ON THE SATELLITES OF MARS. 
BY 
WENTWORTH ERCK, t1.p., F.R.A.S. 
[Read 19th November, 1877.] 
On the 19th of August the astronomical world was startled by the receipt of a tele- 
gram from Washington announcing the discovery of two satellites to the planet 
Mars. 
There was some ground for the incredulity that was at first expressed, for it 
seemed hardly credible that the satellites of our nearest celestial neighbour should 
have been overlooked whilst those of the most distant planets had been long since 
discovered. 
But, when it is considered how many circumstances must combine to render these 
exceedingly minute bodies visible, we shall cease to wonder at their not having 
been discovered earlier. 
Chief among the conditions of visibility are, (1) the position of the planet with 
regard to our earth, and (2) the possession of telescopes of exquisite defining power. 
1. As to the first condition; it is only at opposition that there is any possibility of 
observing Mars to advantage, and the eccentricity of his orbit being considerable, 
it is only at those oppositions which occur when the planet is in perihelion that he 
can be seen to the best advantage. 
Taking our own distance from the Sun ‘as unity, the perihelion opposition dis- 
tance of Mars from us is only 0.38, while his aphelion opposition distance from us is 
0.66, or nearly twice as great. But, unfortunately, these perihelion oppositions 
are rare, owing to the relative periodic times of the Earth and Mars. 
In 1798 there was an opposition very near to perihelion. During the present 
century the oppositions, reckoned according to their proximity to perihelion, are 
those of 1845, 1877, 1830, 1892, 1862, 1860, 1847, 1849. 
The opposition of 1845 was the nearest to perihelion, occurring about ten days 
prior to it, whilst that of 1877, the next nearest, was fourteen days after perihelion. 
But, though at the opposition of 1845 the Earth was nearer to the planet than at 
any other time during the century, yet at that opposition the planet, as seen from 
this latitude, was too low in the heavens for successful observation. 
At the opposition of 1830 he was pretty high in the heavens, but further off, 
whereas at the opposition which has just taken place, his altitude, as seen from our 
latitude, was about 28° when on the meridian, and his distance was very little greater 
than in 1845, so that on the whole the recent opposition was probably the most 
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