Sir William Herschel: His Life and Works

by Edward S. Holden

In his paper of 1792, Herschel shows that the most distant satellite of Saturn—Japetus—turns once on its axis in each revolution about its primary, just as our moon does. He says of this:

"I cannot help reflecting with some pleasure on the discovery of an analogy which shows that a certain uniform plan is carried on among the secondary planets of our solar system; and we may conjecture that probably most of the satellites are governed by the same law; especially if it be founded on such a construction of their figure as makes them more ponderous towards their primary planets."

I believe the last suggestion to have been the first statement of the possible arrangement of matter in satellites, which was afterwards so forcibly maintained by Hansen in his theory of the moon. Hansen's researches show the consequences of such an arrangement, although they do not prove its existence.

It should be recorded that the explanation which is to-day received of the belts and bands upon Jupiter, is, I believe, first found in Herschel's memoir on Venus (1793). His memoir of 1797, on the changeable brightness of the satellites of Jupiter, has already been referred to. The times of the rotation of the satellites on their axes was first determined by Herschel from these observations, which also contain accounts of the curious, and as yet unexplained, phenomena attending their appearances on the disc of the planet.

Herschel discovered in January, 1787, the two brighter satellites of Uranus, now called Oberon and Titania. They are among the faintest objects in the solar system. A later discussion of all his observations led him to the belief that there were four more, and he gives his observations and computations in full. He says that of the existence of additional satellites he has no doubt. Of these four, three were exterior to the most distant satellite Oberon, the other was "interior" to Titania.

It was not until 1834 that even Oberon and Titania were again observed (by Sir John Herschel) with a telescope of twenty feet, similar to that which had discovered them, and not until 1847 was the true state of this system known, when Mr. Lassell discovered Ariel and Umbriel, two satellites interior to Titania, neither of which was Herschel's "interior" satellite. In 1848 and later years Mr. Lassell, by the aid of telescopes constructed by himself, fully settled the fact that only four satellites of this planet existed. In 1874 I examined the observations of Herschel on his supposed "interior" satellite, thinking that it might be possible that among the very few glimpses of it which he recorded, some might have belonged to Ariel and some to Umbriel, and that by combining rare and almost accidental observations of two satellites which really existed, he had come to announce the existence of an "interior" satellite which had no existence in fact. Such I believe to be the case. In 1801, April 17, Herschel describes an interior satellite in the position angle 189°, distant 18″ from the planet. At that instant Umbriel, one of Mr. Lassell's satellites, was in the position 191°, and distant 21″ from Uranus, in the most favorable position for seeing it. The observation of 1794, March 27, may belong to Ariel. At the best the investigation is of passing interest only, and has nothing to do with the question of the discovery of the satellites. Herschel discovered the two brighter ones, and it was only sixty years later that they were properly re-observed by Mr. Lassell, who has the great honor of having added as many more, and who first settled the vexed question of satellites exterior to Oberon, and this with a reflecting telescope made by himself, which is unequalled by any other of its dimensions.

Researches on the Nature of the Sun.

In the introduction to his paper on the Nature and Construction of the Sun and Fixed Stars (1795), Herschel recounts what was known of the nature of the sun at that time. Newton had shown that it was the centre of the system; Galileo and his successors had determined its rotation, the place of its equator, its real diameter, magnitude, density, distance, and the force of gravity on its surface. He says:

"I should not wonder if, considering all this, we were induced to think that nothing remained to be added; and yet we are still very ignorant in regard to the internal construction of the sun." "The [Pg 146] spots have been supposed to be solid bodies, the smoke of volcanoes, the scum floating on an ocean of fluid matter, clouds, opaque masses, and to be many other things." "The sun itself has been called a globe of fire, though, perhaps, metaphorically." "It is time now to profit by the observations we are in possession of. I have availed myself of the labors of preceding astronomers, but have been induced thereto by my own actual observation of the solar phenomena."