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THE PLANET VENUS.

or farther from the Earth ; next, to blacken all that portionof each which, as seen from the Earth , would, according to thecorresponding phase of Venus , be dark; and then to measureour various figures and find which of them has the largestarea unblackened.

The final result of all this explanation is therefore asfollows:—When Venus shines with its maximum brilliancy, asseen by the naked eye, its phase is (as in No. 4 of the abovefigures) very nearly the same as that of the moon when it isabout 3^ days old, or one-fourth full. Venus is at such a timeconsiderably nearer to the Earth than when it is at its greatestapparent distance, or elongation (see Fig . XXXVI.) west oreast of the Sun. But if it come any nearer, the diminution ofits phase more than counteracts the increase in its light whichits greater proximity would otherwise produce. On the otherhand, if it go further away, the phase increases in width, andsoon appears in a telescope to be similar to that of a Moon half-full (as in No. 3 of the above drawings), and presentlysimilar to that of one more than half-full, or gibbous (as inNo. 2); notwithstanding which, the total light received fromit is less. In fact, it may be computed, that the lialf-moonphase, which corresponds to the greatest apparent elongationof Venus from the Sun, sends to us about three-fourths of thegreatest possible amount of its light; while the planet wouldonly appear to shine with one-fourth of its maximum brilliancyif, in spite of the solar glare, it could be seen at its furthestdistance from the Earth upon the opposite side of the Sun,notwithstanding that its whole disc would then be bright.

We have explained in Lecture V. that the combined effect ofthe orbital motions of the Earth and of Venus is to makethe latter appear to travel once round the Sun in 584 days,although its actual revolution in its orbit is accomplished in225 days. An interval of 584 days consequently takes placebetween its being seen in any given position relatively to theSun, and its being so seen again. Once therefore in every 584days ( i.e ., upon an average, since some allowance must be madefor slight irregularities in its movements), the planet will attainits greatest brilliancy as an evening star, and once in the same