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revolving round the Sun , in an orbit nearer to it than that ofthe Earth , has a much longer path to traverse, between beingat its furthest apparent distance upon one side and again uponthe opposite side of the Sun , when it goes round beyond itbetween the two positions, than when it passes, in the mean-time, between the Earth and the Sun . The two periods, in fact,last, upon an average, 43| days and 721 days respectively,for Mercury; 143 and 441 days respectively for Venus .
The whole interval of 116 days in the case of Mercury, or of584 days in that of Venus , which elapses between its being thus
Fig . XXXIX —Comparative positions of Mercury and Venus when seen from the Earth at their greatest elongations from the Sun .
seen twice in succession, in conjunction or opposition, or atextreme eastern or western elongation from the Sun , or in anyother given position relatively to it, is called a synodic period,which term means that the period depends upon the joint effectof the movements of the planet and of the Earth in theirrespective paths.
In like manner, it may be deduced from the Copernicantheory that the period of Mars in its own orbit being 687 days,it must occupy, upon an average (when certain variations inits rate of movement are allowed for) 780 days between beingtwice seen from the Earth in conjunction or opposition, or in
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