THE PLANET MARS.

253

Mars turns upon its axis, as we have already stated, in 24hours 37 minutes 22f seconds. The nearer of the two satellitesgoes round it in 7 hours 39 minutes 14 seconds ; the outer in30 hours 17 minutes 54 seconds, the latter period being almostexactly four times the former;—-a fact which may some dayprove to be of special interest and importance. It followsfrom the above-mentioned statements, that the apparent motionof the outer one, as it would be seen by an observer upon Mars ,although it would be from east to west across the sky, wouldbe very slow. On the contrary, the inner one would possessthe most extraordinary property of appearing to cross the skyday by day from west to east ; i.e., in the reverse direction tothat pursued by all the other heavenly bodies.

Let us endeavour to explain why this would be the case.In order to do so we may recall the statement made inLecture III., page 56, with regard to the way in whichthe apparent daily circuit, which our own Moon wouldotherwise appear to perform from east to west once in every24 hours, is modified by its orbital motion round theEarth .

We showed that in 24 hours the Moon describes about ^tfiof its own path around the Earth from west to east; trom whichit results that its apparent daily motion across the sky fromeast to west is somewhat slower than it else would be, thevelocity which it would otherwise appear to have being dimi-nished by about ^t;h part. At the end of 24 hours after ithas risen on any given day, it does not rise again, but is ingeneral about -^th part of a whole revolution below the horizon.It therefore rises upon an average about ^j-th of 24 hours, orfrom 40 to 50 minutes later, on successive days.

But let us make another supposition, and imagine the Moon to go round the Earth in 48 hours. It would in that case goback just half as quickly as it would otherwise seem to goforward. If it rose at 6 p.m., and (apart from its own motionin its orbit) would appear to go across the sky so as to set at6 a.m., it would, instead of so doing, be seen only half-wayacross at that hour; in other words, it would be upon themeridian at 6 a.m.