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The symbol used for the acceleration due to gravity. At the Earth's surface it averages 9.81 meters/second2, directed towards the Earth's center. In common talk, "g forces" are stresses due to acceleration, e.g. on astronauts or payloads. In the same vein, "zero g" is the condition when no acceleration is sensed, because gravity is already fully employed supplying the centripetal force which holds the object in its orbit (or alternatively from the rotating frame of reference, because gravity is fully balanced by the centrifugal force).
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electromagnetic waves of the highest frequencies known, originally discovered as an emission of radioactive substances. See also radioactivity.
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The study of the shape of the Earth, e.g. its deviations from an exact sphere.
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(GPS) a navigation system using about 20 satellites in 12-hour orbits, distributed evenly around Earth. These satellites continually broadcast their positions. and a small instrument, receiving signals of 3 or more such satellites, can calculate its position within about 10 meters. The system was created by the US military, which can get from it even more accurate positions. The Russian GLONASS system is similar.
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The part of a sundial which casts the shadow, usually a rod or fin pointed at the celestial pole.
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(or "gravitation"), one of 4 main forces in the universe (others: electro-magnetic, and 2 types of nuclear force). Every mass exerts a gravitational pull on any other mass, inversely proportional to the square of the distance between them. This was first proposed by Newton, based on his calculation of orbits of planets and of the Moon.
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Another name for the planetary swing-by maneuver.
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The surface of the Earth is, on the average, in a state of equilibrium between heating and cooling: that is, on the average, the rate at which sunlight heats it equals the rate at which it loses heat.
If no atmosphere existed, all that loss would take place by infra-red radiation from the surface. The Earth's atmosphere, however, absorbs infra-red, which heats it up and slows down the escape of heat. The same process occurs in glass-covered greenhouses, whose panes let sunlight in but absorb the infra-red emitted back, keeping their interior warm even in winter. For that reason, the process is known as the "greenhouse effect."
Some gases which constitute only a small portion of the atmosphere--water vapor, CO2 (carbon dioxide) and CH4 (methane)--are major contributors to the greenhouse effect. Burning coal and oil in the last century has markedly increased the CO2 content of the atmosphere, which is why some scientists credit the warming trend experienced in the last decades of the 20th century to an increased "greenhouse effect."
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Introduced in 1582 by Pope Gregory the 13th, this calendar modifies the Julian calendar for greater precision, decreeing that century years such as 1900 are not leap years, except if the number of centuries is divisible by 4 (e.g. 2000).
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