If there is any one in some degree friendly to Copernicus and the lights of the Samian philosophy, who finds this difficulty only, that he doubts how it can happen, supposing the earth to perform again and again her course among the planets through the ethereal plains, that the moon should keep so constantly by her side, like an inseparable companion, and at the same time fly round and round the actual orb of the earth, just like a faithful dog which goes round and round his master on some journey, now running in front, now deviating to this side or that, in ever-varying mazes, let him look at the planet Jupiter, which, as this telescope shows, certainly carries in its train not one such companion only, like the earth, as Copernicus showed, but actually four, that never leave it, though all the time hastening each in its own orbit. [21] The degree of accuracy attained by Kepler and Galileo with their imperfect instruments will be appreciated by comparing these statements with the determinations of later astronomers. Jupiter is about 1300 times the size of the Earth. Its diameter is about 87,000 miles; time of rotation, 9 h. 55 m. 21 sec.; time of revolution, 4333 days nearly. The angular diameter of the sun, seen from Jupiter, is between 6´ and 7´. The times of revolution of the four satellites are, as already given: (i.) 1 d. 18 h. 28 m., (ii.) 3 d. 13 h. 15 m., (iii.) 7 d. 3 h. 43 m., (iv.) 16 d. 16 h. 32 m. But enough has been said about these matters in my _Discussion with the Sidereal Messenger_. It is time that I should turn to those discoveries which have been made since the publication of Galileo’s _Sidereal Messenger_, and since my _Discussion_ with it, by means of this telescope. [Sidenote: Galileo’s discovery of Saturn’s ring (imperfectly).] It is now just a year since Galileo wrote to Prague, and gave full notice that he had detected something new in the heavens beyond his former discoveries; and that there might not be any one who, with the intention of detracting from his credit, should try to pass himself off as an earlier observer of the phenomenon, Galileo gave a certain space of time for the publication of the new phenomena which any one had seen; he himself meanwhile described his discovery in letters transposed in this manner: _s m a i s m r m i l m e p o e t a l e u m i b u n e n u g t t a u i r a s_. Out of these letters I made an uncouth verse which I inserted in my _Short Account_ in the month of September of last year:— Salve umbistineum[22] geminatum Martia proles. Hail, twin companionship, children of Mars. [22] _Umbistineum._ Apparently this is some German word with a Latin ending, such as _um-bei-stehn_; Kepler fancied that Galileo had discovered two satellites of Mars.