The elements of astronomy; a textbook . ect of the Combination of the Earths Motion withthat of a Planet. — The apparent or geocentric motion of aplanet is therefore made up of two motions, and appears to bethat of a body moving once a year around the circumferenceof a circle equal to the earths orbit, while the centre ofthis circle itself goes around the sun upon the real orbitof the planet, and with a periodic time equal to that of theplanet. 204 EXPLANATION OF TERMS. [§288 Jupiter, for instance, appears to move as in Fig. 74, making 11loops in each revolution, the smaller circle having a di
The elements of astronomy; a textbook . ect of the Combination of the Earths Motion withthat of a Planet. — The apparent or geocentric motion of aplanet is therefore made up of two motions, and appears to bethat of a body moving once a year around the circumferenceof a circle equal to the earths orbit, while the centre ofthis circle itself goes around the sun upon the real orbitof the planet, and with a periodic time equal to that of theplanet. 204 EXPLANATION OF TERMS. [§288 Jupiter, for instance, appears to move as in Fig. 74, making 11loops in each revolution, the smaller circle having a diameter of about one-fifth of the larger one, uponwhich its centre moves, sincethe diameter of Jupiters orbitis about five times that of theearth. As a consequence, wehave an apparent back-and-forth movement of theplanets among the move eastward (tech-nically advance ) part ofthe time, and part of thetime they move westward(technically retrograde ),the arc of retrogression be-ing, however, always less than that of Fig. Geocentric Motion of Jupiter. 289. Explanation of Terms.—Fig. 75 illustrates the mean-ing of a number of terms which are used in describing aplanets position with reference to the sun, viz., Opposition,Quadrature, Inferior and Superior Conjunction, and GreatestElongation. E is the position of the earth, the inner circlebeing the orbit of an inferior planet, while the outer circleis the orbit of a superior planet. In general, the anglePES (the angle at the earth between lines drawn from theearth to the planet and the sun) is the planets a superior planet, it can have any value from zero to 180°;for an inferior, it has a maximum value that the planet can-not exceed, depending upon the diameter of its orbit. 290. Motion of a Planet in Eight Ascension and Longitude. — Starting from the line of superior conjunction, the planet whethersuperior or inferior, moves eastward or direct for a time, but at a §290] MOTION
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