Opposition of Mars. 131 



OPPOSITION OF MAES.— DOUBLE STAES.— OCCUL- 

 TATIONS.— THE COMET. 



BY THE EEV. T. W. WEBB, F.E.A.S. 

 OPPOSITION" OF MAES. 



A planet revolving round the sun in an orbit exterior to our 

 own, and therefore, by the 3rd law of Kepler, with inferior 

 speed, must necessarily be overtaken and passed by us from 

 time to time, and these epochs will be attended with remark- 

 able changes in the planet's apparent movements. So long as 

 our own motion is such in point of direction and swiftness as 

 not to give a false impression of the progress of the exterior 

 planet, it will of course be seen to move, as we and all other 

 planets really move, from W. to E., according to the order 

 of the signs of the zodiac, or, in astronomical language, 

 direct ; but as soon as our position and velocity become such 

 as to make appearances prevail over realities, our neighbour 

 will first seem to slacken his speed, then to stand still, and 

 ultimately, while we are in the act of passing him, to move 

 backwards, or, astronomically speaking, to retrograde ; as we 

 leave him behind his reversed velocity will gradually decrease, 

 till he again become stationary, to recover himself as it were, 

 and afterwards starts afresh, and proceeds quietly on his own 

 way. These phenomena are, of course, the same with those 

 which may be noticed when a train passes an ordinary vehicle, 

 or a steamer overtakes a sailing vessel. The precise position in 

 which our neighbour is passed by us is called his opposition, be- 

 cause he is at that time exactly opposite to the Sun, and comes to 

 the meridian at midnight ; and being then, of course, at his least 

 distance from us, this will be the most favourable opportunity 

 for the study of his physical peculiarities under an enlarged 

 diameter. Such is about to be the case with the planet Mars, 

 whom our readers must have been for some time noticing as the 

 most conspicuous as well as the most ruddy object in the Eastern 

 sky. He becomes stationary on September 2, and from that time 

 will rapidly increase in brilliancy as we are coming up to him, 

 till, on October 5, we shall pass him by; after which date he will 

 fall off in appearance and speed till, on November 7, he reaches 

 the end of his retrogradation, becomes stationary, and subse- 

 quently proceeds in the ordinary direction till we begin to come 

 round arter him again, not, however, to overtake him till after two 

 years and fifty days. Such is the general principle and aspect 

 of these optical changes : but they are not invariable in their 

 amount. They would be so, if the planetary orbits were of a 

 circular form; but since they are elliptical, and the ellipses have 

 no fixed position in space, but are subject to a continual vari- 



