MARS IN THE AUTUMN OF 1877 - 
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and though the greenish line of the seas perhaps extends a little 
more widely (at least for most eyes) it is wanting over large 
tracts usually regarded as marine in character. 
As a general rule it may be considered that a dark marking 
once fairly seen is to be regarded as indicating a sea-region, 
whether commonly seen or not. We cannot but suppose that 
on Mars as on our own earth, there are sea-regions where clouds 
are very prevalent, and where, therefore, we are seldom likely to 
catch the dark hues of the sea. Our observations are after all 
only made under favourable conditions at long intervals ; and 
those of the northern regions of Mars have been as yet very 
imperfect, because when Mars turns his north polar regions 
earthwards, he is near the aphelion of his orbit, or, in other 
words, the summer of Mars’ northern hemisphere, like the 
summer of our own northern hemisphere, occurs near the 
aphelion of the orbit.* This part of my chart of Mars will 
probably require more correction than any other part. In fact, 
from observations by Messrs. Green and Knobel (four of the latter 
I have given in the article Astronomy “Encycl. Brit.”) it appears 
* See PL xxvii. vol. ii. 11 Encycl. Brit./’ (article Astronomy), which I have 
reduced from my chart of the orbits of the planets Mars, Earth, Venus and 
Mercury. 
