9 definitions]
Viking one of two U.S. spacecraft launched by NASA in 1975 to orbit and land on Mars. [1/2 definitions]
zero gravity the condition of apparent weightlessness, as of a body in orbit, when the apparent effect of gravity is zero.