Van Diense, and others. The present writer has elsewhere called attention to the close agreement of the elements of the comets of 1812 and 1846. These coincidences are seen at a glance in the following figure, where the dotted ellipse represents the orbit of the comet of 1812, and the continuous curve that of the fourth comet of 1846.
It seems difficult to regard this general similarity as accidental. A possible explanation may be found in the hypothesis of an ancient comet's separation into parts—a phenomenon known to have occurred in the case of Biela's comet. It has also been pointed out that the paths of both comets very nearly intersect the orbit of Venus; that of 1812 in true anomaly 341°, and that of 1846 in 347°.
On the hypothesis of a common origin it is obvious that these bodies must have entered the solar system at a remote epoch. It seems, therefore, quite remarkable that neither is known to have been observed before 1812. The period of De Vico's comet of 1846 is still too uncertain to be traced backward through former returns; but, with a mean period of the Pons-Brooks comet equal to the interval between the two observed apparitions, we find the dates of former perihelion passages to have been approximately as given below. The
- "Comets and Meteors," Chapter III. The nodal lines are nearly coincident, but the ascending node of the one is at the descending node of the other.