The Planet Mars. 471 



exceedingly favourable as respects altitude ; but the planet will 

 be jet further removed from us than he was in 1864. 



Some misapprehension, by the way, appears to prevail 

 respecting the apparent diameter of Mars in 1867. We find 

 it stated by some of our best authorities that the planet will 

 appear larger than he did in 1864. This is a mistake, though 

 the Nautical Almanack appears to countenance the supposition. 

 In 1864, Mars came to opposition when situated as at M' in 

 Fig. 1, the earth being at E'; this was on December 1st, 1864. 

 In 1867 Mars in opposition will be situated as at M, the earth 

 being at E. If now the distance E M (measured of course from 

 centre to centre) be measured, it will be found to exceed 

 the distance E'M' by more than one- seventh. I would not be 

 understood as placing the accuracy of my figures against the 

 calculations of the Nautical Almanack computers ; though I 

 I may note in passing that much more may be learned from 

 carefully drawn diagrams than is commonly supposed.* As a 

 matter of fact, however, the increased diameter given in the 

 Nautical Almanack results from a change in the assumed value 

 of the planet's actual diameter, and a reference to the columns 

 in which the distance of Mars is noted will at once show that 

 the planet is farther off, and therefore must present a smaller 

 apparent diameter. The actual distance of the planet will be 

 about 58,500,000 miles on January 10, 1867, against less than 

 50,000,000 miles on December 1st, 1864. At the opposition 

 of February 13, 1869 (see Fig. 2), Mars will be yet further off 

 than in 1867. In fact the next three or four oppositions will 

 be very unfavourable ones as respects the distance of Mars, 

 and will be successively less and less favourable as respects 

 his meridian altitude. At the approaching opposition the 

 planet will attain an altitude of 64^° when in opposition. 



But, if we would rightly understand the physical peculi- 



* It seems to me that most works on popular astronomy fail in this respect ; 

 it is as easy, or nearly so, to give diagrams correctly representing the relations 

 to be illustrated, as to give incorrect drawings, and it is surely far more instruc- 

 tive. The mind apprehends much more readily, and retains much more certainly, 

 what is presented to the eye, than what is stated verbally. Many of the most 

 marked features of the solar system, or of astronomy generally, admit, too, of being 

 fairly exhibited in diagrams. Occasionally, indeed, it becomes necess-ary to 

 exaggerate dimensions ; in such a case it is sufficient to call the attention of the 

 re der to the want of proportion in the drawing. But often (almost always, in 

 fact) incorrect drawing is admitted where the just proportions could be easily 

 presented. Take, for instance, the eccentricity of Mars' orbit. In G-uillemin's 

 Heavens we find a figure representing the orbits of the earth and Mars, in which 

 no attempt whatever is made to exhibit this feature, though it is one of the most 

 remarkable and most easily illustrated in the solar system. 



Singularly enough, in the Bridgewater Treatises, Whewell remarks that all 

 readers of works on popular astronomy are familiar with the appearance of the solar 

 system, represented as it is in such works by a number of slightly eccentric 

 ellipses. Where are we to find such works ? 



