The encyclopædia britannica; a dictionary of arts, sciences, literature and general information . of any body, and especially of a heavenly body, revolvinground an attracting centre. If the law of attraction is that ofgravitation, the orbit is a conic section—ellipse, parabola orhyperbola—having the centre of attraction in one of its foci;and the motion takes place in accordance with Keplers laws(see Astronomy). But unless the orbit is an ellipse the bodywill never complete a revolution, but will recede indefinitelyfrom the centre of motion. Elliptic orbits, and a parabolicorbit considered as
The encyclopædia britannica; a dictionary of arts, sciences, literature and general information . of any body, and especially of a heavenly body, revolvinground an attracting centre. If the law of attraction is that ofgravitation, the orbit is a conic section—ellipse, parabola orhyperbola—having the centre of attraction in one of its foci;and the motion takes place in accordance with Keplers laws(see Astronomy). But unless the orbit is an ellipse the bodywill never complete a revolution, but will recede indefinitelyfrom the centre of motion. Elliptic orbits, and a parabolicorbit considered as the special case when the eccentricity of theellipse is 1, are almost the only ones the astronomer has toconsider, and our attention will therefore be confined to them inthe present article. If the attraction of a central body is notthe only force acting on the moving body, the orbit will deviatefrom the form of a conic section in a degree depending on theamount of the extraneous force; and the curve described maynot be a re-entering curve at all, but one winding around so as ORCAGNA 165. to form an indefinite succession of spires. In all the cases whichhave yet arisen in astronomy the extraneous forces are so smallcompared with the gravitation of the central body that the orbitis approximately an ellipse, and the preliminary computations,as well as all determinations in which a high degree of precisionis not necessary, are made on the hypothesis of elliptic are set forth the methods of determining and dealing withsuch orbits. We begin by considering the laws of motion in the orbit itself,regardless of the position of the latter. Let the curve represent an elliptic orbit, AB being the majoraxis, DE the minor axis, and F the focus in which the centre ofattraction is situated, which centre we shall call the sun. From the properties of theellipse, A is thepericentre ornearest point ofthe orbit to thecentre of attrac-tion and B theapocentre or mostdistant point. T
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