CHAPTER XXII
CELESTIAL PHENOMENA SEEN FROM MARS--M'ALLISTER RECEIVES A PRACTICALLESSON IN GRAVITATION
Mars is really an ideal world for an astronomer to live in, its skiesbeing so clear, the air so thin and pure, and the stars shining sobrilliantly.
Besides these advantages, the rapid movements of the two satellites ofthe planet result in a constant succession of celestial phenomena whichafford very frequent opportunities for most interesting observations.Changes in the phases of the two moons, eclipses, occultations,transits, &c., are constantly occurring, so there is nearly alwayssomething to attract our attention to the Martian sky.
We have already seen several of these phenomena, and I will now describewhat we have observed.
Early one evening when we were out with Merna, we looked up at the skyand saw the two moons a considerable distance apart, but approachingeach other from opposite directions, Phobos appearing to move veryrapidly. Both were near the full phase, Deimos being more nearly fullthan Phobos; and we watched them drawing closer and closer together tillPhobos passed right in front of Deimos so as to hide it entirely. Thisis termed an occultation; and both the satellites had become full whenthe occultation occurred; but when they were again clear of each otherboth were beginning to wane.
This sight may be seen anywhere near the Martian equator about every tenhours.
The movements of Phobos seemed very peculiar to us who had lived uponthe earth and seen all the celestial bodies appearing to move in thesame direction.
I have already alluded to the fact that Phobos is only 3700 miles abovethe surface of Mars, and moves so rapidly that it makes more than threecomplete revolutions round the planet whilst the latter is turning onlyonce on its axis.
The effect of this very rapid revolution of the satellite, which has nocounterpart, so far as we know, in our Solar system is that, instead ofrising in the east and setting in the west as all the other heavenlybodies appear to do, Phobos appears to rise in the west, cross the sky,and set in the east.
The moon and planets all actually move from west to east; the apparentreverse of this being caused by the more rapid movement of the earth onits axis, giving the other bodies the appearance of moving from east towest. If, however, our moon is closely watched, and its position withregard to a fixed star carefully noted, it will be found that in thecourse of a short time its real movement has been eastward, and that itsposition with regard to the fixed star has changed, although therevolution of the earth has appeared to carry both westward.
Phobos is 36 miles in diameter. Its actual period of revolution roundthe planet is 7 hours and 39 minutes, but, owing to the movement of Marson its axis in the same direction, it appears to take a few minutesover 11 hours to complete one revolution.
Near the equator, Phobos is seen above the horizon for about 4-1/4hours, and is below it about 6-3/4 hours. According as the place fromwhich it is viewed is farther from the equator so will the time ofvisibility of Phobos be decreased, until when latitude 69 deg. is reachedin either hemisphere, it will cease to become visible at all. This isowing to its nearness to the planet; and, Mars being small, the curveof its sphere is sharp, so that the horizon is more limited than on theearth, and the satellite is shut out from view anywhere above latitude69 deg. by the body of the planet.
Another peculiarity is that, when in the zenith, Phobos appears twice aslarge in area as it does when near the horizon, and notwithstanding itsvery small size, Phobos appears rather larger than our moon, because itis so near to the planet.
The length of the Martian "night" is about 12 hours and 20 minutes, andduring this very short time Phobos may be seen to rise in the west, setin the east, and rise again once more in the west. Consequently it willbe evident that it must travel very rapidly across the sky. It reallymoves over a space of 32-1/2 deg. in a single hour--a great contrast tothe slow and stately movement of our moon, which only passes over half adegree in an hour.
Moreover, Phobos may be seen to rise as a new moon, pass through itsphases to the full, wane, and again become new, all in the course of asingle Martian night; or it may be seen twice full and once new duringthe same time.
Even this does not exhaust the list of phenomena, for, being so closeto Mars, Phobos is very frequently eclipsed by the shadow of the planet.On the other hand, the sun may be eclipsed by Phobos something likefourteen hundred times in the course of a Martian year; and, as alreadymentioned, the other satellite is often occulted by Phobos--sometimeswhen both may be only at the half full phase, and these occultationslook very peculiar.
Deimos, being only 10 miles in diameter and about 12,500 miles from thesurface of the planet, does not give rise to so many phenomena as thenearer satellite: still they are very numerous.
It revolves round the planet in 30-1/4 hours, but appears to take131-1/2 hours to do so, being above the horizon about 60 hours, andbelow it nearly 72 hours. These are the times as seen from the equator;but, as in the case of Phobos, the farther the place is from the equatorthe shorter is the period that Deimos is seen above the horizon, until,when latitude 82 deg. is reached in either hemisphere, it ceases tobecome visible at all.
Our moon, being so very much more distant from our earth, could be seenfrom both the poles.
Deimos also passes nearly twice through all its phases whilst it isabove the horizon, viz. during about 60 hours, and may be seen twicefull and twice new in that time.
Eclipses of Deimos by the planet and occultations of it by the othersatellite are very frequent. Being so small, it can never cause aneclipse of the sun, but it transits the sun as a dark spot about onehundred and twenty times during the Martian year.
This is really a very inadequate list of the phenomena connected withthe satellites, but it will be seen that the number is enormouscompared with the few eclipses of the sun or moon seen on the earthduring the course of one year. Certainly Mars is an astronomer's world!
Merna heard my statements respecting these movements and phenomena as Iexplained them to my two friends; and when I had finished, he remarked,"You seem to be fairly well posted in these matters, sir?"
"Yes," I said; "thanks to our astronomers, both professional andamateur, all these things have been very carefully calculated; and, withthe exception of a few doubtful points, we probably know nearly as muchabout them as the Martians themselves do."
M'Allister then turned to me and said, "Professor, you told us that thetwo satellites of Mars revolved round the planet in a certain time, butin each case you afterwards said they appeared to take a much longertime to do so. I'm rather puzzled to understand how that can be."
"It's really a simple matter, M'Allister," I answered, "and I think Ican make it clear to you. While the satellite is making one revolutionround the planet the latter is turning on its axis in the same directionas the satellite is moving, following it up in fact; and you will Ithink understand that in these circumstances the people on that part ofthe planet where the moon is visible must necessarily keep it in viewfor a longer period than would be the case if the planet were notrevolving in the same direction.
"You have been used to being on board a ship; so suppose your vessel wassteaming twelve miles an hour and there was another vessel at anchorjust twelve miles ahead of you, you would reach it in just one hour,would you not?"
"Yes, certainly I should," replied M'Allister.
"Now," I continued, "suppose that the other vessel, instead of being atrest, was moving away from you at the rate of six miles an hour; afteryou had steamed one hour it would still be six miles ahead of you, andit would take you exactly another hour to catch it up. So you would bejust double the time reaching it when moving as compared with the timerequired to do so when it was at anchor. This is very similar to thecases of the satellites of Mars, and much the same thing happens inregard to Mars and the earth. If they are opposite to each other at acertain point, Mars will have taken much more than one revolution roundits orbit before they will be opposite to each other again, because
theyare both moving in the same direction. Do you see it now?" I asked.
"Yes, Professor," he replied. "I know now, because you have cleared itall up. It's simple enough when one understands it."
Merna then asked me if I would like to see some of their astronomicalinstruments, and, on my replying that I should very much like to do so,he took us to an observatory where Corontus was at work.
I was at once struck by the small size of the telescopes; and, oninquiring about them, Corontus told me that very large instruments hadlong become obsolete, for these small ones could be used for all thepurposes for which a large one had been required, and gave betterresults.
I examined one of them and found, to my surprise, that it embodied thevery ideas that I had long been trying to carry into effect. With thisview I had made many experiments, as it seemed to me that it ought to bepossible to construct an instrument of moderate and convenientdimensions which would show as much as our monsters will show, and yetbe capable of being used with low powers when occasion required. I hadendeavoured to attain this result by the aid of electricity, but failedto do so. Evidently I had missed something, but here was the thingitself in successful working, as I found upon testing it.
On looking at some drawings of Saturn, which were hanging up in theobservatory, I noticed that this planet was depicted with two faintouter rings which do not appear on our drawings of the planet. One ofthese rings has, however, been discovered by M. Jarry-Desloges, but theoutermost ring is still unknown to our observers. This ring is a verybroad one, its particles being widely scattered, hence its extremefaintness.
The Martians have also discovered two planets far beyond the orbit ofNeptune, and their knowledge of the other planets and also of the sunand the stars is far ahead of ours.
I was also shown a comet which had recently become visible through theirtelescopes, and found from its position that it was undoubtedly Halley'scomet, for which our astronomers were so eagerly watching. I wonderedwhether any of them had been fortunate enough to discover it early inAugust, as the Martian astronomers did. Its last appearance was in theyear 1835.
John remarked that "He thought Halley's comet might be termed 'Britain'sComet,' for several of its appearances had coincided with theoccurrence of very important events and turning-points in our nationalhistory, such as the Battle of Hastings, the Reformation, &c.," and headded, "as it will be a conspicuous object in our skies in 1910, Iwonder whether any important event will occur in our country? In 1835,when it last appeared, we had a political crisis!"
"Well, John," I replied, "I do not attach much importance to comets asaffecting mundane affairs; we have got rather beyond such beliefs asthat. Besides, when we left England early in August things were going onall right in our political world, and there was no indication of anyserious crisis."
"Still," said John, "it would be rather curious if we did have a crisisnext year; and I should not be surprised!"
As we were walking home next day, M'Allister suddenly tripped over somelittle projection and fell prone to the ground. John ran to hisassistance and raised him up, at the same time asking "If he were hurt?"
"No, not at all," said M'Allister; "I seemed to fall so lightly that Iscarcely felt it when I touched the ground."
"Ah, M'Allister!" I exclaimed, "if you had fallen like that upon ourearth, I think you would not have come off quite scatheless. You see,upon Mars the gravitation is much less than on the earth, being onlythree-eighths of what it is there, so one does not fall so swiftly, norso heavily, as on the earth.
"You can prove that very easily. Just take up a stone and hold it outhigher than your head, and let it fall; at the same time note, by thesecond hand of your watch, how long it takes for the stone to reach theground."
He did so, and said that "As near as he could tell, the stone was justabout one second of time in passing from his hand to the ground."
"Just so," I replied. "On Mars a falling body only moves through a spaceof about six feet in the first second of time. On the earth, however,the gravitation is so much greater that a falling body passes through aspace of a little over sixteen feet during the first second.
"In addition to that, although you weighed twelve stones when on theearth, you only weigh about four and a half stones here upon Mars. Nowyou can understand why it was you seemed to fall so lightly."
"Yes, Professor," he replied, "and I'm glad I fell here, and not uponthe earth!"
Then, picking up the stone again and throwing it high in the air, hewatched its fall, and turning to me, remarked, "Professor, you werequite right; that stone seemed to be quite a long time coming downagain, much longer than it would have been on our own world."
"Well, M'Allister," I replied, "now you know for certain that upon asmall planet gravitation really is much less than upon a larger planetof the same kind.
"That's another little wrinkle for you, and you have found it all outthrough tripping over a stone!"
"Losh, mon," replied he, "I seem to have learnt something almost everyday since I have been here; even a tumble down teaches me something!"
I then drew his attention to the birds flying near us, and pointed outthat they had a much wider spread of wing than our birds have, and thatthis was owing to the fact that the air being so thin a wide spread ofwing was absolutely necessary to support them in the air and enable themto fly. I further explained that, if the gravitation upon Mars were asgreat as upon the earth, the birds' wings must necessarily have beenstill larger, as the pull of the planet would have been so much thegreater, and would thus have prevented the birds from flying at all insuch thin air if their wings had been small.
"M'Allister," I then remarked, "you will, no doubt, have noticed thesame thing with regard to those large and beautiful butterflies we haveseen. Why, the outspread wings of the largest must have measured ten ortwelve inches across, and many of the smaller varieties were more thansix inches across. I wonder what our naturalists would say if they couldsee some specimens of these large and splendidly coloured insects!"
"Well, Professor," he answered, "I never saw such large butterfliesanywhere else, not even when I was in the tropics on our own world. Ithad never occurred to me that gravitation, or even the density of theair, had anything to do with their size. Even now I do not understandhow it is the small insects are able to fly, for they are heavy fortheir size, and do not possess very large wings, yet they can move veryswiftly."
"Let me explain then," I answered. "Large birds can only move theirwings with comparative slowness, and it is therefore necessary thattheir wings should be large to enable them to keep their balance and beable to fly. Their wings are somewhat in the nature of aeroplanes, andthey shift them to different angles to take advantage of the varyingcurrents of air.
"In the case of humming-birds and small insects, the wings are capableof intensely rapid vibrations, so rapid indeed that, when flying, thewings are almost, if not quite, invisible. This intensely rapid movementenables them to fly, and is somewhat analogous to the rapid movements ofthe vertical spiral screws, which you have seen on some of the Martianair-ships that screw their way up into the air.
"Such rapid movements would not be suited to larger creatures, becausetheir muscular powers would have to be so enormously great that theirbodies would require to be larger and heavier in proportion. They wouldthus be very unwieldy."