There are other interesting glimpses to be had of an older world in the moon than that whose scarred face is now beautified for us by distance.
Not far from Theophilus and the other great crater-mountains just described, at the upper, or southern, end of the level expanse called the "Sea of Nectar," is a broad, semicircular bay whose sh.o.r.es are formed by the walls of a partially destroyed crater named Fracastorius.
It is evident that this bay, and the larger part of the "Sea of Nectar,"
have been created by an outwelling of liquid lavas, which formed a smooth floor over a portion of the pre-existing surface of the moon, and broke down and submerged a large part of the mountain ring of Fracastorius, leaving the more ancient walls standing at the southern end, while, outlined by depressions and corrugations in the rocky blanket, are certain half-defined forms belonging to the buried world beneath.
Near Copernicus, some years ago, as Dr. Edward S. Holden pointed out, photographs made with the great Lick telescope, then under his direction, showed, in skeleton outline, a huge ring buried beneath some vast outflow of molten matter and undiscerned by telescopic observers.
And Mr. Elger, who was a most industrious observer and careful interpreter of lunar scenery, speaks of "the undoubted existence of the relics of an earlier lunar world beneath the smooth superficies of the _maria_."
Although, as already remarked, it seems necessary to a.s.sume that any life existing in the moon prior to its great volcanic outburst must have ceased at that time, yet the possibility may be admitted that life could reappear upon the moon after its surface had again become quiet and comparatively undisturbed. Germs of the earlier life might have survived, despite the terrible nature of the catastrophe. But the conditions on the moon at present are such that even the most confident advocates of the view that the lunar world is not entirely dead do not venture to a.s.sume that anything beyond the lowest and simplest organic forms--mainly, if not wholly, in the shape of vegetation--can exist there. The impression that even such life is possible rests upon the acc.u.mulating evidence of the existence of a lunar atmosphere, and of visible changes, some apparently of a volcanic character and some not, on the moon"s surface.
Prof. William H. Pickering, who is, perhaps, more familiar with the telescopic and photographic aspects of the moon than any other American astronomer, has recorded numberless instances of change in minute details of the lunar landscapes. He regards some of his observations made at Arequipa as "pointing very strongly to the existence of vegetation upon the surface of the moon in large quant.i.ties at the present time." The mountain-ringed valley of Plato is one of the places in the lunar world where the visible changes have been most frequently observed, and more than one student of the moon has reached the conclusion that something very like the appearances that vegetation would produce is to be seen in that valley.
Professor Pickering has thoroughly discussed the observations relating to a celebrated crater named Linne in the _Mare Serenitatis_, and after reading his description of its changes of appearance one can hardly reject his conclusion that Linne is an active volcanic vent, but variable in its manifestations. This is only one of a number of similar instances among the smaller craters of the moon. The giant ones are evidently entirely extinct, but some of the minor vents give occasional signs of activity. Nor should it be a.s.sumed that these relatively slight manifestations of volcanic action are really insignificant. As Professor Pickering shows, they may be regarded as comparable with the greatest volcanic phenomena now witnessed on the earth, and, speaking again of Plato, he says of its evidences of volcanic action:
"It is, I believe, more active than any area of similar size upon the earth. There seems to be no evidences of lava, but the white streaks indicate apparently something a.n.a.logous to snow or clouds. There must be a certain escape of gases, presumably steam and carbonic acid, the former of which, probably, aids in the production of the white markings."[19]
[Footnote 19: Annals of Harvard College Observatory, vol. x.x.xii, part ii, 1900.]
To Professor Pickering we owe the suggestion that the wonderful rays emanating from Tycho consist of some whitish substance blown by the wind, not from Tycho itself, but from lines of little volcanic vents or craters lying along the course of the rays. This substance may be volcanic powder or snow, in the form of minute ice crystals. Mr. Elger remarks of this theory that the "confused network of streaks" around Copernicus seems to respond to it more happily than the rays of Tycho do, because of the lack of definiteness of direction so manifest in the case of the rays.
As an encouragement to amateur observers who may be disposed to find out for themselves whether or not changes now take place in the moon, the following sentence from the introduction to Professor Pickering"s chapter on Plato in the Harvard Observatory Annals, volume x.x.xii, will prove useful and interesting:
"In reviewing the history of selenography, one must be impressed by the singular fact that, while most of the astronomers who have made a special study of the moon, such as Schroeter, Maedler, Schmidt, Webb, Neison, and Elger, have all believed that its surface was still subject to changes readily visible from the earth, the great majority of astronomers who have paid little attention to the subject have quite as strenuously denied the existence of such changes."
In regard to the lunar atmosphere, it may be said, in a word, that even those who advocate the existence of vegetation and of clouds of dust or ice crystals on the moon do not predicate any greater amount, or greater density, of atmosphere than do those who consider the moon to be wholly dead and inert. Professor Pickering himself showed, from his observations, that the horizontal refraction of the lunar atmosphere, instead of being less than 2"", as formerly stated, was less than 0.4"".
Yet he found visual evidence that on the sunlit side of the moon this rare atmosphere was filled to a height of four miles with some absorbing medium which was absent on the dark side, and which was apparently an emanation from the lunar crust, occurring after sunrise. And Messrs.
Loewy and Puiseux, of the Paris Observatory, say, after showing reasons for thinking that the great volcanic eruptions belong to a recent period in the history of the moon, that "the diffusion of cinders to great distances infers a gaseous envelope of a certain density.... The resistance of the atmosphere must have been sufficient to r.e.t.a.r.d the fall of this dust [the reference is to the white trails, like those from Tycho], during its transport over a distance of more than 1,000 kilometers [620 miles]."[20]
[Footnote 20: Comptes Rendus, June 23, July 3, 1899.]
We come now to a brief consideration of certain peculiarities in the motions of the moon, and in the phenomena of day and night on its surface. The moon keeps the same side forever turned toward the earth, behaving, in this respect, as Mercury does with regard to the sun. The consequence is that the lunar globe makes but one rotation on its axis in the course of a month, or in the course of one revolution about the earth. Some of the results of this practical ident.i.ty of the periods of rotation and revolution are ill.u.s.trated in the diagram on page 250. The moon really undergoes considerable libration, recalling the libration of Mercury, which was explained in the chapter on that planet, and in consequence we are able to see a little way round into the opposite lunar hemisphere, now on this side and now on the other, but in the diagram this libration has been neglected. If it had been represented we should have found that, instead of only one half, about three fifths of the total superficies of the moon are visible from the earth at one time or another.
[Ill.u.s.tration: PHASES AND ROTATION OF THE MOON.]
Perhaps it should be remarked that in drawing the moon"s...o...b..t about the earth as a center we offer no contradiction to what was shown earlier in this chapter. The moon does travel around the earth, and its...o...b..t about our globe may, for our present purpose, be treated independently of its motion about the sun. Let the central globe, then, represent the earth, and let the sun be supposed to shine from the left-hand side of the diagram. A little cross is erected at a fixed spot on the globe of the moon.
At _A_ the moon is between the earth and the sun, or in the phase of new moon. The lunar hemisphere facing the earth is now buried in night, except so far as the light reflected from the earth illuminates it, and this illumination, it is interesting to remember, is about fourteen times as great--reckoned by the relative areas of the reflecting surfaces--as that which the full moon sends to the earth. An inhabitant of the moon, standing beside the cross, sees the earth in the form of a huge full moon directly above his head, but, as far as the sun is concerned, it is midnight for him.
In the course of about seven days the moon travels to _B_. In the meantime it has turned one quarter of the way around its axis, and the spot marked by the cross is still directly under the earth. For the lunar inhabitant standing on that spot the sun is now on the point of rising, and he sees the earth no longer in the shape of a full moon, but in that of a half-moon. The lunar globe itself appears, at the same time from the earth, as a half-moon, being in the position or phase that we call first quarter.
Seven more days elapse, and the moon arrives at _C_, opposite to the position of the sun, and with the earth between it and the solar orb. It is now high noon for our lunarian standing beside the cross, while the earth over his head appears, if he sees it at all, only as a black disk close to the sun, or--as would sometimes be the case--covering the sun, and encircled with a beautiful ring of light produced by the refraction of its atmosphere. (Recall the similar phenomenon in the case of Venus.) The moon seen from the earth is now in the phase called full moon.
Another lapse of seven days, and the moon is at _D_, in the phase called third quarter, while the earth, viewed from the cross on the moon, which is still pointed directly at it, appears again in the shape of a huge half-moon.
During the next seven days the moon returns to its original position at _A_, and becomes once more new moon, with "full earth" shining upon it.
Now it is evident that in consequence of the peculiar law of the moon"s rotation its days and nights are each about two of our weeks, or fourteen days, in length. That hemisphere of the moon which is in the full sunlight at _A_, for instance, is buried in the middle of night at _C_. The result is different than in the case of Mercury, because the body toward which the moon always keeps the same face directed is not the luminous sun, but the non-luminous earth.
It is believed that the moon acquired this manner of rotation in consequence of the tidal friction exercised upon it by the earth. The tidal attraction of the earth exceeds that of the sun upon the moon because the earth is so much nearer than the sun is, and tidal attraction varies inversely as the cube of the distance. In fact, the braking effect of tidal friction varies inversely as the sixth power of the distance, so that the ability of the earth to stop the rotation of the moon on its axis is immensely greater than that of the sun. This power was effectively applied while the moon was yet a molten ma.s.s, so that it is probable that the moon has rotated just as it does now for millions of years.
As was remarked a little while ago, the moon traveling in an elliptical orbit about the earth has a libratory movement which, if represented in our picture, would cause the cross to swing now a little one way and now a little the other, and thus produce an apparent pendulum motion of the earth in the sky, similar to that of the sun as seen from Mercury. But it is not necessary to go into the details of this phenomenon. The reader, if he chooses, can deduce them for himself.
But we may inquire a little into the effects of the long days and nights of the moon. In consequence of the extreme rarity of the lunar atmosphere, it is believed that the heat of the sun falling upon it during a day two weeks in length, is radiated away so rapidly that the surface of the lunar rocks never rises above the freezing temperature of water. On the night side, with no warm atmospheric blanket such as the earth enjoys, the temperature may fall far toward absolute zero, the most merciful figure that has been suggested for it being 200 below the zero of our ordinary thermometers! But there is much uncertainty about the actual temperature on the moon, and different experiments, in the attempt to make a direct measurement of it, have yielded discordant results. At one time, for instance, Lord Rosse believed he had demonstrated that at lunar noon the temperature of the rocks rose above the boiling-point of water. But afterward he changed his mind and favored the theory of a low temperature.
In this and in other respects much remains to be discovered concerning our interesting satellite, and there is plenty of room, and an abundance of original occupation, for new observers of the lunar world.
CHAPTER IX
HOW TO FIND THE PLANETS
There is no reason why everybody should not know the princ.i.p.al planets at sight nearly as well as everybody knows the moon. It only requires a little intelligent application to become acquainted with the other worlds that have been discussed in the foregoing chapters, and to be able to follow their courses through the sky and recognize them wherever they appear. No telescope, or any other instrument whatever, is required for the purpose. There is but one preliminary requirement, just as every branch of human knowledge presupposes its A B C. This is an acquaintance with the constellations and the princ.i.p.al stars--not a difficult thing to obtain.
Almost everybody knows the "Great Dipper" from childhood"s days, except, perhaps, those who have had the misfortune to spend their youth under the glare of city lights. Some know Orion when he shines gloriously in the winter heavens. Many are able to point out the north star, or pole star, as everybody should be able to do. All this forms a good beginning, and may serve as the basis for the rapid acquirement of a general knowledge of the geography of the heavens.
If you are fortunate enough to number an astronomer among your acquaintance--an amateur will do as well as a professor--you may, with his aid, make a short cut to a knowledge of the stars. Otherwise you must depend upon books and charts. My Astronomy with an Opera-Gla.s.s was prepared for this very purpose. For simply learning the constellations and the chief stars you need no opera-gla.s.s or other instrument. With the aid of the charts, familiarize yourself with the appearance of the constellations by noticing the characteristic arrangements of their chief stars. You need pay no attention to any except the bright stars, and those that are conspicuous enough to thrust themselves upon your attention.
Learn by observation at what seasons particular constellations are on, or near, the meridian--i.e., the north and south line through the middle of the heavens. Make yourself especially familiar with the so-called zodiacal constellations, which are, in their order, running around the heavens from west to east: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, and Pisces. The importance of these particular constellations arises from the fact that it is across them that the tracks of the planets lie, and when you are familiar with the fixed stars belonging to them you will be able immediately to recognize a stranger appearing among them, and will correctly conclude that it is one of the planets.[21] How to tell which planet it may be, it is the object of this chapter to show you. As an indispensable aid--unless you happen already to possess a complete star atlas on a larger scale--I have drawn the six charts of the zodiacal constellations and their neighbors that are included in this chapter.
[Footnote 21: In our lat.i.tudes, planets are never seen in the northern quarter of the sky. When on the meridian, they are always somewhere between the zenith and the southern horizon.]
[Ill.u.s.tration: CHART NO. 1.--FROM RIGHT ASCENSION 0 HOURS TO 4 HOURS; DECLINATION 30 NORTH TO 10 SOUTH.]
Having learned to recognize the constellations and their chief stars on sight, one other step, an extremely easy one, remains to be taken before beginning your search for the planets--buy the American Ephemeris and Nautical Almanac for the current year. It is published under the direction of the United States Naval Observatory at Washington, and can be purchased for one dollar.
This book, which may appear to you rather bulky and formidable for an almanac, contains hundreds of pages and scores of tables to which you need pay no attention. They are for navigators and astronomers, and are much more innocent than they look. The plain citizen, seeking only an introduction to the planets, can return their stare and pa.s.s by, without feeling in the least humiliated.
[Ill.u.s.tration: CHART NO. 2.--FROM RIGHT ASCENSION 4 HOURS TO 8 HOURS; DECLINATION 30 NORTH TO 10 SOUTH.]
In the front part of the book, after the long calendar, and the tables relating to the sun and the moon, will be found about thirty pages of tables headed, in large black letters, with the names of the planets--Mercury, Venus, Mars, Jupiter, Saturn, etc. Two months are represented on each page, and opposite the number of each successive day of the month the position of the planet is given in hours, minutes, and seconds of right ascension, and degrees, minutes, and seconds of north and south declination, the sign + meaning north, and the sign - south.
Do not trouble yourself with the seconds in either column, and take the minutes only when the number is large. The hours of right ascension and the degrees of declination are the main things to be noticed.
Right ascension, by the way, expresses the distance of a celestial body, such as a star or a planet, east of the vernal equinox, or the first point of Aries, which is an arbitrary point on the equator of the heavens, which serves, like the meridian of Greenwich on the earth, as a starting-place for reckoning longitude. The entire circuit of the heavens along the equator is divided into twenty-four hours of right ascension, each hour covering 15 of s.p.a.ce. If a planet then is in right ascension (usually printed for short R.A.) 0 h. 0 m. 0 s., it is on the meridian of the vernal equinox, or the celestial Greenwich; if it is in R.A. 1 h., it will be found 15 east of the vernal equinox, and so on.
[Ill.u.s.tration: CHART NO. 3.--FROM RIGHT ASCENSION 8 HOURS TO 12 HOURS; DECLINATION 30 NORTH TO 10 SOUTH.]
Declination (printed D. or Dec.) expresses the distance of a celestial body north or south of the equator of the heavens.
With these explanations we may proceed to find a planet by the aid of the Nautical Almanac and our charts. I take, for example, the ephemeris for the year 1901, and I look under the heading "Jupiter" on page 239, for the month of July. Opposite the 15th day of the month I find the right ascension to be 18 h. 27 m., neglecting the seconds. Now 27 minutes are so near to half an hour that, for our purposes, we may say Jupiter is in R.A. 18 h. 30 m. I set this down on a slip of paper, and then examine the declination column, where I find that on July 15 Jupiter is in south declination (the sign - meaning south, as before explained) 23 17" 52"", which is almost 23 18", and, for our purposes, we may call this 23 20", which is what I set down on my slip.
[Ill.u.s.tration: CHART NO. 4.--FROM RIGHT ASCENSION 12 HOURS TO 16 HOURS; DECLINATION 10 NORTH TO 30 SOUTH.]
Next, I turn to Chart No. 5, in this chapter, where I find the meridian line of R.A. 18 h. running through the center of the chart. I know that Jupiter is to be looked for about 30 m. east, or to the left, of that line. At the bottom and top of the chart, every twenty minutes of R.A.
is indicated, so that it is easy, with the eye, or with the aid of a ruler, to place the vertical line at some point of which Jupiter is to be found.
[Ill.u.s.tration: CHART NO. 5.--FROM RIGHT ASCENSION 16 HOURS TO 20 HOURS; DECLINATION 10 NORTH TO 30 SOUTH.]