It is rather curious to what extent we have a feeling of kinship with Neptune, notwithstanding he dwells forever in far-off space where we cannot expect even to have a glimpse of him without the aid of a telescope. Uranus, the other very distant planet, is so nearly within the limit of ordinary vision that we have always a hope that, by some lucky chance of situation or atmosphere, we may some day be able to see him face to face, and know for ourselves what manner of planet this is which, though a member of our own cosmic family, remains always just beyond easy exchange of glances with us; and so we in a measure keep a lookout for him that gives us a sense of his reality.

With Neptune there can be no feeling of this sort to keep us with a lively interest in him, and yet he is hardly less real to us than Uranus, and we have a more intimate sense of nearness to him than we have for any fixed star. Far away as he is, the distance between us is short compared with the many trillions of miles farther that we must go to reach the nearest star, and in thinking of him we always have a sense of this. Then, however aloof he may keep himself from this cozy little bunch of planets near the sun, of which the earth is one, he is still of the same parentage with us, and his life history is part of our family history, so that we can never feel indifferent to what concerns him.

Close as Neptune is to us in kinship and distance, as astronomical distances go, we never knew of his existence until sixty-six years ago. He is to us almost a recent arrival in the solar domain, but we know that he has been here as long as we have; and whether he was detached before we were from the great nebula which gave birth to us all, or at about the same time, we know that for long ages before there were eyes on the earth to see him he was, as he still is, circling slowly and majestically around our common center of control.

The discovery of Neptune in 1846 created truly a sensation in astronomical circles. And, unlike most sensational happenings, it fully justified the extreme interest it aroused. The computation that led to it was a mathematical triumph, and the final result was a most splendidly convincing proof of the theory of gravitation. For the place of this hitherto unknown planet was found by means of computations based on the fact that at certain times Uranus went a little out of his way, thus showing some disturbing body outside of his orbit pulling him slightly from the course he would otherwise take. The deviation was not much—only about one and three-fourths of a minute, which is equal to about one-seventeenth of the apparent diameter of the moon, or one-sixth of the distance between Mizar and Alcor, situated at the bend of the handle of the Big Dipper, two stars that it is difficult for some eyes to separate.7 But this slight irregularity of Uranus was enough to set at least two able men at work in an effort to locate the disturbing cause. These two men were Adams, of England, and Leverrier, of France.

The result of Adams’s work was announced to the Astronomer Royal in England in the autumn of 1845; but the actual search for the planet in the place predicted was delayed until the following summer. In the mean time Leverrier had completed his work and had communicated with astronomers in Berlin, directing them where to look for the planet. The facilities for that sort of work were then better in Berlin than in England; and within half an hour after the search was begun, on the night of September 23, 1846, the new planet was discovered a little more than half a degree from the exact position Leverrier had found for it. It was first recognized as having a sensible disc, and within a day its motion was apparent. No wonder the astronomical world was thrilled by this achievement!

Although the planet was actually discovered by following the directions of Leverrier, it was found that it might have been seen months before if the English astronomers had shown more promptness in using the computations of Adams; and there has always been a disposition among astronomers, both in France and in England, to give both men credit for their extraordinary achievement, though, naturally, there is somewhat more stress laid upon the work of each in his own country. The newly discovered body was at first named for its discoverer, Leverrier, but a sense of justice to Adams prevailed to such an extent that in the end a less commemorative name was chosen, and the planet was called after Neptune, the son of Saturn and the brother of Jupiter—a name more fitting, on the whole, for a member of this planet family, whose other members all bear the names of some of the ancient deities. The trident (?), Neptune’s three-pronged spear, is the symbol of the planet.

The mean distance of Neptune from the sun is more than two and a half billion miles (2,790,000,000), and his orbit is so nearly circular that the variation between his perihelion and aphelion distance is only about fifty million miles. His orbit is, in fact, less eccentric than that of any other planet except Venus. His immense distance from the sun, of course, deprives him of any great amount of heat or light from that source as compared with the other planets. The sun would appear to an observer on Neptune a little smaller than Venus appears to us. But so great is the intensity of its radiance that even as so diminutive a sun as that it would give to Neptune more than six hundred times as much light as our full moon gives to us. This, however, would be as much as nine hundred times less light than we get from the sun. Such light as the planet receives from the sun reaches it after a journey of a little more than four hours.

Of the heat the planet has, either inherent or acquired from the sun, we do not know much. The normal temperature at that distance from the sun would be more than three hundred and sixty degrees below zero, Fahrenheit, and there is not much to indicate in what state the planet is with reference to its own heat. Investigations thus far made do not show it to be so intensely hot as Jupiter and Saturn undoubtedly are; but with its heavily vapor-laden atmosphere it could not have the frigidity normal to a black, unprotected body at its distance from the sun.

Neptune is thought to have an immense atmosphere, and, like the other outer planets, one of a composition not wholly familiar to us. Consequently we do not know as yet just what this atmosphere does for the planet. It has a fairly good reflecting power, though the planet, on the whole, is darker in color than Jupiter or Saturn. Its color is of that bluish cast which sometimes suggests a leaden appearance. The color, as well as the fact that Neptune is denser than any of the other outer planets, indicates that it may be in a more advanced stage of development than at least Jupiter and Saturn are, and perhaps than Uranus is.

That Neptune has made greater progress toward solidity (though it is still very far from that state) than the other outer planets is suggested also by its size; for, as we have seen, the smaller planets develop more rapidly than the larger ones. The diameter of Neptune is a little less than thirty thousand (29,827) miles. The planet is somewhat smaller, therefore, than Uranus, and much smaller than Jupiter or Saturn. But as compared with the earth, the largest of the inner planets, it is a vastly greater body. Its mass is seventeen times more than that of the earth; its surface is as much as sixteen times more extensive than the earth’s; and its volume is more than eighty times greater than the volume of the earth.

Of the time of Neptune’s rotation on its axis very little is known. That little, however, indicates a slower rotation than the other planets seem to have, and the alternations of day and night on Neptune are, therefore, probably less swift than on Jupiter and on Saturn. The planet is too far away for us to see its surface markings with any distinctness, but there are indirect processes by which we can get approximate information concerning the facts about rotation. One of these processes is by observation of the motions of the satellites. Of these useful bodies Neptune, fortunately, has one—a very excellent moon about the size of our own. It has some eccentricities, such as revolving about the planet in the opposite direction from that which the more conventional satellites follow, and having an orbit a good deal inclined to the plane of the equator of the parent body. But it is a very interesting moon to astronomers, and will no doubt in time help to make clear some things in the history of Neptune which are now not quite understood.

Being so far from the sun, Neptune moves, of course, very slowly in comparison with the nearer planets, though his speed is at the rate of three and a half miles a second, which, after all, does not denote any high degree of sluggishness. His change of position in the sky amounts to a little more than two degrees a year; so that in an ordinary lifetime he does not make any very great progress along the zodiac.

When Neptune was discovered he had just left the constellation Capricornus, and in the sixty-six years that his movements have been followed he has passed through Aquarius, Pisces, Aries, Taurus, and is now (1912) in Gemini, very near Castor and Pollux. The time required for his circuit around the sun is nearly one hundred and sixty-five (164.6) years, so that he remains for about thirteen years in each constellation. He will complete one sidereal period, dating from the time of his discovery, in the year 2011.

The apparent motion of Neptune is direct a little more than six months in the year, and retrograde a little more than five months, so that it seems to present the old mental arithmetic problem of the climber that fell back so much every time after he had climbed a certain number of feet. But the falling back in the case of Neptune is an illusion, as we know. He keeps straight on in his journey, as we may see if we watch him from year to year, and his change of position is so slight during any year that the change of direction is hardly noticeable.

Neptune is as bright as an eighth-magnitude star, and it is possible to see him with a good field-glass. The difficulty is in distinguishing him from a star, for his disc does not show except through a telescope. If one has such a glass, however, it will be worth while to direct it toward that part of the ecliptic just under Castor and Pollux any time within the next two or three years, and a sight of this yet strange brother planet may be the reward. He will be in opposition on January 14, 1913, and thereafter about two days later each year.