Astronomy for Everybody/Part 4/Chapter 9

So far as yet known, Neptune is the outermost planet of our solar system. In size and mass it is not very different from Uranus, but its greater distance, 30 astronomical units, instead of 19.2, makes it fainter and harder to see. It is far below the limit of visibility by the naked eye, but quite a moderate-sized telescope would show it if one could only distinguish it from the numerous stars of similar brightness that stud the heavens. This needs astronomical appliances of a more refined and complex sort.

The disk of Neptune is to be made out only with a telescope of considerable power. It is then seen to be of a bluish or leaden tint, perceptibly different from the sea-green of Uranus. Of course nothing can be known by direct observation about its rotation on its axis. Its spectrum shows bands like those of Uranus, and it seems likely that the two bodies are much alike in their constitution.

The discovery of Neptune in 1846 is regarded as one of the most remarkable triumphs of mathematical astronomy. Its existence was made known by its attraction on the planet Uranus before any other evidence had been brought out. The history of the circumstances leading to the discovery is so interesting that we shall briefly mention its main points.

During the first twenty years of the nineteenth century Bouvard, of Paris, an eminent mathematical astronomer, prepared new tables of the motions of Jupiter, Saturn, and Uranus, then supposed to be the three outermost planets. He took the deviations of these planets, produced by their attraction on each other, from the calculations of Laplace. He succeeded fairly well in fitting his tables to the observed motions of Jupiter and Saturn, but found that all his efforts to make tables that would agree with the observed positions of Uranus were fruitless. If he considered only the observations made since the discovery by Herschel, he could get along; but no agreement could be obtained with those made previously by Flamsteed and Lemonnier, when the planet was supposed to be a fixed star. So he rejected these old observations, fitted his orbit into the modern ones, and published his tables. But it was soon found that the planet began to move away from its calculated position, and astronomers began to wonder what was the matter. It was true that the deviation, measured by a naked eye standard, was very small; in fact, if there had been two planets, one in the real and one in the calculated position, the naked eye could not have distinguished them from a single star. But the telescope would have shown them well separated.

Thus the case stood until 1845. At that time there lived in Paris a young mathematical astronomer, Leverrier, who had already made a name in his science, having ​communicated to the Academy of Sciences some researches which gave Arago a very high opinion of his abilities. Arago called his attention to the case of Uranus and suggested that he should investigate the subject. The idea occurred to Leverrier that the deviations were probably caused by the attraction of an unknown planet outside of Uranus. He proceeded to calculate in what orbit a planet should move to produce them, and laid his result before the Academy of Sciences in the summer of 1846.

It happened that, before Leverrier commenced his work, an English student at the University of Cambridge, Mr. John C. Adams, had the same idea and set about the same work. He got the result even before Leverrier did, and communicated it to the Astronomer Royal. Both computers calculated the present position of the unknown planet, so that, were it possible to distinguish it from a fixed star, it would only have been necessary to search in the region indicated in order to find the planet. Unfortunately, however, Airy was incredulous as to the matter, and did not think the chance of finding the planet sufficient to go through the laborious operation of a search until his attention was attracted by the prediction of Leverrier, and the close agreement between the two computers was remarked.

The problem of finding the planet was now taken up. Very thorough observations were made upon the stars in the region by Professor Challis at the Cambridge Observatory. I must explain that, as it was not easy with the imperfect instruments of that time to distinguish so ​small a planet from the great number of fixed stars which studded the heavens around it, it was necessary to proceed by determining the position of as many stars as possible several times, in order that, by a comparison of the observations, it could be determined whether any of them had moved out of its place.

While Mr. Challis was engaged in this work it occurred to Leverrier that the astronomers of Berlin were mapping the heavens. He therefore wrote to Encke, the director of the Berlin Observatory, suggesting that they should look for the planet. Now it happened that the Berlin astronomers had just completed a map of that part of the sky in which the planet was located. So, on the very evening after the letter was received, they took the map to the telescope and proceeded to search about to see if any object was seen in the telescope which was not on the map. Such an object was very soon found, and, by comparing its position with that of the stars around it, it seemed to have a slight motion. But Encke was very cautious and waited for the discovery to be confirmed on the night following. Then it was found to have moved so much that no doubt could remain, and he wrote Leverrier that the planet actually existed.

When this news reached England, Professor Challis proceeded to examine his own observations, and found that he had actually observed the planet on two occasions. Unfortunately, however, he had not reduced and compared his observations, and so failed to recognise the object until after it had been seen at Berlin.

The question of the credit due to Adams gave rise to ​much controversy, Arago in France claiming that, in the history of the affair, the name of Adams should not even be mentioned—the whole credit should go to Leverrier. This he did on the principle that it was not the person who first did a thing, but he who first published it, who should receive the credit. But the English claimed that, as Adams had actually preceded Leverrier and, if he had not printed his paper, had at least communicated it to public authorities, and had enabled Challis to see, although not to recognise, the planet, he should get his due share of credit. The whole question thus raised was one of honour, and subsequent astronomers have taken the very proper course of honouring both men all they could for so wonderful a work.

Of course the newly found planet was observed by astronomers the world over. The result was that Mr. Lassell soon found that Neptune was accompanied by a satellite. This object was observed at the few observatories then possessing telescopes of sufficient power to make it out. Its time of revolution was found to be nearly six days.

The most curious feature of this satellite is that, contrary to the rule in the case of all the bodies of the solar system except Uranus, it moves from east toward west. In the case of Uranus we cannot consider the motion as being east or west, we should rather call it a north and south motion.

It would be very interesting to know whether the ​planet Neptune revolves on its axis in the same direction as the satellite moves. But this cannot be determined, because it is so distant and its disk so faint and diffuse that no markings can be detected upon it. Indeed, if we reflect that the rotation of a planet so near us as Venus has never been certainly determined, we may easily see how hopeless is the prospect of determining that of Neptune.

But, in spite of this, there is remarkable evidence that the planet has a rapid rotation. It is found that the orbit of the satellite is very slowly changing its position from year to year. During the half century since observations commenced, this change amounts to several degrees. The only way in which it can be accounted for is by supposing that Neptune, like the earth and the other rapidly rotating planets, is an oblate ellipsoid, and that the plane of the planet's equator does not coincide with that of the orbit of the satellite. In time the astronomer will be able to learn from this motion the position of the poles and equator of the planet Neptune, but this may require a century of observation, or even several centuries.