Part 11

An ellipse may be very much elongated, or almost circular, but still have these properties which are essential to it, and in the case of the orbits of the planets, they are so nearly circular, that if drawn a few inches wide, would hardly be detected to differ from the circle. The orbit of the earth is about one thirtieth part longer than broad.

The rate at which the planets revolve round the sun is not equable, that is, their progress is not through equal distances at equal times, but a line drawn from the planet to the sun would always pass over equal spaces or "areas" in equal times, for example, in fig. 5, if the area of the angle included in A B C be equal to that included in A D E, then a planet would pass from B to C and from D to E in equal times.

Mercury is the smallest of the greater planets, and the nearest to the sun. Its diameter is about 3000 miles, and it is about one-fifteenth part the volume of the earth; its distance from the sun is 36,770,000 miles, and it performs its revolution in eighty-eight days.

This planet is but seldom seen with the naked eye; for, being so (comparatively) near to the sun, it sets before dark, and does not rise till the grey of morning makes it scarcely visible. Nothing notable has been discovered on its surface.

Venus is the second in rotation from the sun, and revolves round it, at a distance of 68,750,000 miles, in 224-1/2 days. The diameter of Venus is about 7760 miles, it is therefore nearly the volume of the earth. This is the brightest planet seen in the heavens, for although much smaller than many others, its comparative nearness to the sun and earth causes it to appear larger and brighter to the eye. Venus is the evening and morning star; for when to the west of the sun, it rises before it, and is then called "the morning star" (or, formerly, "Lucifer"), but when it is to the east of the sun, it sets after twilight is gone, and is called "the evening star," or "Hesperus."

These two planets, Mercury and Venus, have "phases" (like the moon), or certain positions in which the whole of the side illuminated by the sun is seen from the earth, and other positions in which it is seen sideways; the planet is then said to be in "quadrature," as may be seen at Q Q, fig. 6. Mercury and Venus, being between the sun and the earth, are called "inferior" planets, while those whose orbits are outside that of the earth are called "superior" planets. When either of the inferior planets are between the earth and the sun, they are said to be in "inferior conjunction" (I C, fig. 6), and when on the opposite side, or behind the sun, so that a straight line from it to the earth would have to pass through the sun, then it is called being in "superior conjunction" (S C); at mid-distance, either east or west, it is said to be in "eastern" or "western quadrature."

The third planet from the sun is the Earth, distant about 95,000,000 miles, with a diameter of 7925 miles, so that the most lofty mountains (five miles high) bear about the same proportion to it as would an elevation one-fourteenth of an inch in height to a ball of ten feet in diameter; therefore, with all its valleys and high hills, the surface of the earth is smoother in proportion to its bulk than the rind of an orange. The earth is not a perfect sphere, but has a slightly flattened form, as though it had been compressed at the poles. The diameter at the poles is the smallest, and is 7899 miles, while that at the equator (its greatest) is 7925 miles, being a difference of twenty-six miles; but this is so small, when compared with the size of the earth, that if an exact model of it were made, four inches in diameter, it would require the most accurate measurement to determine that it was not a perfect sphere.

The form of the earth has been demonstrated by accurate experiments and calculations, but there are many things we may observe, convincing us of its rotundity; one of the most evident of these is the fact that as a ship at sea goes from the land, the hull first disappears, then the lower sails, and lastly the tops, while a ship approaching the shore shows first the topsails, and lastly the hull; see fig. 7, where A shows a ship on the horizon, B hull-down, C out of sight. In the same way, the tops of distant mountains are seen at sea long before the lower lands. Another proof is drawn from the fact, that ships have been sailed quite round the earth; by steering as nearly as possible in one direction, they have arrived at the place from which they started. A third proof of the earth's rotundity is found in the form of its shadow, when the moon is eclipsed by it; this shadow, as thrown by the earth upon the moon, is circular, in whatever position the earth may be, and a sphere is the only solid form which can in all positions cast a circular shadow.

The earth turns upon its axis every 23 hours, 56 minutes, and 4 seconds, which constitutes a day, and makes one revolution round the sun in every 365-1/4 days, which make up a year; but the earth's axis is not at a right angle to an imaginary line drawn from the earth to the sun, but at an angle of 23 deg. 28 min. to it, as shown at fig. 8, where the line from N to S is the earth's axis of rotation, the dotted line leading to S is the sun's direction, and E Q is the equator.

The earth travels through space at a rate of more than a million-and-a-half miles per day, besides the distance which each object upon it is carried in its diurnal rotation (24,000 miles). At first thought it would seem impossible that such a rapid motion should not be felt; but as the air and clouds, and every object belonging to the earth, moves with it, and this motion is perfectly smooth, it can only be perceived by looking at objects independent of the earth. If sailing on the water, when it is quite smooth and the land not very near, the ship seems to be stationary and the objects on shore appear to pass along in an opposite direction to that in which the ship is going. It is the same with the earth; the sun, which appears to move, is stationary, while the earth is going round it; and the result is, while the earth turns round on its axis the whole firmament appears to move, we lose sight of those stars which are to the westward, which are then said to "set," while those to the eastward, constantly coming into sight, are said to "rise," the whole appearing to pass from east to west, while in reality the earth's surface is passing from west to east. At the equator, or that part of the earth which lies midway between the poles, the sun and stars appear to rise and set perpendicularly to the horizon and at equal times above and below it, it is therefore 12 hours day and 12 hours night; in places not situated on the equator, they do not rise and set perpendicularly to the horizon, but form portions of circles at greater or less altitudes in the heavens, and the nearer the observer is to either of the poles, the smaller the circle, and the more of it is seen; so that, at the poles, any star situated exactly overhead does not appear to move at all. This place is called the north pole of the heavens, and the nearest star to it is called the "polestar," which neither rises nor sets, and, as it always keeps the same position, serves as a guide to mariners. All this is the effect of the diurnal motion of the earth, its annual motion not affecting the position of the stars, they being at such an immense distance that the circle which the earth forms in passing round the sun--although nearly 200,000,000 miles across--is as a mere point in proportion. But not so with the sun: as before stated, at the equator it is twelve hours above the horizon and the same time below it; now, as we approach towards the poles, it forms at each rotation an arc above the horizon, lower and lower, until at the poles its daily rising and setting is lost altogether, and there would never be daylight there, provided that the earth was only subject to the diurnal motion (fig. 9); but as the earth turns upon an oblique axis, the north and south poles share the blessing of daylight between them, so that through one-half of the earth's orbit the north pole is towards the sun, as shown in the figure, and the south pole during the other half. There is therefore at the poles but one day and one night in the year--the day being summer and the night winter. In less northern climates, as England, this exists to a much smaller extent; for half the year the sun is longer above than below the horizon, constituting our summer, the other half of the year the sun is longer below the horizon than above it, and this is winter. Hence, in each case, midway between, the sun is twelve hours above and twelve below the horizon, which occurs in spring and autumn, being called the "equinoxes" (equal nights); the vernal or spring equinox takes place on the 21st of March, the autumnal on the 21st of September. The times just between these, when the sun is the longest and shortest time below the horizon, are called the summer and winter solstices, and occur on the 21st of December and the 21st of June.

The earth has a smaller sphere or "satellite," circulating round it, this is the Moon; it is placed at a distance of about 238,000 miles from the earth, is about one-fiftieth of its bulk or volume, and revolves round it every 27-1/2 days, keeping always the same side towards it, so that the other side has never been seen. This is effected by rotation on its axis, which takes place once for every circuit round the earth; if it did not rotate, or kept the same side always to the same point of the heavens, then, when it had half-way revolved, the other side would be turned towards the earth. There are good reasons for believing that the moon has no atmosphere or air around it, for the concave edge (when only a small portion of the half illuminated by the sun is seen) instead of being gradually shaded off into darkness, as would be the case if there were an atmosphere, is well defined and uneven, showing the tops of some of the mountains and their shadows (fig. 10). Nor does there appear to be any clouds, and as a consequence no water; neither has anything been observed by the aid of the best telescopes which could be considered sea, on the contrary, the whole surface seems torn up and rent into chasms and immense jagged mountains, enclosing circular portions like gigantic walls. The darkened parts which are always seen on the surface of the moon were formerly supposed to be seas, but of late, by the use of improved instruments, they have been seen to be rough like the other parts, and cannot therefore be water (fig. 11). As there appears to be no water, it may be confidently inferred that there are no inhabitants.

The moon having always one half illuminated by the sun, while the other half is dark, presents different "phases" to us on the earth, according to the position in which we view it; thus, in fig. 12, E represents the central position of the earth, and S the direction from which the light of the sun comes, the outer circle of moons represents the various real positions of that luminary with respect to the earth and sun, and the inner circle shows its appearance when in these various positions. When the moon is between the earth and sun, as at a, it is said to be in "conjunction," and as the darkened side is towards the earth, of course it is not visible, this is "new moon" (a corruption of "no moon"); when at b b, but a small part of the illuminated half is seen, and it is then said to be "crescentic" (first and last quarter); at c c, half of the bright side is seen, the moon is said to be at "quadrature," and the appearance is that of a "half-moon;" at d d, the greater part of the bright side is seen, and it is called "gibbous" and appears as a "three-quarter-moon," and at e the whole of the illuminated side is seen; it is then "full-moon," and is said to be in "opposition."

Beyond the earth the planet Mars (fig. 13) moves in its orbit round the sun; it is the smallest, but one, of the larger planets, having a diameter of but 4085 miles, and being only about one-seventh the magnitude of the earth; it makes its revolution in 687 days, at a distance of 144,780,000 miles from the sun.

Owing to the brilliancy and proximity to the sun of Venus and Mercury, together with other causes, no rotation has been observed in them, but as this exists in all the other planets there is no doubt they also rotate, but the nearness of Mars to the earth when in opposition (that is, when the earth is between it and the sun), has caused its rotation to be distinctly visible; the observation of this has been from time to time greatly favoured by certain dark spots which have remained stationary a sufficient time to determine the question very accurately. Mars takes 24 hours and 37 minutes to turn on its axis, or pretty nearly the time the earth does. Its resemblance is still more increased by the axis of rotation being oblique, from all which it is inferred that there is a day and night, winter and summer, and variation of climate very nearly resembling that of our own world, and there are round bright parts situated at the poles of Mars, which enlarge when it is winter there and diminish when summer, just as would the snows of arctic regions, and these are therefore supposed to be portions of the surface of Mars which are actually covered with snow (fig. 13). As to the question of the planets being inhabited, of course it can never be answered with certainty, but it is a great deal more likely that they are than that the moon is, which, having neither water nor atmosphere, can hardly be supposed to give habitation to any beings similar or analogous to those on earth, while Mars possesses a climate not greatly differing from that of the earth, and has both air and water. Mars has, at certain positions of the earth with respect to it, a partially "gibbous" form, that is, a small portion of the non-illuminated part comes within the lines of our vision it is then of the form seen in (fig. 14), but this can only occur when the earth and planet occupy positions somewhat near to that represented in fig. 15, in which S is the sun, E E opposite positions of the earth, and M Mars.

Jupiter is by far the largest of the planets, it is 87,030 miles in diameter, and placed at the enormous distance of 494,000,000 miles from the sun. This great planet takes 12 years and 52 days to perform its circuit; it turns upon its axis in 9 hours 55 minutes, a surprisingly short time considering the immensity of its bulk. As a result of this rapid motion Jupiter is very far removed from the form of a true sphere, for the oblate form of heavenly bodies is caused by their rotatory motion, and the centrifugal force set up by it. In fig. 16 is an outline of the earth and Jupiter, showing their relative size. It has no phases, like those planets which are nearer to the sun than the earth, its great distance preventing this, as may be seen in fig. 17, in which the earth (E E) is placed at the two widest lateral positions of its orbit, but the earth is too near the sun, in proportion to the great distance of Jupiter, to allow any part of the latter to come within the range of vision, except that which is illuminated by the sun. This planet is high up in the heavens the greater number of nights in the year, and is therefore a very conspicuous object. It also presents a most beautiful appearance through a good telescope, its vast size causing it to look larger than those which are much nearer; it has several shadowy belts across it, which are supposed to be openings in the strata of clouds which surround it, drawn into ring-like forms by the rotation of the planet, these are shown in fig. 18. It being probable that this great planet is surrounded by strata of the densest clouds which only open in the tropical region, its inhabitants therefore, (if there be any) get but a glimpse of the firmanent and its stars through them, in those situations at or near to the planet's equator. These dense clouds serve a very useful purpose in regions so very far removed from the source of heat, for if radiation were permitted to go on freely from the surface of the planet the sun's rays would be too feeble to compensate for it, and the cold would be intense; but the clouds reflect back the heat radiated from the surface and keep in what little heat is received at that great distance. Jupiter has four satellites or moons, which revolve round it as our moon does round this earth.

Far beyond Jupiter rolls another stupendous orb called Saturn, not so large as Jupiter, but still immense, being 847 times the bulk of the earth. It is placed at the distance of 906 millions of miles from the sun. An idea of this may be formed from the fact that light, which travels at the rate of nearly 200,000 miles a second or 12,000,000 a minute, takes about an hour and a quarter to pass from the sun to Saturn. It performs its journey round the sun only in 29-1/2 years, which are therefore but as one year, yet all this time it is moving at the rate of nearly 22,000 miles an hour, so immense is the orbit it has to traverse, but it revolves on its axis in about 10-1/2 hours, so that the nights and days are extremely short while the years are prodigiously long. There is every reason to believe that it has changes of seasons and variation of climate similar to those in our world, but, being so far from the sun, they must be altogether more severe than ours. The most extraordinary part of this great globe is its possession of three (perhaps more) great flattened rings, which surround it, one within the other; these rings are of immense size and width, but very thin, the great breadth through all from the inner to the outer edges being about 30,000 miles, while their thickness cannot exceed 250. These rings are placed at a right angle to the planet's axis of rotation and revolve with it, so that when the planet is at the equinox, the edge of these rings is turned towards the sun, they can then be seen only by the most powerful telescopes, forming a faint streak on each side of the orb of the planet (fig. 19), but as they become inclined they appear as a very long ellipse, the ends of which project in loop-like forms on either side, giving rise to the notion of the planet having two handles (fig. 20). This ellipse becomes broader and broader as the plane of the rings forms a greater angle with the line of vision. For a short time before and after the equinoxes of Saturn, the rings become invisible, owing to the earth and sun being on opposite sides of them, as may be seen in fig. 21, so that the darkened side is turned towards the earth and the edge, which is the only part illuminated, is towards the sun.

The inner ring of Saturn is supposed to be composed of watery vapour, as it is somewhat transparent, but the outer ones are solid, which is shown by the shadow they cast upon the planet, and the shadow it casts upon them in different positions (figs. 22 and 23). Besides these rings Saturn has eight satellites or moons, which revolve in a plane nearly parallel to that of the rings and exterior to them. It has been calculated that Saturn weighs only 100 times more than the earth, although it is somewhere about 900 times larger, from which it is concluded that the substance of which Saturn is made must be about one-ninth the density of this earth, half the density of water, or about the same as cork. Saturn is very much flattened at the poles, so much so that the equatorial diameter is a tenth more than the axial diameter, which difference is distinctly visible through good glasses.

Still further into space, at double the distance of Saturn, or nearly 1822 millions of miles from the sun, another great world or planet revolves round it, it is called Uranus. This planet is not so large as either Jupiter or Saturn, but is of considerable magnitude, being eighty times that of this world, it takes eighty-four of our years to complete its vast circle round the sun, which are therefore equivalent to but one year of Uranus.

It has several satellites, four of which have been discerned perfectly, but it is doubtful whether there have not been two more seen. This orb was first recognised as a planet by Sir Wm. Herschell, after whom it was for some time named.

Another, and the most remote planet in our system, is Neptune, revolving at the immense distance of 2850 millions of miles from the sun, and taking more than 164 years to perform the journey. This planet is only to be seen by the most powerful glasses, and was discovered under very peculiar circumstances, not (like other planets) by chance; its existence was recognised as necessary to account for certain "perturbations" or deviations in the orbit of Uranus, which was found to take a course differing from what it should, according to computation, and which were only to be accounted for by supposing that another planet existed far out in space, which affected the course of Uranus by its attraction. The idea of finding out where this orb should be, occurred to M. Le Verrier and Mr. Adams, independent of each other; they both arrived at nearly similar conclusions, for the positions assigned to the supposed planet so nearly agreed with each other and with its real position, that their calculations have been looked on as the greatest feat of astronomical research. It was discovered by Dr. Galle of Berlin, upon his hearing from Le Verrier the position in which at that time it should be sought for.

THE PLANETOIDS.

Upon observing the relative distances of the planets from each other in passing outwards from Mercury, it will be found that each one is placed about double the distance of the one next before it (not exactly, but sufficiently near to form a coincidence almost amounting to a law), they are as follows:--

Mercury 36,700,000 Venus 68,770,000 The Earth 95,000,000 Mars 144,780,000 Jupiter 494,000,000 Saturn 906,000,000 Uranus 1822,000,000 Neptune 2850,000,000

It will be seen, however, that in this table of distances there is one great exception to the rule, namely the distance between Mars and Jupiter, which is nearly double what it should be according to this rule, giving rise to the idea that a planet ought to be placed between them. Professor Bode was so convinced that something of the kind was necessary to complete the harmony of the series, that he caused to be instituted a search for the supposed planet, and according to his conjecture one was discovered as nearly as possible in the situation indicated, but upon more accurate information being obtained, this planet was found to be so small (scarcely 150 miles diameter) that it could hardly be considered as a planet ranging with the others, and having so large a space of the heavens to itself; shortly afterwards another small planet was discovered, having nearly the same orbit. From time to time others have been discovered, to the number of forty-two, up to the present date. Dr. Olbers upon discovering the second of these small bodies, where one great one was anticipated, put forth the curious hypothesis, that one large planet had really existed there at some former time, that it had been shattered into pieces by some accident, such as an internal explosion, and that more of these pieces would be found; it is curious how accurately this prediction has been verified.

If we suppose the original planet to have been liquid when shattered to pieces, it would fully account for the fragments being spherical, just as globules of mercury assume this form from their own attraction of gravitation, and that the planets, our own earth amongst them, are liquid, is very nearly proved by their forms, which are exactly such as fluid masses rotating on an axis would assume, moreover the specific gravity of some of the planets is but little more than that of water, and with respect to our earth, the evidence of internal heat, increasing as we descend below the surface, &c., show almost beyond doubt that at the present time it is in a liquid state (molten) covered over by a few miles' thickness of hardened crust, which bears such a small proportion to the bulk of the globe, that the whole may be considered as liquid.

THE STELLAR SYSTEM.

The thousands of stars which spangle the heavens are all part of one "system;" but it has been found by the aid of the telescope that this system is but one out of many. Our system of stars occupies a space somewhat in the form of a thick lens or much-flattened sphere, but others are of very different forms, and some have but little regularity of form at all; our sun is one of the stars of this system. It is not known whether there are planets revolving round the other stars, the distance being far too great for any telescope to render them visible. Our sun occupies a somewhat central position in the system. The stars are classed into sizes, as first magnitude, second magnitude, &c., on to the thirteenth or fourteenth magnitude, but all beyond the fifth magnitude (by far the greater number) are only visible by the aid of the telescope. There are about 5000 stars visible to the naked eye. But fourteen stars of the first magnitude are in our hemisphere of the heavens, and about fifty of the second, but the number of stars of each magnitude increases prodigiously in the higher numbers, so that those stars capable only of being seen by the aid of powerful telescopes, amount to many millions; these are chiefly situated in a great belt which encircles the heavens, called the "Milky Way," which is caused by the line of vision passing through the breadth of our starry system, and consequently meeting with a greater number of stars than in other directions, where it only crosses its thickness.

The other systems of stars, called "nebulæ," from their resemblance to little clouds, were supposed to consist of luminous matter of but little density, and which might at some future period be condensed into stars; but the improved power and construction of telescopes have enabled astronomers to resolve many of these nebulæ into clusters of stars, and there is but little doubt that all could be thus resolved, were the telescope of sufficient power; and thus it appears that in the infinity of space collections of systems are placed, each one too distant from the others to be calculated or written in numbers, but each consisting of thousands of suns many hundred times greater than this earth, and many millions of miles from each other. What an idea of space does this afford, and how soon do all our narrow notions of possibility and impossibility vanish before such facts accomplished by the hands of God!