It is evident that if the plane of the moon"s...o...b..t were to correspond with that of the earth, as they all lie in the plane of the page (Fig. 61), the moon must pa.s.s between the centres of the earth and sun, and exactly behind the earth at every revolution. Such successive and total darkenings would greatly derange all affairs dependent on light. It is easily avoided. Venus does [Page 158] not cross the disk of the sun at every revolution, because of the inclination of the plane of its...o...b..t to that of the earth (see Fig.
41, p. 107). So the plane of the orbit of the moon is inclined to the orbit of the earth 5 8" 39"; hence the full-moon is often above or below the earth"s shadow, and the earth is below or above the moon"s shadow at new moon. It is as if the moon"s...o...b..t were pulled up one-quarter of an inch from the page behind the earth, and depressed as much below it between the earth and the sun. The point where the orbit of the moon penetrates the plane of the ecliptic is called a node. If a new moon occur when the line of intersection of the planes of orbits points to the sun, the sun must be eclipsed; if the full-moon occur, the moon must be eclipsed. In any other position the sun or moon will only be partially hidden, or no eclipse will occur.
If the new moon be near the earth it will completely obscure the sun. A dime covers it if held close to the eye. It may be so far from the earth as to only partially hide the sun; and, if it cover the centre, leave a ring of sunlight on every side. This is called an annular eclipse. Two such eclipses will occur this year (1879).
If the full-moon pa.s.ses near the earth, or is at perigee, it finds the cone of shadow cast by the earth larger, and hence the eclipse is greater; if it is far from the earth, or near apogee, the earth"s shadow is smaller, and the eclipse less, or is escaped altogether.
There is a certain periodicity in eclipses. Whenever the sun, moon, and earth are in a line, as in the total eclipse of July 29th, 1878, they will be in the same position after the earth has made about eighteen revolutions, [Page 159] and the moon two hundred and sixteen--that is, eighteen years after. This period, however, is disregarded by astronomers, and each eclipse calculated by itself to the accuracy of a second.
How terrible is the fear of ignorance and superst.i.tion when the sun or moon appear to be in the process of destruction! how delightful are the joys of knowledge when its prophesies in regard to the heavenly bodies are being fulfilled!
MARS.
The G.o.d or war; Its sign [Symbol], spear and shield.
MEAN DISTANCE FROM THE SUN, 141,000,000 MILES. DIAMETER, 4211 MILES.
REVOLUTION, AXIAL, 24H. 37M. 22.7S.; ORBITAL, 686.98 DAYS. VELOCITY PER MINUTE, 899 MILES. SATELLITES, TWO.
[Ill.u.s.tration: Fig. 62.--Apparent Size of Mars at Mean and Extreme Distances.]
At intervals, on an average of two years one month and nineteen days, we find rising, as the sun goes down, the reddest star in the heavens. Its brightness is exceedingly variable; sometimes it scintillates, and sometimes it shines with a steady light. Its marked peculiarities demand a close study. We find it to be Mars, the fiery G.o.d of war. Its...o...b..t is far from circular. At perihelion it is 128,000,000 miles from the sun, and at aphelion 154,000,000; hence its mean distance is about 141,000,000. So great a change in its distance from the sun easily accounts for the change in its brilliancy. Now, if Mars and the earth revolved in circular orbits, the one 141,000,000 miles from the sun, and the other 92,000,000, they would approach at conjunction within 49,000,000 miles of each other, and at opposition be 233,000,000 miles apart.
But Mars at perihelion may be only 128,000,000 miles from the sun, and earth at [Page 160] aphelion may be 94,000,000 miles from the sun. They are, then, but 34,000,000 miles apart. This favorable opportunity occurs about once in seventy-nine years. At its last occurrence, in 1877, Mars introduced to us his two satellites, that had never before been seen by man. In consequence of this greatly varying distance, the apparent size of Mars differs very much (Fig.
62). Take a favorable time when the planet is near, also as near overhead as it ever comes, so as to have as little atmosphere as possible to penetrate, and study the planet. The first thing that strikes the observer is a dazzling spot of white near the pole which happens to be toward him, or at both poles when the planet is so situated that they can be seen. When the north pole is turned toward the sun the size of the spot sensibly diminishes, and the spot at the south pole enlarges, and _vice versa_. Clearly they are ice-fields. Hence Mars has water, and air to carry it, and heat to melt ice. It is winter at the south pole when Mars is farthest from the sun; therefore the ice-fields are larger than at the north pole.
It is summer at the south pole when Mars is nearest the sun. Hence its ice-fields grow smaller [Page 161] than those of the north pole in its summer. This carrying of water from pole to pole, and melting of ice over such large areas, might give rise to uncomfortable currents in ocean and air; but very likely an inhabitant of earth might be transported to the surface of Mars, and be no more surprised at what he observed there than if he went to some point of the earth to him unknown. Day and night would be nearly of the same length; winter would linger longer in the lap of spring; summer would be one hundred and eighty-one days long; but as the seas are more intermingled with the land, and the divisions of land have less of continental magnitude, it may be conjectured that Mars might be a comfortable place of residence to beings like men. Perhaps the greatest surprise to the earthly visitor would be to find himself weighing only four-tenths as much as usual, able to leap twice as high, and lift considerable bowlders.
_Satellites of Mars._
The night of August 11th, 1877, is famous in modern astronomy.
Mars has been a special object of study in all ages; but on that evening Professor Hall, of Washington, discovered a satellite of Mars. On the 16th it was seen again, and its...o...b..tal motion followed.
On the following night it was hidden behind the body of the planet when the observation began, but at the calculated time--at four o"clock in the morning--it emerged, and established its character as a true moon, and not a fixed star or asteroid. Blessings, however, never come singly, for another object soon emerged which proved to be an inner satellite. This is extraordinarily near [Page 162]
the planet--only four thousand miles from the surface--and its revolution is exceedingly rapid. The shortest period hitherto known is that of the inner satellite of Saturn, 22h. 37m. The inner satellite of Mars makes its revolution in 7h. 39m.--a rapidity so much surpa.s.sing the axial revolution of the planet itself, that it rises in the west and sets in the east, showing all phases of our moon in one night. The outer satellite is 12,579 miles from Mars, and makes its revolution in 30h. 18m. Its diameter is six and a quarter miles; that of the inner one is seven and a half miles. This can be estimated only by the amount of light given.
ASTEROIDS.
ALREADY DISCOVERED (1879), 192. DISTANCES FROM THE SUN, FROM 200,000,000 TO 315,000,000 MILES. DIAMETERS, FROM 20 TO 400 MILES. Ma.s.s OF ALL, LESS THAN ONE-QUARTER OF THE EARTH.
The sense of infinite variety among the countless number of celestial orbs has been growing rapidly upon us for half a century, and doubtless will grow much more in half a century to come. Just as we paused in the consideration of planets to consider meteors and comets, at first thought so different, so must we now pause to consider a ring of bodies, some of which are as small in comparison to Jupiter, the next planet, as aerolites are compared to the earth.
In 1800 an a.s.sociation of astronomers, suspecting that a planet might be found in the great distance between Mars and Jupiter, divided the zodiac into twenty-four parts, and a.s.signed one part to each astronomer for a thorough search; but, before their organization could commence work, Piazzi, an Italian astronomer of Palermo, [Page 163] found in Taurus a star behaving like a planet. In six weeks it was lost in the rays of the sun. It was rediscovered on its emergence, and named Ceres. In March, 1802, a second planet was discovered by Olbers in the same gap between Mars and Jupiter, and named Pallas. Here was an embarra.s.sment of richness. Olbers suggested that an original planet had exploded, and that more pieces could be found. More were found, but the theory is exploded into more pieces than a planet could possibly be. Up to 1879 one hundred and ninety-two have been discovered, with a prospect of more.
Between 1871-75 forty-five were discovered, showing that they are sought for with great skill. In the discovery of these bodies, our American astronomers, Professors Watson and Peters, are without peers.
Between Mars and Jupiter is a distance of some 339,000,000 miles.
Subtract 35,000,000 miles next to Mars and 50,000,000 miles next to Jupiter, and there is left a zone 254,000,000 miles wide outside of which the asteroids never wander. If any ever did, the attraction of Mars or Jupiter may have prevented their return.
Since the orbits of Mars and Jupiter show no sign of being affected by these bodies for a century past, it is probable that their number is limited, or at least that their combined ma.s.s does not approximate the size of a planet. Professor Newcomb estimates that if all that are now discovered were put into one planet, it would not be over four hundred miles in diameter; and if a thousand more should exist, of the average size of those discovered since 1850, their addition would not increase the diameter to more than five hundred miles.
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That all these bodies, which differ from each other in no respect except in brilliancy, can be noted and fixed so as not to be mistaken one for another, and instantly recognized though not seen for a dozen years, is one of the highest exemplifications of the accuracy of astronomical observation.
JUPITER.
The king of the G.o.ds; sign [Symbol], the bird of Jove.
DISTANCE FROM THE SUN, PERIHELION, 457,000,000 MILES; APHELION, 503,000,000 MILES. DIAMETER, EQUATORIAL, 87,500 MILES; POLAR, 82,500 MILES. VOLUME, 1300 EARTHS. Ma.s.s, 213 EARTHS. AXIAL REVOLUTION, 9H.
55M 20S. ORBITAL REVOLUTION, 11 YEARS 317 DAYS. VELOCITY, 483.6 MILES PER MINUTE.
[Ill.u.s.tration: Fig. 63.--Jupiter as seen by the great Washington Telescope. Drawn by Mr. Holden.]
Jupiter rightly wears the name of the "giant planet." His...o...b..t is more nearly circular than most smaller planets. He could not turn short corners with facility. We know little of his surface.
His spots and belts are [Page 165] changeable as clouds, which they probably are. Some spots may be slightly self-luminous, but not the part of the planet we see. It is covered with an enormous depth of atmosphere. Since the markings in the belts move about one hundred miles a day, the Jovian tempests are probably not violent. It is, however, a singular and unaccountable fact, as remarked by Arago, that its trade-winds move in an opposite direction from ours.
Jupiter receives only one twenty-seventh as much light and heat from the sun as the earth receives. Its lighter density, being about that of water, indicates that it still has internal heat of its own.
Indeed, it is likely that this planet has not yet cooled so as to have any solid crust, and if its dense vapors could be deposited on the surface, its appearance might be more suggestive of the sun than of the earth.
_Satellites of Jupiter._
In one respect Jupiter seems like a minor sun--he is royally attended by a group of planets: we call them moons. This system is a favorite object of study to everyone possessing a telescope. Indeed, I have known a man who could see these moons with the naked eye, and give their various positions without mistake. Galileo first revealed them to ordinary men. We see their orbits so nearly on the edge that the moons seem to be sliding back and forth across and behind the disk, and to varying distances on either side. Fig. 64 is the representation of their appearance at successive observations in November, 1878. Their motion is so swift, and the means of comparison by one another and the planet so excellent, that they can be seen to change their places, [Page 166] be occulted, emerge from shadow, and eclipse the planet, in an hour"s watching.
[Ill.u.s.tration: Fig. 64.--_a._ Various Positions of Jupiter"s Moons; _b._ Greatest Elongation of each Satellite.]
ELEMENTS OF JUPITER"S SATELLITES.
+-------------------------------------------------------------+Mean Distancefrom Jupiter.Sidereal Period.Diameter.---------------+------------------+----------Miles.Days Hrs. Min.Miles.I. Io260,0001 18 282,352II. Europa414,0003 13 432,099III. Ganymede661,0007 3 593,436IV. Callisto1,162,00016 18 52,929+-------------------------------------------------------------+
It is seen by the above table that all these moons are larger than ours, one larger than Mercury, and the asteroids are hardly large enough to make respectable moons for them. They differ in color: I. and II. have a bluish tinge; III. a yellow; and IV. is red.
The amount of light given by these satellites varies in the most sudden and inexplicable manner. Perhaps it may be owing to the different distributions of land and water on them. The ma.s.s of all of them is .000171 of Jupiter.
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If the Jovian system were the only one in existence, it would be a surprising object of wonder and study. A monster planet, 85,000 miles in diameter, hung on nothing, revolving its equatorial surface forty-five miles a minute, holding four other worlds in steady orbits, some of them at a speed of seven hundred miles a minute, and the whole system carried through s.p.a.ce at five hundred miles a minute. Yet the discovery of all this display of power, skill, and stability is only reading the easiest syllables of the vast literature of wisdom and power.
SATURN.
The G.o.d or time; sign [Symbol], his scythe.
MEAN DISTANCE FROM THE SUN, 881,000,000 MILES. DIAMETER, POLAR, 66,500 MILES; EQUATORIAL, 73,300 MILES. AXIAL REVOLUTION, 10H.
14M. PERIODIC TIME, 29T YEARS. MOONS, EIGHT.
The human mind has used Saturn and the two known planets beyond for the last 200 years as a gymnasium. It has exercised itself in comprehending their enormous distances in order to clear those greater s.p.a.ces, to where the stars are set; it has exercised its ingenuity at interpreting appearances which signify something other than they seem, in order that it may no longer be deluded by any sunrises into a belief that the heavenly dome goes round the earth.
That a wandering point of light should develop into such amazing grandeurs under the telescope, is as unexpected as that every tiny seed should show peculiar markings and colors under the microscope.
[Ill.u.s.tration: Fig. 65.--View of Saturn and his Rings.]
There are certain things that are easy to determine, such as size, density, periodic time, velocity, etc.; but other things are exceedingly difficult to determine. It requires long sight to read when the book is held [Page 168] 800,000,000 miles away. Only very few, if more than two, opportunities have been found to determine the time of Saturn"s rotation. On the evening of December 7th, 1870, Professor Hall observed a brilliant white spot suddenly show itself on the body of this planet. It was as if an eruption of white hot matter burst up from the interior. It spread eastward, and remained bright till January, when it faded. No such opportunity for getting a basis on which to found a calculation of the time of the rotation of Saturn has occurred since Sir William Herschel"s observations; and, very singularly, the two times deduced wonderfully coincide--that of Herschel being 10h. 16m., that of Mr. Hall being 10h. 14m.
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The density of Saturn is less than that of water, and its velocity of rotation so great that centrifugal force antagonizes gravitation to such an extent that bodies weigh on it about the same as on the earth. All the fine fancies of the habitability of this vaporous world, all the calculations of the number of people that could live on the square miles of the planet and its enormous rings, are only fancy. Nothing could live there with more brains than a fish, at most. It is a world in formative processes. We cannot hear the voice of the Creator there, but we can see matter responsive to the voice, and moulded by his word.
_Rings of Saturn._