104 ON COMETS. 



Mercury and Venus by circles of 4^ and 9 inches respec- 

 tively that of the moon above the earth by one i5th of 

 an inch, and the globe of the earth itself by a dot barely 

 the thousandth part of an inch in size. 



(n.) Strictly speaking, the orbits are not circles they 

 are slightly oval, or, as it is called, elliptic in form, and 

 the sun does not occupy their common centre, but what 

 is called \hefocus of each; that is to say, one of the two 

 pins round which an ellipse may be described by carry- 

 ing a pencil round them confined by a looped string 

 encircling them both. The planetary orbits, moreover, 

 all lie nearly in one plane, or very slightly inclined to 

 that in which the earth performs its annual revolution, 

 which is called the plane of the ecliptic the angle at 

 which the plane of each orbit meets and cuts this, being 

 called its inclination to the ecliptic. They all circulate 

 the same way round the sun, and the farther they are 

 from the sun the slower they move so that while the 

 earth goes round it in 365 days, Mercury occupies only 

 88 in its revolution, while Neptune requires no less than 

 1 68 years to complete one of his circuits. 



(12.) When we come to the comets, however, we find a 

 very different state of things. A comet, it is true, moves 

 round the sun as his centre of motion : not, however, in 

 a circle, or any approach to a circle, but (with a very 

 few, and those highly remarkable exceptions) in an im- 

 mensely elongated, or, as it is termed, a very eccentric 

 ellipse. In consequence, the nearest distances to which 

 they approach the sun bear almost universally an exceed- 

 ingly small proportion to those they attain when most 



