Page:Astronomy for Everybody.djvu/255

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.