+——————————————+————————————————————+———————+————————————————+ | | Mean distance. | | | | Number and +——————————+————————-+ Max. | Date of | | name. | Diameters| Miles. |Elonga-| Discovery. | | |of Uranus.| | tion. | | +——————————————+————————————————————+———————+————————————————+ | | | | ″ | | | 3rd. Ariel | 4·03 | 125,000 | 12 | 1847, Sept. 14.| | 4th. Umbriel | 5·61 | 174,000 | 15 | 1847, Oct. 8.| | 1st. Titania | 9·19 | 285,000 | 33 | 1787, Jan. 11.| | 2nd. Oberon | 12·32 | 382,000 | 44 | 1787, Jan. 11.| +——————————————+————————————————————+———————+————————————————+ ————————————-+——————————+ | | Discoverer. | d h m | | | | | ————————————-+——————————+ | | W. Lassell. | 2 12 29 | O. Struve. | 4 3 27 | W. Herschel.| 8 16 57 | 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 β^2 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. [Illustration: 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.)]