It may be mentioned as an interesting point that some fifty years after the discovery of Uranus by Sir W. Herschel the planet was accidentally rediscovered by his son Sir John Herschel, who mentioned the fact as follows in a letter to Admiral Smyth, written on Aug. 8, 1830:—“I have just completed two 20-foot reflectors, and have got some interesting observations of the satellites of Uranus. The first sweep I made with my new mirror I rediscovered this planet by its disk, having blundered upon it by the merest accident for 19 Capricorni.” Had the father failed to detect this planet in 1781, the discovery might therefore have been made by the son half a century later.

Some spectroscopic observations of Uranus made in 1889 with Mr. Common’s 5-foot reflector, appear to show that the planet “is to a large extent self-luminous.” But Mr. Huggins on June 3 seems to have obtained a different result (see ‘Monthly Notices,’ xlix. p. 404 et seq.).

The Satellites.—For many years it was supposed that Uranus possessed six satellites, all of which were discovered by Sir W. Herschel, but later observations proved that four of these had no existence. They were small stars near the planet. But two of Herschel’s satellites were fully corroborated, and two new ones were discovered by Lassell and Struve. The number of known satellites attending Uranus is four, and it is probable that many others exist, though they are too minute to be distinguished in the most powerful instruments hitherto constructed. The following are the periods, distances, &c., of the known satellites:—

Number
and name.		Mean Distance.Max.
Elong-
ation.
″		Date of
Discovery.		Discoverer. d h m
Diameters
of Uranus.		Miles.
3rd. Ariel4·03125,000121847, Sept. 14.W. Lassell.2 12 29
4th. Umbriel5·61174,000151847, Oct. 8.O. Struve.4 3 27
1st. Titania9·19285,000331787, Jan. 11.W. Herschel.8 16 57
2nd. Oberon12·32382,000441787, Jan. 11.W. Herschel.13 11 7

Titania and Oberon are the two brightest satellites, but none of them can be seen except in large instruments. The two outer ones are said to have been glimpsed in a 4·3-inch refractor, but this feat is phenomenal, and certainly no criterion of ordinary capacity. Sir J. Herschel found them tolerably conspicuous in a reflector of 18 or 20 inches aperture, and mentioned a test-object by which observers might determine whether their telescopes were adequate to reveal them. This test is a minute double star lying between the stars β′ and β2 Capricorni. The magnitudes are 15 and 16, and distance 3″. Relatively to the satellites of Uranus this faint double is a “splendid object.”

From observations with large modern instruments it appears highly probable that the four known satellites, must be considerably larger than any others which may be revolving round the planet. A curious fact in connection with these satellites is that their motions are retrograde.

Fig. 44.

Apparent Orbits of the Satellites of Uranus, as seen in an Inverting Telescope.

(The small circle in the above diagram represents the planet and is on the same scale as the orbits. The arrows show the direction of the motion of the satellites, and the figures indicate the number of days from the time of the last North elongation.)