1Sir Isaac Newton’s BookU*

6 . Each planet moves round the fun in the line, whichwe have mentioned above a under the name of ellipsis ; whichI shall here shew more particularly how to describe. I havethere laid how it is produced in the cone. I shall now fheWhow to form it upon a plane. Fix upon any plane two pm s >as at A and B in fig. 91. To these tye a string ALB of anylength. Then apply a third pin D lo to the string, as to holdit strained; and in that manner carrying this pin about, thepoint of it will describe an ellipfis. If through the points A,B the straight line EABF be drawn, to be terminated atthe ellipsis in the points E and F, this is the longest lin eof any, that can be drawn within the figure, and is call"ed the greater axis of the ellipsis. The line GH* drawnperpendicular to this axis EF, so as to pass through themiddle of it, is called the lesser axis. The two points Aand B are called focus’s. Now each planet moves roundthe fun in a line of this kind, so that the fun is found 1 stone focus. Suppose A to be the place of the sun. Then siis the point, wherein the planet will be nearest of all to thefun, and at F it will be most remote. The point E is call"ed the perihelion of the planet, and F the aphelion. In ^and H the planet is said to be in its middle or mean distance,because the distance AG or AH is truly the middle he"tween AE the least, and AF the greatest distance. In fig- 9 ^'is represented how the greater axis of each orbit is situated J *jrespect of the rest. The proportion between the greatest anleast distances of the planet from the fun is very diss ereIlt:in the different planets. In Saturn the proportion of t ^f

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* Book I. ch. 2 . § 8r»