HALFWAY between the orbit of Mars and that of Jupiter there revolve several thousand bits of matter in regular orbits about the Sun. The whole collection is grouped under the named asteroids. Several of them attain diameters of hundreds of miles, but they are so small, their gravity so weak, their mass so infinitesimal that they are not considered worthy of the title planet, but are called planetoids.
Jupiter, diameter 86,728 miles, mass 316.9 times Earth's. Saturn, diameter 72,430 miles, mass 94.92 times Earth's. What the planetoid Ceres is to Earth, Earth is to Jupiter. Would then, an inhabitant of 86,728-mile Jupiter call Earth a planet? Or would he call it, perhaps, a planetoid?
The entire solar system seems to have been laid out by two different plans, on two different scales, as though God had decided it were fitting and proper that there be a planetary system about this Sun, a Sun of a little better than medium rating, and had, forthwith started to make one. A dab of matter three thousand miles through, forty millions of miles from the Sun, was Mercury. A larger dab some eight thousand miles through and sixty-seven millions of miles from the Sun for Venus. Another eight-thousand-mile dab, ninety millions out, for Earth, and a four-thousand-mile pebble a hundred and fifty million out for Mars.
Then He started to make another three hundred millions out, looked at the size of His creation and shattered it in disgust.
Perhaps He decided that while He was building He'd do a job of it. He started with a will, and with space to work in. He got clear away from that puny, little dust-speck system He'd started, went out a way where space was clear, twice again as far as his farthest, and started right: Jupiter, half a billion miles from the Sun, more than ten times the diameter of Earth, largest of the minor planets, over three hundred times as massive, a hundred times as massive as all four minor planets together!
Saturn, a real planet, nearly a billion miles from the Sun, then Uranus, a billion and three quarters, Neptune, two and three quarter billions of miles out in the void. Perhaps Pluto was an afterthought, a thing halfway between the size of Mars and Earth, three and a half billions of miles from the Sun, what He scrubbed off His hands when He finished making the planets.
A fanciful account, but throughout the solar system there seems to be two distinct scales in use; the Minor Scale, with planets thousands of miles through, and tens of millions of miles from the Sun; and the Major Scale, with planets tens of thousands of miles in diameter, hundreds of millions of miles from the Sun.
|Planet||Distance in Tens of Millions||Diameter in Thousands||Mass (Earth==1)|
|Planet||Distance in Hundreds of Millions||Diameter in Tens of Thousands||Mass (Earth==1)|
In the distances, the Major Planets are more hundreds of millions of miles out than the Minors are tens of millions -- Neptune being almost exactly twice as many hundreds of millions distant as Mars is tens of millions. If it were not for this, the Minor Planets would look almost like a model of the Major Planets done on a one-tenth normal scale, except for the masses which are even more disproportionate. Jupiter would make three hundred-odd Earths; Uranus would make three hundred-odd Mercurys; Saturn a hundred Venuses; Neptune fifty Mars. An inhabitant of Jupiter could say with truth, "We wouldn't call Mercury a first-class moon; we already have two larger than that."
A CURIOUS THING about the Major Planets is their low density. For Jupiter being ten times and more the diameter of Earth has more than one hundred times the area, and more than one thousand times the volume. But it hasn't the thousand-times mass, because the density is 1.33 times that of water, while Earth's is 5.52 on the same scale.
That's another problem, too. Saturn has only .73 density, Uranus and Neptune about the same as Jupiter -- 1.36 and 1.30 respectively. Don't say glibly, "Deep gaseous atmosphere," because it won't work out that way.
Suppose there were a hole drilled into the Earth sixty miles deep. If you climb up a mountain three and one half miles tall, the atmosphere is half as dense at the top as at the bottom. Seven miles up it is one fourth as dense. If it increases at that rate going downward -- it won't quite, but as a matter of fact there are factors which make it increase more rapidly -- then three and one half miles down it is twice as dense, seven miles down four times as dense, ten and one half eight times.
The result will be that at the bottom of that sixty-mile hole you'd have to anchor platinum down to keep it there. In that air, so compressed, it would float! Water would float at about fifty miles, iron a few miles farther down.
But all those Major Planets have similar general characteristics; they are immense -- ten times the size of the Minors, ten times as far out -- for some reason they are not dense; they generally have a large family of satellites.
Another general characteristic of the Majors is their rapid rotation. The day of Jupiter is 9.9 hours; Saturn is 10.2; Uranus 10.7 -- not certain because of observational difficulties. Uranus isn't handy by. Neptune also is uncertain for much the same reason. Neptune about 15 hours.
In contrast, the Minors are distant tens of millions of miles, are thousands of miles in diameter, and rotate in periods comparable to Earth's day -- Mars in almost exactly the time Earth rotates, 24.60 hours.
It has been suggested that this is due to an original high rotational speed of all the planets, and just as Earth's rotation slows down one second in 100,000 years now, so the others have slowed down more or less depending on their masses and on the various influences acting on them.
Jupiter, because of its immense mass, requires an immense braking action. Nine satellites dragging on him haven't slowed him appreciably yet.
Mercury, on the other hand, has slowed down all it can for its year and its day are equal -- 88 days. The terrific gravitational field of the near-by Sun dragged on it till Mercury turns no more with respect to the Sun, facing it always with the same side.
Venus is so cloud-wrapped we know nothing about its rotation. Earth is comparatively massive, far enough from the Sun so that it is not greatly influenced by its tidal action, but seriously affected by the moon.
THERE IS another immensely interesting thing about this rotation, a thing which has led to speculation since it was first discovered. First, the Earth turns on its axis, the equatorial plane being tipped somewhat (23°) to the plane of the orbit in which it revolves about the Sun. The Sun rotates in the same direction also, turning on its axis in about 28 days. (This isn't accurate, for while the Sun's equator turns once in 25 days, the polar regions rotate once in 34 days).
The plane of the Earth's orbit is called the plane of the ecliptic and taken as an arbitrary and handy reference plane. Really as arbitrary as the meridian of Greenwich, since it was chosen for much the same sort of reason. We happen to live here. But taken as a reference plane, the axis of the Sun's rotation is such that the equator is inclined to it at a very slight angle -- about 7° out. All the others are within three degrees of the plane, Uranus less than 50 minutes out.
It was suggested that the asteroids, lying in the plane between the orbits of Mars and Jupiter -- thousands and tens of thousands of jagged, broken pieces of rock and metal would cause trouble if ever interplanetary travel were attempted. Their countless incalculable orbits would make them cosmic buckshot aimed at speeds of miles a second at any body venturing near them. But even they obey this general rule apparently, and by going only a few degrees out of the plane of the ecliptic, they would lie in the great disk of the solar system below.
All but the two inner planets have satellites -- nine for Jupiter, nine for Saturn, four for Uranus, one for Neptune, two for Mars, and Earth has its Moon. We know so little about Pluto we cannot say more than that we haven't seen one yet; but we have scarcely seen Pluto yet. Neptune, we are sure, has more, probably Uranus has, and Saturn and Jupiter may have more so small they have not been detected. Quite a sizeable satellite could be missed if it were circling Neptune, two and a half billion miles away from the only source of light that could make it visible, and practically the same distance from us.
At Jupiter's distance we can detect one fifteen miles in diameter. But Mercury and Venus have none, because, it is suggested, if they ever had any, the Sun's titanic gravitational field pulled them away in ages past.
So incredibly immense is that field that it is the controlling force in some two light years of space, about 12,000,000,000,000 miles in every direction. And this is not a vain rule over utterly empty, matterless space. Out almost to the limits of that control go meteors and comets in immense orbits, looping in any and all directions. The plane of the ecliptic does not contain them; they spread in every plane -- the wanderers of the solar system. Returning at long intervals for a brief visit before plunging again into space, these comets exhibit every conceivable type of motion compatible with the law of gravitation.
And comets come to grief. Perhaps they retire into the depths of space for two, three, or even four million years, 5,000,000,000,000 miles from the Sun. Out there, moving at velocities so low -- astronomically speaking, but still of the order of 1,000 miles an hour -- they barely crawl through space, and the pulls and cross pulls of the stars have time to act on them.
If it happens to have retired in the direction of a near-by star, such as Alpha Centauri, one may be pulled out of its orbit sufficiently to change from a closed eliptical[ed. sic] orbit, to an open parabola that leads it out and onward forever, never to return. Perhaps in the ages that pass they come back at the wrong moment and pass close to Jupiter, or one of the other Major Planets.
Jupiter's own gravitational field is titanic, so immense that fifteen million miles from him he holds a satellite in a grip the Sun cannot break. If a comet passes close to this giant, its orbit is badly wrenched, Jupiter may "capture" it, not as a satellite but in an orbit so broken, so small, that thenceforth the comet can never retire into space, and comets don't last long near the Sun. Their structure is too tenuous to stand the buffeting strains of the immense cross pulls of the planets.
If we could see gravitational lines of force, the solar system would appear a network of titanic springs, stretching and yielding but always holding, gravitational arms reaching from every planet to every planet, dragging and pulling at it, stiffening the whole system into a locked, firm system. Comets, collections of meteorlike material floating in space, held together by loose bonds of infinitely weak mutual gravitation, cannot endure such strains. Within a generation or two the captured comet is apt to disappear.
METEORS never fall to Earth from space. They fall to the Sun, and the Earth happens to get in the way. Countless billions of minute meteors strike the Sun, but they have no effect on it for all their numbers. Bright as meteors are, Lindemann and Dobson believe that 10,000 first-magnitude meteors could be held in one hand! Yet each one of them in its brief flight gives up energy at the rate 100,000 ordinary incandescent lights would.
It has been suggested that the merest push would topple the delicate balance of the planets; that if one were destroyed the others would crash to oblivion, because of the disruption of their fine balance. The "delicate push" would go precisely as far toward upsetting the planetary balance as throwing a cigarette stub off the Normandie would toward upsetting her balance.
The gravitational attraction between the Earth and the Sun has been compared to a steel cable of equal strength. The steel would have to be some thousands of miles thick. Any structure laced together with thousand-mile thick steel cables takes considerable pushing to reach unbalance. Further, if some inconceivable force pushed one of the planets a million miles Sunward, releasing that force would be an instant signal for the planet to spring back. If you depress one side of a balanced scale, releasing it permits it to regain equilibrium, naturally. If you completely destroyed the Earth, it would stop perturbing, or bothering, the other planets. And that is just about all it would mean -- to the other planets.
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