132 Opposition of Mars. 



ation in the direction of their longer axes, ever}?- succeeding op- 

 position will take place under fresh relations, oscillating between 

 the most favourable circumstances, when the Earth is in its 

 aphelion or furthest from the Sun, and the planet nearest, in its 

 perihelion, and the least favourable, in the reverse of these con- 

 ditions. With regard to Mars, the difference is very consider- 

 able. His orbit is so elliptical, that when furthest he is dis- 

 tant from the Sun 158,754,000 miles, when nearest, only 

 131,656,000; so that his best opposition, so to speak, brings 

 him more than 27,000,000 of miles nearer to us than his worst, 

 from the form of his own orbit ; while the excentricity of ours 

 may increase or diminish this quantity by nearly 3,250,000 

 more. His apparent diameter of course varies in a correspond- 

 ing proportion from little more than 13" to upwards of 30" : at 

 the coming epoch it will amount to 21""8. The greatest expan- 

 sion of disc will, in consequence of the mutual obliquity of the 

 two orbits, take place on September 29 ; but the difference for 

 a considerable time will not be material, as it will range above 

 20'' from September 5 till October 20 ; at the first of which 

 dates there will be a slight phasis or defalcation from the perfect 

 circle on the W., as at the latter on the E. limb. Had the oppo- 

 sition taken place' between the aphelion of the Earth on July 1, 

 and the perihelion of Mars on August 4, it would have been 

 still more auspicious, and had those epochs coincided, as they 

 nearly did in August 1719, the planet would have assumed 

 unwonted magnitude and splendour; but the present is by 'no 

 means an unfavourable opportunity, being little inferior to the 

 opposition of 1830, which Beer and Madler found so advanta- 

 geous for the delineation of his physical aspect. In another 

 respect too it will be suitable for the observers of Europe, as 

 the planet is a little N. of the equator ; on some other occa- 

 sions it has had such wide S. decimation when we passed it as 

 to be fairly visible only in equatorial or southern latitudes : in 

 such a position, the most powerful telescope in our northern 

 observatories would show it far less satisfactorily than a much 

 smaller one upon the coming occasion. Such of our readers as 

 are possessed of instruments of moderate capability will there- 

 fore prepare themselves for the examination of that fine ruddy 

 disc, which, in its appearance of continents, and seas, and 

 snowy poles, and atmospheric obscurations, as well as in its 

 diurnal rotation and its change of seasons, presents so great 

 and interesting a resemblance to our own globe. On these 

 points a few remarks will be offered in a future paper. 



P.S. August 15. A brief interruption last night in our 

 wonderfully persistent cloudy veil exhibited the S. polar snows 

 very distinctly, though of no great extent. There was much 

 dark mottling on the disc, but no decided spot. 



