Astronomy for Everybody/Part 4/Chapter 5

The seeming gap in the solar system between the orbits of Mars and Jupiter naturally attracted the attention of astronomers as soon as the distances of the planets had been accurately laid down. It became very striking when Bode announced his law. There was a row of eight numbers in regular progression, and every number but one represented the distance of a planet. That one place was vacant. Was the vacancy real, or was it only because the planet which filled it was so small that it had escaped notice?

This question was settled by Piazzi, an Italian astronomer who had a little observatory in Palermo in Sicily. He was an ardent observer of the heavens, and was engaged in making a catalogue of all the stars whose positions he could lay down with his instrument. On January 1, 1801, he inaugurated the new century by finding a star where none had existed before; and this star soon proved to be the long-looked-for planet. It received the name of Ceres, the goddess of the wheat field.

It was a matter of surprise that the planet should be so small; and when its orbit became known it proved to be very eccentric. But new revelations were soon to come. Before the new planet had completed a revolu​tion after its discovery, Dr. Olbers, a physician of Bremen, who employed his leisure in astronomical observations and researches, found another planet revolving in the same region. Instead of one large planet there were two small ones. He suggested that these might be fragments of a shattered planet, and that, if so, more would probably be found. The latter part of the conjecture proved true. Within the next three years two more of these little bodies were discovered, making four in all.

Thus the matter remained for some forty years. Then, in 1845, Hencke, a German observer, found a fifth planet. The year following a sixth was added, and then commenced the curious series of discoveries which, proceeding year by year, are now carrying the number known rapidly past five hundred.

Up to 1890 these bodies had been found by a few observers who devoted especial attention to the search, and caught the tiny stars as the hunter does game. They would lay traps, so to speak, by mapping the many small stars in some small region of the sky near the ecliptic, familiarise themselves with their arrangement, and then watch for an intruder. Whenever one appeared, it was found to be one of the group of minor planets, and the hunter put it into his bag.

Quite a succession of planet-hunters appeared, some of them little known for any other astronomical work. The most successful of these in the fifties was Gold​schmidt, of Paris, a jeweller if I mistake not. Three were discovered by Professor James Ferguson at the Washington Observatory. Palisa, of Vienna, made a record for himself in this work. In this country Professors C. H. F. Peters, of Clinton, and James C. Watson, of Ann Arbor, were very successful. The last three observers carried the number above the two hundred mark.

About 1890 the photographic art was found to offer a much easier and more effective means of finding these objects. The astronomer would point his telescope at the sky and photograph the stars with a pretty long exposure, perhaps half an hour, more or less. The stars proper would be taken on the negative as small round dots. But if a planet happened to be among them it would be in motion, and thus its picture would be taken as a short line, and not as a dot. Instead of scanning the heavens the observer had only to scan his photographic plate, a much easier task, because the planet could be recognised at once by its trail.

Recently a dozen or more of these bodies have been found nearly every year. Of course the unknown ones are smaller and more difficult to find as the years elapse. But there is as yet no sign of a limit to the number. Most of those newly discovered are very minute, yet the number seems to increase with their smallness. Even the larger of these bodies are so small that they appear only as star-like points in ordinary telescopes, and their disks are hard to make out even with the most powerful instruments. So far as can be determined, ​the diameters of the largest ones, naturally the earliest discovered, are only three or four hundred miles. The size of the smallest can be inferred only in a rough way from their brightness. They may be twenty or thirty miles in diameter.

The orbits of these bodies are for the most part very eccentric. In the case of Polyhymnia, the eccentricity is about 0.33, which means that at perihelion it is one third nearer the sun than its mean distance, and at aphelion one third more. It happens that its mean distance is just about three astronomical units; its least distance from the sun is therefore two, its greatest four, or twice as great as the least.

The large inclination of most of the orbits is also noteworthy. In several cases it exceeds twenty degrees, in that of Pallas it is twenty-eight degrees.

Olbers' idea that these bodies might be fragments of a planet which had been shattered by some explosion is now abandoned. The orbits range through too wide a space ever to have joined, as they would have done if the asteroids had once formed a single body. In the philosophy of our time these bodies have been as we see them since the beginning. On the theory of the nebular hypothesis the matter of all the planets once formed rings of nebulous substance moving round the sun. In the case of all the other planets the material of these rings gradually gathered around the densest point of the ring, thus agglomerating into a single body. But ​it is supposed that the ring forming the minor planets did not collect in this way, but separated into innumerable fragments.

There is a curious feature of the orbits of these bodies which may throw some light on the question of their origin. I have explained that the planetary orbits are nearly exact circles, but that these circles are not centred on the sun. Now imagine ourselves to look down upon the solar system from an immense height, and suppose that the orbits of the minor planets were visible as finely drawn circles.
Fig. 35.—Separation of the Minor Planets into Groups. These circles would appear to interlace and cross each other like an intricate network, filling a broad ring of which the outer diameter would be nearly or quite double the inner one.

But suppose we could pick all these circles up, as if they were made of wire, and centre them all on the sun, without changing their size. The diameters of the larger ones would be double those of the smaller, so that the circles would fill a broad space, as shown in the figure. Now, the curious fact is ​that they would not fill the whole space uniformly, but would be collected into distinct groups. These groups are shown
Fig. 36.—Distribution of the Orbits of the Minor Planets. on the figures of their orbits, given above, and, on a different plan, and more completely, in the second figure, which is arranged on a plan explained as follows: Every planet performs its revolution in a certain number of days, which is greater the farther the planet from the sun. Since the complete circumference of the orbit measures 1,296,000″, it follows that if we divide this number by the time of revolution, the quotient will show through what angle, on the average, the planet moves along its orbit in one day. This angle is called the mean motion of the planet. In the case of the minor planets it ranges from 400″ to more than 1,000″, being greater the shorter the time of revolution and the nearer the planet is to the sun.

Now we draw a vertical line and mark off on it values of the mean motion, from four hundred to one thousand seconds, differing by ten seconds. Between each pair of marks we make as many points as there are planets having mean motions between the limits. For example, between 550″ and 560″ there are three dots. This means ​that there are three planets having mean motions between 550″ and 560″. There are also four planets between 560″ and 570″, and one between 570″ and 580″. Then there are no more till we pass 610″, when we find six planets between 610″ and 620″, followed by a multitude of others.

Examining the diagram we are able to distinguish five or six groups. The outermost one is between 400″ and 460″, and is nearest to Jupiter. The times of revolution are not far from eight years. Then there is a wide gap extending to 560″, when we have a group of ten planets between 540″ and 580″. From this point downwards the planets are more numerous, but we find very sparse or empty points at 700″, 750″, and 900″. Now the most singular feature of the case is that these empty spaces are those in which the motion of a planet would have a simple relation to that of Jupiter. A planet with a mean motion of 900″ would make its circuit round the sun in one third the time that Jupiter does; one of 600″ in half the time; one of 750″ in two fifths of the time. It is a law of celestial mechanics that the orbits of planets having these simple relations to another undergo great changes in the course of time from their action on each other. It was therefore supposed by Kirkwood, who first pointed out these gaps in the series, that they arose because a planet within them could not keep its orbit permanently. But it is curious that there is no gap, but on the contrary, a group of planets whose mean motion is nearly two thirds of that of Jupiter. Hence the view is doubtful.

One of these bodies is so exceptional as to attract our special attention. All the hundreds of minor planets known up to 1898 moved between the orbits of Mars and Jupiter. But in the summer of that year Witt, of Berlin, found a planet which, at perihelion, came far within the orbit of Mars—in fact within fourteen million miles of the orbit of the earth. He named it Eros. The eccentricity of its orbit is so great that at aphelion the planet is considerably outside the orbit of Mars. Moreover the two orbits, that of the planet and of Mars, pass through each other like two links of a chain, so that if the orbits were represented of wire they would hang together.

Owing to the inclination of its orbit, this planet seems to wander far outside the limits of the zodiac. When nearest the earth, as it was in 1900, it was for a time so far north that it never set in our middle latitudes, and passed the meridian north of the zenith. This peculiarity of its motion was doubtless one reason why it was not found sooner. During its near approach in the winter of 1900-‘01 it was closely scrutinised and found to vary in brightness from hour to hour. Careful observation showed that these changes went through a regular period of about two and a half hours. At this interval it would fade away a little with great uniformity. Some observers maintained that it was fainter at every alternate minimum of light, so that the real period was five hours. It was supposed that this indi​cated that the object was really made up of two bodies revolving round each other — perhaps actually joined into one. But it seems more likely that the variations of light were due to there being light and dark regions on the surface of the little planet, which therefore changed in brightness according as bright or dark regions predominated on the surface of the hemisphere turned toward us. The case was made perplexing by the gradual disappearance of the variations after they had been well established by months of observation. There seems to be some mystery in the constitution of this body.

From a scientific point of view Eros is most interesting because, coming so near the earth from time to time, its distance may be measured with great precision, and the distance of the sun as well as the dimensions of the whole solar system thus fixed with greater exactness than by any other method. Unfortunately, the nearest approaches occur only at very long intervals. What is most tantalising is that there was such an approach in 1892 before the object was recognised. At that time it was photographed a number of times at the Harvard Observatory, but was lost in the mass of stars by which it was surrounded. Its distance was, astronomically, only sixteen hundredths, or some fifteen millions of miles, while the nearest approaches of Mars are nearly forty millions. There will not be another approach so near for more than sixty, perhaps not for more than a hundred years.

In 1900 it approached the earth within about thirty ​millions of miles, and a combined effort was made at various observatories to lay down its exact position from night to night among the stars by photography, with a view to determining its parallax. But the planet was faint, the observations were difficult, and it is not yet known what measure of success was reached.

Variations of light which might be due to a rotation on their axes have been suspected in the case of other asteroids besides Eros, but nothing has yet been settled.