HALLE Y'S COMET. 



175 



and only time, would emerge from it in another direction, and quit it for ever. 

 It will perhaps be asked, if the orbits must be conic sections, with the sun in the 

 focus, how is it that the planetary orbits are considered as circles ? The fact 

 is, the planetary orbits are not. strictly circular, though very nearly so ; they 

 are ellipses, which are so slightly oval, that, when exhibited in a drawing, they 

 would not be perceived to be so, unless their length and breadth were ac- 

 curately measured. The centre of the sun, also, is in their focus, and not in 

 their centre ; but owing to their slightly oval form, the distance of the focus 

 from the centre is very inconsiderable compared with their whole magnitude. 



To obtain a correct notion of the form of an ellipse, let a flexible string be 

 attached to two points, such as A and B, and let a pencil be looped in it so 

 that when the string is stretched the pencil will be at D ; the string extending 

 from A to D, and from D to B. Let the pencil be moved, carrying the loop 



with it. It will pass successively to the points C, E, M, &c., &c., describing 

 the oval curve, D, C, E, M, L. This curve is called an ellipse. The points 

 A and B are called its foci, and the point 0, at the middle of the distance A 

 B, is called its centre. The ellipse will be more or less oval as the string is 

 less or greater than the distance A B. 



Such is the general form of the curves in which the comets move. If the 

 entire ellipse except the part D, L, G, were blotted out, it would be very dif- 

 ficult to distinguish the arc D, L, G, from that of a parabola or hyperbola. 



On the appearance of a comet then, the first question which presents itself 

 to the astronomical inquirer is, whether the same comet has ever appeared be- 

 fore ? and the only means which he possesses of answering this inquiry is, by 

 ascertaining, from such observations as may be made during its appearance, 

 the exact "path it follows ; and this being known, to discover, by the principles 

 i of geometry, the nature of its orbit. If the orbit be found to be an ellipse, then 

 the return of the comet would be certain, and the time of the return would be 

 known by the magnitude of the ellipse. If the path, on the other hand, should 

 appear to be either a parabola or hyperbola, then it would be equally certain that 

 the comet had never been before in our system, and would never return to it. 



