Page:The story of the comets.djvu/85

V. comet can only be determined with any certainty when the "elements" of its orbit have been ascertained, and that the question of elements is of sufficient importance to need a separate chapter. But without forestalling what will be said there something more may conveniently be said here in dealing with the discovery of comets.

When a comet has been found it must be confessed that astronomers are always in a little flutter pending the inquiry whether the new comet is really a new one, or one that has been seen before, showing itself again to our ken for the second or third time. When the 3 observations necessary for determining its orbit have been made the computers set to work at once to see what is the size and shape and position of its orbit. These facts being ascertained resort is had to a catalogue of previous comets, which is searched in order to see whether the elements of the new comet's orbit bear any resemblance to those of any old comet. If any striking resemblance should be noticed between the longitude of the perihelion, the longitude of the ascending node, the inclination and the perihelion distance of the new comet and of any old comet, there is a primâ facie probability that the new comet is really the old one come again. Accordingly, further observations, prolonged through several weeks, are anxiously awaited in order to see whether they yield results which tally with the first provisional orbit. If they do, so much the better. If there is evidence to show that the new body is moving in an elliptic orbit, that is to say in an orbit which enables the comet to go round and round the Sun, then it becomes possible to assign a period for the comet's revolution round the Sun. This done, the question of identity with some comet already seen becomes very interesting. To quote a medical phrase, "an acute crisis" has been reached; but it is not safe at this stage to jump at conclusions as to identification because both the old and the new comets may have been subjected to disturbances of their orbits (called technically "perturbations") which may have considerably, or even completely, changed the shape and character of either or both orbits.