In his next paper, however, read January 11, 1781, a subject is approached which shows a different kind of thought. It is the first obvious proof of the truth of the statement which he made long afterwards (1811), when he said: "A knowledge of the construction of the heavens has always been the ultimate object of my observations."
The title of this paper was _Astronomical Observations on the Rotation of the Planets round their Axes, made with a view to determine whether the Earth's diurnal motion is perfectly equable_. Here the question is a difficult and a remote one, and the method adopted for its solution is perfectly suitable in principle. It marks a step onward from mere observations to philosophizing upon their results. In practical astronomy, too, we note an advance. Not only are his results given, but also careful estimates of the errors to be feared in them, and a discussion of the sources of such errors. The same volume of the _Philosophical Transactions_ which contains this paper, also contains another, _Account of a Comet_, read April 26, 1781. This comet was the major planet _Uranus_, or, as HERSCHEL named it, _Georgium Sidus_. He had found it on the night of Tuesday, March 13, 1781. "In examining the small stars in the neighborhood of H _Geminorum_, I perceived one that appeared visibly larger than the rest; being struck with its uncommon appearance, I compared it to H _Geminorum_ and the small star in the quartile between _Auriga_ and _Gemini_, and finding it so much larger than either of them, I suspected it to be a comet." The "comet" was observed over all Europe. Its orbit was computed by various astronomers, and its distance from the sun was found to be nineteen times that of our earth. This was no comet, but a new major planet. The discovery of the amateur astronomer of Bath was the most striking since the invention of the telescope. It had absolutely no parallel, for every other major planet had been known from time immemorial.[13]
The effect of the discoveries of GALILEO was felt almost more in the moral than in the scientific world. The mystic number of the planets was broken up by the introduction of four satellites to _Jupiter_. That _Venus_ emulated the phases of our moon, overthrew superstition and seated the Copernican theory firmly. The discovery of "an innumerable multitude of fixed stars" in the Milky Way confounded the received ideas. This was the great mission of the telescope in GALILEO'S hands.