Lectures on astronomical theories . lanet wouldmake rather more than half a revolution during the7 years, so that a great number of these septennial re-appearances might occur before the planets situation inthe zodiac would cause the comet to leave the solar orbitat that particular time of the year when its return wasexpected, and so prevent its being seen from the earthat the time of its usual re-appearance. * And moreover the comet must certainly have its periods of verticalelevation and depression which, instead of coinciding vrhh those of theearth, may be in opposition thereto, and hence c
Lectures on astronomical theories . lanet wouldmake rather more than half a revolution during the7 years, so that a great number of these septennial re-appearances might occur before the planets situation inthe zodiac would cause the comet to leave the solar orbitat that particular time of the year when its return wasexpected, and so prevent its being seen from the earthat the time of its usual re-appearance. * And moreover the comet must certainly have its periods of verticalelevation and depression which, instead of coinciding vrhh those of theearth, may be in opposition thereto, and hence considerably increase theapparent difference in the relative situation.) 36 halleys comet. (6) Notices from the Record of various ^s Comet. Fig. 9* represents the supposed orbit of Halleys comet,and is a fair illustration of the elliptical orbit of extremeeccentricity, which is now attributed to cometary obsei-ve that the comet, having nearly reached itsperihelion, makes about one-third of a revolution arourid. the sun in moving from A to B^ but having arrived at Byand still being comparatively very near to the sun, it nolonger obeys the restraining power of tlio suns gravi-tating influence, but recedes in an almost direct line to agreat distance, then, describing a slight curve towardsthe major axis of the ellipse, it gradually approaches its(supposed) aphelioli (7. Notwithstanding that the cometwhen at B, comparatively close to the sun, vv-as unaffected * From Dicks Sidereal Heavens. halleys comet. 37 by the enormous gravitating force to which it must havebeen at that place subjected, now, when near C at thevery great distance >S^. C, it becomes suddenly and sensi-tively attentive to the comparatively very feeble influenceof the sun and describes the short curve shown at C (thesupposed aphelion) ; but, here again, it appears quite evi-dent that if the velocity of the comet at this place is sosmall and the suns influence sufficiently great to cause the
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