30 
POPULAR SCIENCE REVIEW. 
more of his north polar regions towards us. In April, when 
Mars will be passing through midsummer of his northern hemi- 
sphere (and therefore the polar cap of his southern hemisphere 
will be probably very large), the north pole will not be much 
more fully presented to us than at opposition, since the earth 
will have moved on beyond E. Thus observers will have a very 
good view of the south polar cap — a much better view, in fact, 
as respects extent of snowsurface^directed towards the earth, than 
they could have if Mars were in opposition during this part of 
his year. The planet, at about this time, will present his greatest 
phase of gibbosity. Being much farther from the earth than in 
January, more powerful telescopes will be required to reveal 
minor details. In fact, his diameter will be reduced to about 7". 
The path of Mars during the first five months of 1867 is ex- 
hibited in fig. 2. He retrogrades below the two bright stars 
Castor and Pollux, reaching his stationary point in the middle 
of February. After this he advances — growing meanwhile 
gradually less bright — through Gemini and Cancer, passing in 
May very close to the visible cluster Prsesepe in the last-named 
constellation. In no part of this course will there be the least 
difficulty in tracing the planet, as at first his superior light, and 
afterwards his red colour and the absence of scintillation, will 
markedly distinguish him from neighbouring objects. 
Observers armed with suitable instruments will have the 
opportunity of aiding in the determination of that element 
which may be called the fundamental element of astronomy — the 
measuring-line of space — the sun’s distance from the earth. 
Mars does not afford us quite such powerful means as Venus 
(in transit) for the purpose in view — first, because he never 
approaches us so near as Venus (in inferior conjunction); and, 
secondly, because his apparent position viewed from any given 
part of the earth is not determinable by a natural index-face 
such as the solar disc. However, he approaches so near as to 
have a sensible parallax ; and his apparent path brings him near 
enough to many stars to afford very sufficient indications of 
such change of apparent position. In fact, so largely has 
instrumental astronomy improved, that determinations of the 
solar distance afforded by Mars in opposition (especially if near 
perihelion), and Venus in inferior conjunction, are more trust- 
worthy than determinations obtained from the last transit of 
Venus would have been (with the instruments in use in 1787) 
even with far more satisfactory observations than appear to have 
been actually obtained. A series of observations made by Mr. 
Pogson at Madras, with an instrument very ill adapted to the 
investigation, resulted in giving for the sun’s distance 88,950,000 
miles, a result much nearer the distance (about 91,650,000 
miles) now generally supposed to be the true one, than the dis- 
