Astronomy for Everybody/Part 4/Chapter 6

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Jupiter, the "giant planet," is, next the sun, the largest body of the solar system. It is, in fact, more than three times as large, and about three times as massive as all the other planets put together. Yet, such is the preponderating mass of our central luminary that the mass of Jupiter is less than one thousandth part that of the sun.

This planet is in opposition in September, 1903, October, 1904, November, 1905, and so on for several years afterward, about a month later every year. Near the time of opposition it may easily be recognised in the evening sky, both by its brightness and its colour. It is then, next to Venus, the brightest star-like object in the heavens. It can easily be distinguished from Mars by its whiter colour. If we look at it with a telescope of the smallest size, even with a good ordinary spy-glass, we shall readily see that instead of being a bright point, like a star, it is a globe of very appreciable dimensions. We shall also see what look like two shadowy belts crossing the disk. These were noticed and pictured two hundred years ago by Huygens. As greater telescopic power was used it was found that these seeming belts resolved themselves into very variegated cloud-like forms, and that they vary, not only from month to month, but even from ​night to night. By careful observation on the aspect which they present from hour to hour, and from night to night, it was found that the planet rotates on its axis in about 9 hours 55 minutes. The astronomer may therefore in the course of a single night see every part of the surface of the planet presented to his view in succession.

Two features presented by the planet will at once strike the careful observer with the telescope. One of these is that the disk does not seem uniformly bright, but gradually shades off near the limb. The latter, instead of being bright and hard is somewhat soft and diffuse. In this respect the appearance forms quite a contrast to that presented by the moon or Mars. The shading off toward the edge is sometimes attributed to a dense atmosphere surrounding the planet. While this is possible, it is by no means certain.

The other feature to which we allude is an ellipticity of the disk. Instead of being perfectly round, the planet is flattened at the poles, like our earth, but in a much greater degree. The most careful observer, viewing the earth from another planet, would see no deviation from the spherical form, but, viewing Jupiter, the deviation is very perceptible. This is owing to its rapid rotation on its axis, which causes its equatorial regions to bulge out, as, to a smaller degree, in the case of the earth.

The features of Jupiter, as we see them with a telescope, are almost as varied as those of the clouds which we see in our atmosphere. There are commonly elon​gated strata of clouds, apparently due to the same cause that produces stratified clouds on the earth, namely, currents of air. Among these clouds round white spots are frequently seen. The clouds are sometimes of a rosy tinge, especially those near the equator. They are darkest and most strongly marked in middle latitudes, both north and south of the equatorial regions. It is this that produces the appearance of dark belts in a small telescope.

The appearance of Jupiter is, in almost every point, very different from that of Mars or Venus. Comparing it with Mars, the most strongly marked difference consists in the entire absence of permanent features. Maps of Mars may be constructed and their correctness tested by observations generation after generation, but owing to the absence of permanence, no such thing as a map of Jupiter is possible.

Notwithstanding this lack of permanence, features have been known to endure through a number of years, and some of them may be permanent. The most remarkable of these was the great red spot, which appeared in middle latitudes, on the southern hemisphere of the planet, about the year 1878. For several years it was a very distinct object, readily distinguished by its colour. After ten years it began to fade away, but not at a uniform rate. Sometimes it would seem to disappear entirely, then would brighten up once more. These changes continued but, since 1892, faintness or invisibility has been the rule. If the spot finally disappeared, it was in so uncertain a way that no exact date for the last observation ​

​of it can be given. Some observers with good eyes still report it to be visible from time to time.

The question of the constitution of this curious planet is still an unsettled one. There is no one hypothesis that readily explains all the facts, which suggest many points, but prove few, unless negatively

Perhaps the most remarkable feature of the planet is its small density. Its diameter is about eleven times that of the earth. It follows that, in volume, it must exceed the earth more than thirteen hundred times. But its mass is only a little more than three hundred times that of the earth. It follows from this that its density is much less than that of the earth; as a matter of fact, it is only about one third greater than the density of water. A simple computation shows that the force of gravity at its surface is between two and three times that at the surface of the earth. Under this enormous gravitation we might suppose its interior to be enormously compressed, and its density to be great in comparison. Such would certainly be the case were it made up of solid or fluid matter of the same kind that composes the surface of the earth. From this fact alone the conclusion would be that its outer portions at least were composed of aeriform matter. But how reconcile this form with the endurance of the red spot through twenty-five years? This is the real difficulty of the case.

Nevertheless, the hypothesis is one which we are forced to accept without great modification. Besides the evi​dence of vapour as shown by the constantly changing aspect of the planet, we have another almost conclusive piece of evidence in the law of rotation. It is found that Jupiter resembles the sun in that its equatorial region rotates in less time than the regions north of middle latitude, although the circuit they have to make is longer. This is probably a law of rotation of gaseous bodies in general. It seems, therefore, that Jupiter has a greater or less resemblance to the sun in its physical constitution, a view which quite corresponds with its aspect in the telescope. The difference in the time of rotation at the equator and in middle latitudes is, so far as we yet know, about five minutes. That is to say, the equatorial region rotates in nine hours fifty minutes and those in middle latitudes in nine hours fifty-five minutes. This corresponds to a difference of velocity of the motion between the two amounting to about two hundred miles an hour; a seemingly impossible difference were the surface liquid.

It is a singular fact that no well-defined law of rotation in different latitudes has yet been made out, as has been done in the case of the sun. Were we to accept the recults of the meagre observations at our disposal we might be led to the conclusion that the difference of time is not a gradually varying quantity, as we go from the equator toward the poles, but that the change of five minutes occurs very near a certain latitude and almost suddenly. But we cannot assume this to be the case without more observations than are yet on record. The subject is one of which an accurate investigation is greatly to be desired.

​Yet another resemblance between Jupiter and the sun is that they are both brighter in the centre of their disk than toward the circumference. In the case of Jupiter, the shading off is very well marked. The extreme circumference especially is more softened than that of any of the other planets.

The apparent resemblance between the surfaces of these bodies, taken in connection with the brightness of the planet, has led to the question whether Jupiter may not be, in whole or in part, self-luminous. This again is a question which needs investigation. The idea that the planet can emit much light of its own seems to be negatived by the fact that the satellites completely disappear when they pass into its shadow. We may therefore say with entire certainty that Jupiter does not give enough light to enable us to see a satellite by that light alone. We can hardly suppose that this would be the case if the satellite received one per cent as much light from the planet as it does from the sun. It is also found that the light which Jupiter sends out is somewhat less than that which it receives from the sun. That is to say, all the light which it gives out, when estimated in quantity, may be reflected light, without supposing the planet brighter than white bodies on the surface of the earth. But this still leaves open the question whether the white spots, sometimes so much brighter than the rest of the planet, may not give us more light than can fall upon them. This also is a question not yet investigated.

The hypothesis which best lends itself to all the facts seems to be that the planet has a solid nucleus, of which ​the density may be as great as that of the earth or any other solid planet, and that the small average density of the entire mass is due to the vapourous character of the matter which surrounds this nucleus. In all probability the nucleus is at a very high temperature, even approximating that at the surface of the sun, but this temperature gradually diminishes as we ascend through the gaseous atmosphere, as we suppose to be the case with the sun; hence it may happen that, at the surface, none of the material that we see is at a high enough temperature to radiate a sensible amount of heat.

On the whole we may describe Jupiter as a small sun of which the surface has cooled off till it no longer emits light.

When Galileo first turned his little telescope on the planet Jupiter he was delighted and surprised to find it accompanied by four minute companions. Watching them from night to night, he found them to be in revolution around their central body as, upon the theory not fully accepted in his time, the planets revolve around the sun. This remarkable resemblance to the solar system was a strong point in favor of the Copernican Theory.

These bodies can be seen with a common spy-glass, or even a good opera glass. It has even been supposed that good eyes sometimes see them without optical assistance. They are certainly as bright as the smallest stars visible to the naked eye, yet the glare of the planet would seem to be an insuperable obstacle to their visibility, even to ​the keenest vision. A story has been told, by Arago, I think, of a woman who professed to be able to see them at any time and even pointed out their positions. It was found, however, that she described them as on the opposite side of the planet to that on which they were really situated. It was then found, or inferred, that she took the positions from an astronomical ephemeris, in which diagrams of them were given, but in which the pictures were made upside down in order that the satellites might be seen as in an ordinary inverting telescope. But it seems quite likely that, when the two outer satellites chance to be nearly in the same straight line, they may be visible by their combined light.

From the measures of Barnard it may be inferred that these bodies range somewhere between two and three thousand miles in diameter. Hence, they do not differ greatly from our moon in size.

Only four satellites were known until 1892; then Barnard, with the great Lick telescope, discovered a fifth, much nearer the planet than the four others. It makes a revolution in a little less than twelve hours, the shortest periodic time known except that of the inner satellite of Mars. Still, however, it is a little longer than the rotation time of the planet. The next outer one, or the innermost of the four previously known, still called the first satellite, revolves in about one day eighteen and a half hours, while the outer one requires nearly seventy days to perform its circuit.

In its visibility the fifth satellite is the most difficult known object in the solar system. Through only a few ​of the most powerful telescopes of the world has it ever certainly been seen by the human eye. Its orbit is decidedly eccentric. Owing to the ellipticity of the planet, it possesses the remarkable peculiarity that its major axis, and, therefore, the perihelion point of its orbit, performs a complete revolution in about a year.

It has sometimes been questioned whether these satellites are round bodies, like the planets and most other satellites. Some observers, especially Barnard and W. H. Pickering, noticed curious changes in the form of the first satellite as it was crossing the surface of the planet. At one time it looked like a double body. But Barnard, by careful and repeated study, showed that this appearance was partly due to the varying shade of the background on which the satellite was seen projected upon the planet, and partly to the differences in the shade of various parts of the satellite itself.

During their course around the planet these bodies present many interesting phenomena, which can be observed with a moderate sized telescope. These are their eclipses and transits. Of course Jupiter, like any other opaque body, casts a shadow. As the satellites make their round they nearly always pass through the shadow during that part of their course which is beyond the planet. Exceptions sometimes occur in the case of the fourth and most distant satellite, which may pass above or below the shadow, as our moon passes above or below that of the earth. When a satellite enters the shadow, it is seen to fade away gradually, and finally to disappear from sight altogether.

​For the same reason the satellites generally pass across the disk of the planet in that part of their course which lies on this side of it. The general rule is that, when a satellite has impinged on the planet, it looks brighter than the latter, owing to the darkness of the planet's limb. But, as it approaches the central regions, it may look darker than the background of the planet. Of course this does not arise from any change in the brightness of the satellite, but only from the fact, already mentioned, that the planet is brighter in its central regions than at its limb.

Yet more interesting and beautiful is the shadow of a satellite which, under such circumstances, may often be seen upon the planet, looking like a black body crossing alongside the satellite itself. Such a shadow is shown in the picture of Jupiter on page 204.

The phenomena of Jupiter's satellites, including their transits and those of their shadows, are all predicted in the astronomical ephemerides, so that an observer can always know when to look for an eclipse or transit.

The eclipses of the inner of the four older satellites occur at intervals of less than two days. By noting their times, an observer in unknown regions of the earth can determine his longitude more easily than by any other method. He has first to determine the error of his watch on local time by certain simple astronomical observations, quite familiar to astronomers and navigators. He thus finds the local time at which an eclipse of the satellite takes place. He compares this with the time predicted in the ephemeris. The difference gives his longitude ​according to the system set forth in our chapter on Time and Longitude.

The principal drawback of this method is that it is not very accurate. Observations of the time of such an eclipse are doubtful to a large fraction of a minute. This corresponds to 15 minutes of longitude, or 15 nautical miles at the equator. In the polar regions the effect of the error is much smaller, owing to the convergence of the meridians. The method is, therefore, most valuable to polar explorers.