. Astronomy for students and general readers . ie greaterwill be the change, so long as it passes in front of it. If it passes behind, thereverse effect willfollow, and the mo-tion will be accele-rated. The orbit willthen be changed intoa hyperbola. The or-bit .finally describedafter the comet leavesour system will de-pend upon whetherits velocity is accele-rated or retarded bythe combined attraction of all the planets. All the studies which have been made of comets seemto show that they originally moved in parabolic orbits, andwere brought into elliptic orbits in this way by the attrac-tion o
. Astronomy for students and general readers . ie greaterwill be the change, so long as it passes in front of it. If it passes behind, thereverse effect willfollow, and the mo-tion will be accele-rated. The orbit willthen be changed intoa hyperbola. The or-bit .finally describedafter the comet leavesour system will de-pend upon whetherits velocity is accele-rated or retarded bythe combined attraction of all the planets. All the studies which have been made of comets seemto show that they originally moved in parabolic orbits, andwere brought into elliptic orbits in this way by the attrac-tion of some planet. The planet which has thus broughtin the greatest number is no doubt Jupiter. In fact, theorbits of several of the periodic comets pass very near totha,t planet. It might seem that these orbits ought almostto intersect that of the planet which changed them. Thiswould be true at first, but owing to the constant change inthe position of the cometary orbit, produced by the at-traction of the planets, the orbits would gradually move. Fig. 108.—ATTKACTION OP PLANET ONCOMET. ORIOm OF COMETS. 403 away from each other, so that in time there might be noapproach whatever of the planet to the comet. A remarkable case of this sort was afforded by a cometdiscovered in June, 1770. It was observed in all nearlyfour months, and was for some time visible to the nakedeye. On calculating its orbit from all the observations,the astronomers were astonished to find it to be an ellipsewith a period of only five or six years. It ought thereforeto have appeared again in 1776 or 1777, and should havereturned to its perihelion twenty times before now, andshould also have been visible at returns previous to that atwhich it was first seen. But not only was it never seenbefore, but it has never been seen since ! The reason ofits disappearance from view was brought to light on cal-culating its motions after its first discovery. At its re-turn in 1776, the earth was irot in the right part of i
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