NEW WA YS OF MEASURING THE SUN'S DISTANCE. 71 



The asteroids present several important advantages over 

 even Mars and Venus. 



Of course, none of the asteroids approach so near to trie 

 earth as Mars at his nearest. His least distance from the sun 

 being about 127 million miles, and the earth's mean dis- 

 tance about 92 millions, with a range of about a million and 

 a half on either side, owing to the eccentricity of her orbit, it 

 follows that he may be as near as some 35 million miles 

 (rather less in reality) from the earth when the sun, earth, 

 and Mars are nearly in a straight line and in that order. 

 The least distance of any asteroid from the sun amounts to 

 about 167 million miles, so that their least distance from the 

 earth cannot at any time be less than about 73,500,000 

 miles, even if the earth's greatest distance from the sun 

 corresponded with the least distance of one of these closely 

 approaching asteroids. This, by the way, is not very far from 

 being the case with the asteroid Ariadne, which comes within 

 about 169 million miles of the sun at her nearest, her place 

 of nearest approach being almost exactly in the same direc- 

 tion from the sun as the earth's place of greatest recession, 

 reached about the end of June. So that, whenever it so 

 chances that Ariadne comes into opposition in June, or that 

 the sun, earth, and Ariadne are thus placed 



Sun Earth Ariadne, 



Ariadne will be but about 75,500,000 miles from the earth. 

 Probably no asteroid will ever be discovered which ap- 

 proaches the earth much more nearly than this; and this 

 approach, be it noticed, is not one which can occur in the 

 case of Ariadne except at very long intervals. 



But though we may consider 80 millions of miles as a 

 fair average distance at which a few of the most closely 

 approaching asteroids may be observed, and though this 



Jupiter's being 5 '20, and Mars's I '52 j its period falls short of 8 years 

 by only two months, the average period of the asteroidal family being 

 only about 4$ years'. Five others, Cybele, Freia, Sylvia, Camilla, and 

 Hermione, travel rather nearer to Jupiter than to Mars ; but the remain- 

 ing 166 travel nearer to Mars, and most of them much nearer. 



