120 THE EVOLUTION OF SATELLITES. 



au interestiug account of what had been conjectured, partly in jest 

 and partly in earnest, as to the existence of satellites attending- that 

 planet. He quotes Kepler as writing, after the discovery of the satel- 

 lites of Jupiter, " I am so far from disbelieving the existence of the 

 four circumjovial planets" (that is, satellites) "that I long for a tele- 

 scope to anticipate you, if possible, in discovering two around Mars, 

 six or eight around Saturn, as the proportion seems to require, and 

 perhaps one "each around Mercury and Venus." This was, of course, 

 serious, although based on fantastic considerations. At a later date 

 Swift poured contempt on men of science in his account of the inhab- 

 itants of Laputa, whom he describes as dexterous enough on a piece 

 of paper and in the management of the rule, the pencil, and the 

 dividers, but as a clumsy, awkward, and unhandy people, and per- 

 l^lexed in their conceptions upon all subjects except mathematics and 

 music. He writes, however, of the Laputans, "They have likewise 

 discovered two lesser stars or satellites which revolve about Mars, 

 whereof the innermost is distant from the center of the primary 

 exactly three of his diameters, and the outermost five." In one of his 

 satires Voltaire also represents au imaginary traveler from Sirius as 

 making a similar discovery. 



These curious prognostications were at length verified by Prof. 

 Asaph Hall in the discovery of two satellites, which he named Phobos 

 and Deimos — Fear and Panic, the dogs of war. The period of Deimos 

 is about thirty hours, and that of Phobos about eight hours, while the 

 Martian day is of nearly the same length as our own. The month of 

 the inner minute satellite is thus less than a third of the planet's day; 

 it rises to the Martians in the west, and i^asses through all its phases 

 in a few hours. Sometimes it must even rise twice in a single Martian 

 night. As we here find au illustration of the condition foreseen for 

 our own planet and satellite, it seems legitimate to suppose that solar 

 tidal friction has slowed down the planet's rotation. The ultimate fate 

 of Phobos must almost certainly be absori)tion by the planet. 



Several of the satellites of Jupiter and Saturn present faint inequal- 

 ities of coloring, and telescopic examination has led astronomers to 

 believe that they always present the same face to their planets. The 

 theory of tidal friction would certainly lead us to expect that these 

 enormous j)lanets would have worked out the same result for these 

 relatively small satellites that the earth has effected in the moon. 



The efficiency of solar tidal friction must be far greater in its action 

 on the planets Mercury and Venus than on the earth. The determina- 

 tion of the periods of rotation of these planets thus becomes a matter of 

 much interest. But the markings on their disks are so obscure that 

 their rates of rotation have remained under discussion for many years. 

 Until recently the prevailing opinion was that in each case the day 

 was of nearly the same length as our own; but a few years ago Schiap- 

 arelli, of Milan, an observer endowed with extraordinary acuteness of 



