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Number between 0 and 1, gauging the elongation of elliptic orbit. The eccentricity e of the orbital ellipse is one of the "orbital elements" characterizing it.
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A line around the middle of the celestial sphere, connecting the points occupied by the Sun over the year. The moon and the visible planets also appear to move very close to that line, which cuts the celestial equator at an angle of about 23.5o . See plane of the ecliptic.
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(EM field) the regions of space near electric currents, magnets, broadcasting antennas etc., regions in which electric and magnetic forces may act. Generally the EM field is regarded as a modification of space itself, enabling it to store and transmit energy. See also (below) "electromagnetic wave" and magnetic field.
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"electromagnetic radiation" -- a combination of oscillating magnetic and electric fields, spreading in wavelike fashion through space at a speed of about 300 000 km.sec. James Clerk Maxwell's theory in 1864 suggested that light was such a wave, and today we know that such waves include all forms of light--also infra-red and ultra-violet, as well as radio waves, microwaves, x-rays and gamma rays.
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a lightweight particle, carrying a negative electric charge and found in all atoms. Electrons can be energized or even torn from atoms by light and by collisions, and they are responsible for many electric phenomena in solid matter and in plasmas.
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A closed curve resembling a flattened circle (the shadow of a circle tilted towards the light is an ellipse). May be defined:
As the collection of points whose distances (R1, R2) from two given points (the foci of the ellipse--in singular, focus) add up to the same sum.
Or else , in polar coordinates (r,f), as the curve whose points satisfy a relation r = a(1 - e)/(1 + e cosf) where a is the semi-major axis, half the width in the direction through the two foci. One of the foci is then at the origin and e is the eccentricity, a number ranging from 0 (circle) to 1 (parabola).
Or else, in cartesian coordinates with the origin halfway between the foci, as the curve of all points (x,y) whose coordinates satisfy (x/a)2 + (y/b)2 = 1
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Ability to perform work, i.e. to advance against resistance, for instance lift a body against gravity, or drag it against friction. See also Work.
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A circle around a point which (in the simplest form of Ptolemy's system) moved steadily around the celestial sphere. Greek astronomers proposed that planets moved along epicycles around the Sun or around other points which circled around the sky; later additional corrections were added. The theory of epicycles was the earliest explanation for the irregular apparent motion of the planets--prograde (forward), then retrograde
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Among the two mutually perpendicular axes of a telescope, the one that points at the celestial pole. To keep a star in view, the telescope must be rotated around this axis at the same rate as the Earth turns.
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(of forces) -- A situation when more than one force acts on a body, but because the sum of forces is zero, no motion results.
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the time of the year (around March 21 and September 23) when the position of the Sun in the sky (following the ecliptic) crosses the celestial equator. To a good approximation, the length of the day and night are then equal, and the Sun rises exactly in the east and sets exactly in the west. . Equinox is viewed as the beginning of spring and fall.
The term is also used for each of the two points on the celestial sphere at which the ecliptic and the celestial equator intersect, i.e. the points occupied by the Sun at equinox.
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The first US artificial satellite, launched 31 January 1958 by a 4-stage modified military rocket. Provided the earliest observations of the Earth's radiation belt.
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