| 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. It revolves around the earth from west to east in about 28 days. The moon joins the earth in its orbit around the sun. The moon does not give off light. Instead, its shine comes from the light it reflects from the sun. [1/2 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] |
| revolution |
motion in orbit around a point, or one completed cycle of this orbiting motion. [1/4 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 spacecraft that is sent into orbit around a planet or other heavenly body to gather or send back information. [1/3 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 spacecraft in orbit for a long period of time. Space stations are used for scientific study and for launching 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. |