CHAPTER IV.

CERTAIN STATISTICAL INFORMATION RELATING

TO COMETS.

Dimensions of the Nuclei of Comets. Of the Coma.-Comets contract and expand on approaching to, and receding from, the Sun.-Exemplified by Encke's in 1838.-Lengths of the Tails of Comets.-Dimensions of Cometary orbits.— Periods of Comets.-Number of Comets recorded.—Duration of visibility of Comets.- Unknown Comet found recorded on a photograph of the Eclipse of the Sun of May 17, 1882.

THE following are the real diameters, in English miles, of

the nuclei of some of the comets which have been satisfactorily measured within the last hundred years:—

The dimensions of the come, or heads, of comets also vary

It should be remarked that the real dimensions of comets are found to vary greatly at different periods of the same apparition, for there is no doubt that many of these bodies contract as they approach the Sun, and expand again as they recede from it-a fact first noticed by Kepler in the case of the great comet of 1611. The following measurements of Encke's comet in 1838, when approaching the Sun, will illustrate this:

6

Another point of considerable interest in regard to the dimensions of comets is raised by the question, Do they waste away?' and it seems that the answer to this must be in the affirmative. It has been supposed that Halley's comet as described by contemporary writers 1500 or more years ago was possessed of a much larger and more brilliant tail than it has exhibited during the last 2 centuries. And probably there is some significance in the fact that none of the well-known short-period comets are

noted for tails or ever exhibit more than what may be called apologies for tails.

The tails of comets, more especially of those visible to the naked eye, are often of stupendous length, as the following table will show:

We have already seen that the period of the shortest comet yet known is but little more than 3 years: this is in striking contrast to the periods exhibited in the following table, which are however so vast as to deserve little reliance:

A significant fact with respect to the periods of the known periodical comets has already been mentioned, namely that there seems some disposition on the part of these comets to become associated with particular planets. It is not improbable that, as our knowledge becomes enlarged, some very interesting facts may come to light, which are at present hidden.

e See pp. 401, and 444, ante.

From the earliest period up to the present time, the number of comets of which there is any trustworthy record is somewhat over 900; but as it is only within the last 100 years that optical assistance has been made generally available in a systematic search for them, the real number of those that have appeared is probably not less than several thousands, especially when we consider that there have doubtless been many, visible only in the Southern hemisphere.

Comets remain visible for periods varying from a few days to more than a year, but the most usual time is 2 or 3 months. Much depends on the apparent position of the comet with respect

to the Earth and the Sun, and much on its own intrinsic lustre. Among the comets which remained longest in sight, are the following:

There are some few comets which have only been seen on one or two occasions, unfavourable weather preventing more extended observation of them. Fig. 229 is a case in point. It represents a comet seen during the totality of the solar eclipse of 1882, which was never seen again, and as to whose history and fate we know nothing.

ECLIPSE OF THE SUN OF MAY 17, 1882, SHOWING AN UNKNOWN COMET. (Ranyard.)