Neptune, the most distant known planet, and the eighth in order of distance from the sun, omitting the asteroids. Neptune travels at a mean distance of about 2,745,998,000 m. from the sun. The eccentricity of his orbit is less than that of any other in the solar system (excluding the asteroids) save that of Venus. It amounts only to 0.008720; that is, his greatest, mean, and least distances are proportional respectively to 1.008720, 1, and 0.991280. But though the eccentricity itself is small, yet the distance of Neptune is so enormous that the actual displacement of the centre of his orbit measured in miles is considerable. His greatest distance from the sun amounts to 2,771,190,000 m., his least to 2,720,806,000 m., and the linear eccentricity amounts to 25,592,-000 m. As the earth's mean distance from the sun is 91.430.000 m., the distance of Neptune from the earth varies between 2,863,000,000 and 2,629,000,000 m. The inclination of his orbit to the plane of the ecliptic is 1° 47'. He travels around the sun in a mean sidereal period of 60,126.7200 days or 164 tropical years and 220.8 days.

So slow is his motion around the sun that his synodical period only exceeds a year by 2.25 days; that is to say, in the course of one year from opposition he advances through so small an arc that the earth overtakes him and he is again in opposition in 2¼ days. His diameter is estimated at about 37,- 000 m., though very little reliance can be placed on any telescopic measures of so distant an orb. His volume exceeds the earth's about 105 times; but his density (assuming his diameter to be correctly estimated) is only 0.16 of that of the earth; accordingly his mass only exceeds that of the earth about 16¾ times. - The most interesting part of the history of this planet is that which relates to its discovery, which cannot but be regarded as among the greatest triumphs of astronomy. After Uranus had been watched for about a quarter of a century, it was noticed that the path pursued by this planet was not strictly in accordance with calculations in which the perturbations produced by Jupiter and Saturn were duly taken into account. Accordingly Bouvard, the author of excellent tables of the three planets, Jupiter, Saturn, and Uranus, expressed the opinion that a planet of considerable size exists outside Uranus, and by its action on Uranus produced these perturbations.

The Rev. T. J. Hussey of Hayes seems even earlier to have adopted this view, and he wrote in 1834 to Sir G. B. Airy, the present English astronomer royal, suggesting that the external planet might be detected by its action. Airy replied that he did not think the irregularity of Uranus was in such a state as to give the smallest hope of making out the nature of any external action on the planet. He expressed his belief also that there was no such action, and that the earlier observations of Uranus must have been erroneous. In 1837 and in 1842 the astronomer royal was again addressed by E. Bouvard, nephew to the Bouvard just named, and by Bessel, on the subject of an external planet, but he expressed the same opinion as before. In 1844 Prof. Challis applied to Airy for the Greenwich observations of Uranus, for "a young friend of his, Mr. J. C. Adams, who was at work on the theory of Uranus." These were sent, and in September, 1845, Challis wrote to say that "Mr. Adams had completed his calculation of the perturbations of Uranus by a supposed ulterior planet." In October, 1845, Adams left his paper with Airy, in which the place was indicated where the new planet would probably be found.

Airy seems to have placed little reliance on the calculations of Adams, who in fact had then but recently taken his degree, and took no measures to verify his calculations. In June, 1846, Le-verrier published his own independent calculation of the place of a disturbing planet, very nearly agreeing with the place assigned by Adams in 1845. Then Airy admitted that Adams might after all have accomplished the task which he himself had deemed beyond the power of any mathematician; and at his request Challis undertook to search for the new planet. He actually saw it, yet failed to recognize it. In the mean time Dr. Galle of Berlin had found the planet on Sept. 23, 1846. The announcement now made by Airy that Adams had anticipated Leverrier in the calculation of the place of the disturbing planet led to angry complaints from Arago and some other French astronomers. But of course no doubt exists as to the justice of the claim made, when too late, for Adams; in fact Airy admitted that "a publication of the elements of the planet's orbit obtained in October, 1845 " (from Mr. Adams) "might have led to the discovery of the planet in November, 1845." The planet eventually received the name Neptune. It was found to have a distance far less than either Adams or Leverrier had hypothetically assigned to it (following Bode's law). In fact, instead of having a distance represented by 388 (where the earth's distance is 10), the actual mean distance of Neptune amounts to only 300. The motions and period of a planet at this mean distance are quite unlike those of the hypothetical planet of Adams and Leverrier; and Prof. Peirce of Harvard college published in 1848 a paper tending to show that the success of the two European mathematicians was merely a lucky accident.

In one sense this was doubtless true. Fifty years hence the position of Neptune will be utterly unlike that which the hypothetical Neptune would by that time have reached. But as the calculations of Adams and Leverrier were based on the perturbations produced by Neptune while traversing but a small portion of his orbit, and as his apparent position in that portion is nearly the same as that of the hypothetical Neptune, it does not appear that the credit due to Adams and Leverrier can be appreciably diminished. "Were it not, indeed, for the lucky chance that a planet's action could explain the observed discrepancies, whether that planet moved in the actual or hypothetical orbit, neither Adams nor Leverrier could have solved the problem without first abandoning their estimate of the mean distance. But it has not been shown that in this case they would not have modified their assumption to correspond with the facts before them. An investigation of the orbits of Neptune and Uranus, with general tables of the planet's motions, has been published by Prof. S. Newcomb in the " Smithsonian Contributions to Science" (May, 1865, and October, 1873). In these papers he discusses the elements of the orbit of Neptune, the existence of an extra-Neptunian planet, the theoretical place of the planet between the years 1600 and 2000, and many other questions of great interest, with a profundity and elegance which has elicited the highest commendation.

At least one satellite attends on Neptune, discovered by Mr. Lassell of Liverpool in 1847. It travels around the planet in a period of 5d. 21h. 8m., at a distance from his centre equal to 12 times his estimated radius, the maximum observed elongation of the satellite from Neptune's centre amounting to 18".