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20 of the Earth’s orbit to Mars’ centre, and pass a plane similarly perpendicular to that, it will cut off the hemisphere we see at any moment from the one we do not. As the two lines do not in general coincide, it will appear that in certain positions, in fact in all but two, Mars must present to us a face partly steeped in daylight, partly shrouded in night; in short, that he shows gibbous like the Moon when she is between the half and the full. This accounts for the look of the drawings made during June, 1894, in which from a seventh to a sixth of the disk is wanting on the left. By drawing lines from his centre to more than one position occupied by the Earth it will be seen that this lacking lune reaches a maximum when the Earth as viewed from Mars is at extreme elongation from the Sun, and that the amount of the phase at such time exactly equals the number of degrees of this elongation. For example, on the sixteenth of last June the lacking lune amounted to 47°, that is, the Earth was then evening star upon the Martian twilight skies at an angular distance of 47° from the Sun, about what Venus seems to us at her extreme elongation, in fact, to Mars we occupy much the same astronomical position that Venus does to us.

To Huyghens we owe the first really