DISCOVERY OF PLANETS.
(Vol. vii., p. 84.)
Leonora says, "supposing that the recently-discovered planets obey the same laws as the larger ones, they must be at all times apparently moving within the zodiac;" and she asks for an explanation of the fact of their not having been discovered before.
Ancient astronomers having observed that the moon, and the planets visible to them, were never seen at more than a small angular distance north or south from the plane of the earth's orbit, they drew two circles parallel to the ecliptic, at the distance which experience had shown them to be sufficient for comprehending the apparent places of those heavenly bodies at all times; and to the intervening space they gave the name of zodiac. But there is no law of matter, or, in other words, it is no necessary consequence of gravitation or planetary action, which confines the planets' orbits within the zodiac. The fact can only be ascribed to the will of Him who first projected them into their intended paths; though that will had doubtless some wise and calculated end in view.
It was further observed, in the last century, that the increasing distance of each successive planet
from the sun would follow an uniform rule, if there were not one wanting between Mars and Jupiter, to fill up the series. This put astronomers upon the search, and led to the discovery, in 1801, of four small planets, all at nearly the requisite distance, but moving in paths inclined to the ecliptic at such large angles as carry them beyond the zodiac, though they necessarily move across it. From hence it was inferred that they were portions of a planet originally harmonising, in size, position, and orbitual path, with the rest of our system, but burst into fragments by an internal explosion, at some time prior to man's recorded observations of the heavenly bodies. This supposition gains strength from the continued discovery of more and still smaller fragments, each still moving as a planet at nearly the same distance from the sun; and each seeming to proclaim that there was a world, probably larger than our earth, amongst whose inhabitants sin entered, as amongst us; but for whom mercy was not in like manner procured.
As to the discoverer of a previously unknown planet, your inquirer should be told, that more is necessary than its merely coming within the field of an observer's telescope, even if it attracts his notice. Some years before 1781, the year in which Herschel discovered the planet which should perpetuate his name, Lalande had noted down an observation of a star, of a certain magnitude, in a position where afterwards no such star could be found, but where calculations since made, from the known orbit of that planet, prove that it must then have been. By failing to continue his observation of it, till it should have changed its place amongst the fixed stars, Lalande lost the discovery. And though Herschel's much more powerful telescope enabled him to perceive, on a first inspection, that it had a defined disc, more observations were required to enable him to say that it could not be a comet shorn of his beams: whilst, as to the last discovered planets, I think we have been told that their apparent size is but that of a star of the ninth order, in decreasing magnitude; and no part of the heavens has been so accurately mapped as to give an observer reason to conclude, from catching sight of one of these planetary fragments, that he has detected an obscure wanderer not usually seen in that locality. But if its appearance leads his practised eye to suspect that it shines with but borrowed light, and that induces him to continue his nightly watch, he receives his reward, if it be so, and announces the existence of another planet.
Henry Walter.