Neptune

Neptune  is farthest from the sun, the distance between the two bodies being about 2,750,000,000 miles. At this immense distance it will, according to Kepler's laws, take a long time to travel once around its orbit, and this time has been found to be one hundred and sixty-five of our years. Although it is ninety-seven times as large as the earth, yet, on account of its enormous distance from us it can only just be seen, even with a powerful telescope. Neptune possesses one satellite, which moves around the planet in rather less than six days.
Another, and the most remote planet in our system, is Neptune, revolving at the immense distance of 2850 millions of miles from the sun, and taking more than 164 years to perform the journey. This planet is only to be seen by the most powerful glasses, and was discovered under very peculiar circumstances, not (like other planets) by chance; its existence was recognised as necessary to account for certain "perturbations" or deviations in the orbit of Uranus, which was found to take a course differing from what it should, according to computation, and which were only to be accounted for by supposing that another planet existed far out in space, which affected the course of Uranus by its attraction. The idea of finding out where this orb should be, occurred to M. Le Verrier and Mr. Adams, independent of each other; they both arrived at nearly similar conclusions, for the positions assigned to the supposed planet so nearly agreed with each other and with its real position, that their calculations have been looked on as the greatest feat of astronomical research. It was discovered by Dr. Galle of Berlin, upon his hearing from Le Verrier the position in which at that time it should be sought for.

Discovery of the Planet Neptune

This noble discovery marked in a signal manner the maturity of astronomical science. The proof, or at least the urgent presumption, of the existence of such a planet, as a means of accounting (by its attraction) for certain small irregularities observed in the motions of Uranus, was afforded almost simultaneously by the independent researches of two geometers, Mr. Adams of Cambridge, and M. Leverrier of Paris, who were enabled from theory alone  to calculate whereabouts it ought to appear in the heavens, if visible, the places thus independently calculated agreeing surprisingly. Within a single degree  of the place assigned by M. Leverrier's calculations, and by him communicated to Dr. Galle of the Royal Observatory at Berlin, it was actually found by that astronomer on the very first night after the receipt of that communication, on turning a telescope on the spot, and comparing the stars in its immediate neighbourhood with those previously laid down in one of the zodiacal charts. This remarkable verification of an indication so extraordinary took place on the 23d of September 1846.[1]—Sir John Herschel's Outlines.

Neptune revolves round the sun in about 172 years, at a mean distance of thirty,—that of Uranus being nineteen, and that of the earth one: and by its discovery the solar system has been extended one thousand millions of miles  beyond its former limit.

Neptune is suspected to have a ring, but the suspicion has not been confirmed. It has been demonstrated by the observations of Mr. Lassell, M. Otto Struve, and Mr. Bond, to be attended by at least one satellite.

One of the most curious facts brought to light by the discovery of Neptune, is the failure of Bode's law to give an approximation to its distance from the sun; a striking exemplification of the danger of trusting to the universal applicability of an empirical law. After standing the severe test which led to the discovery of the asteroids, it seemed almost contrary to the laws of probability that the discovery of another member of the planetary system should prove its failure as an universal rule.

[1] Professor Challis, of the Cambridge Observatory, directing the Northumberland telescope of that institution to the place assigned by Mr. Adams's calculations and its vicinity on the 4th and 12th of August 1846, saw the planet on both those days, and noted its place (among those of other stars) for re-observation. He, however, postponed the comparison  of the places observed, and not possessing Dr. Bremiker's chart (which would at once have indicated the presence of an unmapped star), remained in ignorance of the planet's existence as a visible object till the announcement of such by Dr. Galle.