## Intra-Mercurial Planets and the Zodiacal Light

It is very probable, indeed almost certain, that there are masses of matter revolving around the sun within the orbit of Mercury. 

__Motion of the Perihelion of Mercury's Orbit.__---Leverrier, in 1859, from a discussion of all the observed transits of Mercury, found that the perihelion of its orbit has a movement of nearly 38 " a century. This is more than can be accounted for by the action of the known planets, and, _so far as known_, could be explained only by the attraction of a planet, or ring of small planets, revolving inside this orbit nearly in its plane, with a mass about half as great as that of Mercury itself.

We say "_so far as known_," because an alternative hypothesis has been proposed, viz ., that the law of gravitation, though strictly true for bodies at rest is not absolutely so for bodies in motion; that when bodies are moving towards each other the attraction is less by a minute fraction than if they were at rest. The hypothesis is known as the electro-dynamic theory of gravi- tation, but has at present very little to support it. If, however, it were true, then the peculiar motion of the apsides of Mercury's orbit would be a necessary consequence.

Subsequent investigations by a number of mathematicians have fully confirmed Leverrier's results; Mercury's orbit is beyond question affected as it would be if there were an intra-Mercurial planet, or a number of them.

__Dr. Lescarbault's Observation: Vulcan.__---A certain country physician, living some eighty miles from Paris, Dr. Lescarbault, on the publication of Leverrier's result, announced that he had actually seen this planet crossing the sun nine months before, on the 26th of March of that year, 1859. He was visited by Leverrier, who became satisfied of the genuineness of his observations, and the doctor was duly congratulated and honored as the discoverer of "Vulcan," which name was assigned to the supposed new planet. An interesting account of the matter may be found in Chambers' "Descriptive Astronomy"; and in many of the works published from twenty to twenty-five years ago, as well as in some more recent ones, " Vulcan " is assigned a place in the solar system, with a distance of about 13,000000 miles and a period of 19+ days. Lescarbault described it as having an apparent diameter of about 7 ", which would make it over 2500 miles in diameter.

Nevertheless, it is nearly certain that Vulcan does not exist. There are various opinions which we need not here discuss as to the explanation of this pseudo-discovery. But the planet, if real, ought since 1859 to have been visible on the sun's face at certain definite times which Leverrier calculated and published; and it has never been seen, though very carefully looked for. Small, round, dark objects have from time to time been indeed reported on the sun's disc, which in the opinion of the observers at the time were not sun spots; but most of these observations were made by amateurs with comparatively little experience, with small telescopes, and with no measuring apparatus by which they could certainly determine whether or not the spot seen moved like a planet. In most of these cases photographs or simultaneous observations made elsewhere by astronomers of established reputation, and having adequate apparatus, have proved that the problematical " dots " were really nothing but ordinary small sun spots, and the probability is that the same explanation applies to the rest .

__Eclipse Observations.__---A planet large enough to be seen distinctly on the sun by a 24-inch telescope, such as Lescarbault used, would be a conspicuous object at the time of a solar eclipse, and most careful search has been made for the planet on such occasions; but so far, although stars of the third and fourth magnitudes, and even of the fifth, have been clearly seen by the observers within a few degrees of the eclipsed sun, no planet has been found.

One apparent exception occurred in 1878. During the eclipse of that year, Professor Watson observed two starlike objects ( of the fourth magnitude ), which he thought at the time could not be identified with any known stars consistently with his observations. Mr. Swift, also, at the same eclipse, reported the observations of two bright points very near the sun; but these from his statement could not ( both ) have been identical with Watson's stars. Later investigations of Dr. Peters have shown that the assumption of a very small and very likely error in Professor Watson's circle-readings ( which were got in a very ingenious, but rather rough way, without the use of graduations ) would enable his stars to be identified with 0 and Cancri, and it is almost certain that these were the stars he saw. Mr. Swift's observations remain unexplained. With this exception, the eclipse observations all give negative results, and astronomers generally are now disposed to consider the " Vulcan question " as settled definitely and adversely.

At the same time it is extremely probable that there are a number, and perhaps a very great number, of intra-Mercurial asteroids. A body two hundred miles in diameter near the sun would have an angular diameter of only about 1 ", as seen from the earth, and would not be easily visible on the sun's disc, except with very large telescopes. It would not be at all likely to be picked up accidentally. Objects with a diameter of not more than forty or fifty miles would be almost sure to escape observation, either at a transit or during a solar eclipse.