    
|
 Helping San Diego, California and beyond since 1997 |
|
|
Helping San Diego, California and beyond since 1997 |
Click here and add this page to your favorites!
|
Encyclopedia Britannica - Main :: PIG-POL |
|
|
PLANETS, MINOR . The minor planets, commonly known as asteroids or planetoids, form a remarkable group of small planetary bodies, of which all the known members but three move between the orbits of Mars and Jupiter. Until recently they were all supposed to be contained within the region just mentioned; but the discovery of one, which at perihelion comes far within the orbit of Mars, and of two others, which at aphelion pass outside the orbit of Jupiter, shows that no well-defined limit can be set to the zone containing them. Before the existence of this group was known, the apparent vacancy in the region occupied by it, as indicated by the arrangement of the planets according to Bode's law, had excited remark and led to the belief that a. planet would eventually be found there. Towards the end of the 18th century the conviction that such a planet existed was so strong that an association of astronomers was formed to search for it. The first discovery of the looked-for planet was not, however, made by any member of this association, but by Giuseppe Piazzi of Palermo. On the 1st of January 18o1 he noted a small star in Taurus, which, two days later, had changed its place, thus showing it to be a planet. Shortly after Piazzi's discovery the body was lost in the rays of the sun, and was not again seen until near the following opposition in 18o11802. The orbit was then computed by C. F. Gauss, who found its mean distance from the sun to correspond with Bode's law, thus giving rise to the impression that the gap in the system was filled up. The planet received the name Ceres. On the 28th of March 1802 H. W. M. Olbers (17581840) discovered a second planet, which was found to move in an orbit a little larger than that of Ceres, but with a very large eccentricity and inclination. This received the name of Pallas. The existence of two planets where only one was expected led Olbers to his celebrated hypothesis that these bodies were fragments of a larger planet which had been shattered by an internal convulsion; and he proposed that search should be made near the common node of the two orbits to see whether other fragments could be found. Within the next few years two other planets of the group were discovered, making four. No others were found for more than a generation; then on the 8th of December 1845 a fifth, Astrea, was discovered by K. L. Hencke of Driesen. The same observer added a sixth
Up to 1890 discoveries of these bodies were made by skilful search with the telescope and the eye. Among the most successful discoverers were Johann Palisa of Vienna, C. H. F. Peters (18131890) of Clinton, New York
recent
Among the distinctive features of the planets of this group one is their small size. None exists which approaches either Mercury or the moon in dimensions. The two largest, Ceres and Juno, present at opposition a visible disk about 1" in diameter, corresponding to about 400 miles. The successively discovered ones naturally have, in the general average, been smaller and smaller. Appearing only as points of light, even in the most powerful telescopes, nothing like a measure of their size is possible. It can only be inferred from their apparent magnitude that the diameters of those now known may range from fifteen or twenty miles upwards to three or four hundred, the great majority being near the lower limit. There is yet no sign of a limit to their number or minuteness. From the in-creasing rate at which new ones approaching the limit of visibility are being discovered, it seems probable that below this limit the number of unknown ones is simply countless; and it may well be that, could samples of the entire group be observed, they would include bodies as small as those which form the meteors which so frequently strike our atmosphere. Such being the case, the question may arise whether the total mass of the group may be so great that its action on the major planets admits of detection. The computations of the probable mass of those known, based upon their probable diameter as concluded from the light which they reflect, have led to the result that theircombined action must be very minute. But it may well be a question whether the total mass of the countless unknown planets may not exceed that of the known. The best answer that can be made to this question is that, unless the smaller members of the group are almost perfectly black, a number great enough to produce any observable effect by their attraction would be visible as a faintly illuminated band in the sky. Such a band is occasionally visible to very keen eyes; but the observations on it are, up to the present time, so few and uncertain that nothing can positively be said on the subject. On the other hand, the faint " Gegenschein" opposite the sun is sometimes regarded as an intensification of this supposed band of light, due to the increased reflection of the sun's light when thrown back perpendicularly (see ZODIACAL LIGHT). But this sup-position, though it may be well founded, does not seem to fit with all the facts. All that can he said is that, while it is possible that the light reflected from the entire group may reach the extreme limit of visibility, it seems scarcely possible that the mass can be such as to produce any measurable effect by its attraction. Another feature of the group is the generally large inclinations and eccentricities of the orbits. Comparatively few of these are either nearly circular or near any common plane. Considering the relations statistically, the best conception of the distribution of the planes of the orbits may be gained by considering the position of their poles on the celestial
celestial
A similar law holds true of the eccentricities and the perihelia. These may both be defined by the position of the centre of the orbit relative to the sun. If a be the mean distance and e the eccentricity of an orbit, the geometry of the ellipse shows that the centre of the orbit is situated at the distance ae from the sun, in the direction of the aphelion of the body. When the centres of the orbits are laid down on a diagram it is found that they are not scattered equally around the sun but around a point lying s in the direction of the centre of the A orbit of Jupiter. The statistical law J governing these may be seen from fig I. Here S represents the position of the sun, and J that of the centre of the orbit of Jupiter. The direction JS roduced is that of the perihelion FIG. 1. of Jupiter, which is now near longitude 12. As the perihelion moves by its secular variation, the line SJ revolves around S. Theory then shows that for every asteroid there will be a certain point A near the line SJ and moving with it. Let C be the actual position of the centre of the planetoid. Theory shows that C is in motion around A as a centre in the direction shown by the arrow, the linear eccentricity ae being represented by the line SC. It follows that e will be at a minimum when AC passes through S, and at a maximum when in the opposite direction. The position of A is different in the case of different planetoids, but is generally about two-thirds of the way from S to J. The lines AC for different bodies are at any time scattered miscellaneously around the region A as a centre. AC may be called the constant of eccentricity of the planetoid, while SC represents its actual but varying eccentricity, C Grouping of the Planetoids.A curious feature of these bodies is that when they are classified according to their distances from the sun a tendency is seen to cluster into groups. Since the mean distance and mean motion of each planet are connected by Kepler's third law, it follows that this grouping may also be described as a tendency toward certain times of revolution or certain values of the mean motion around the sun. This feature was first noticed by D. Kirkwood in 187o, but at that time the number of planetoids known was not sufficient to bring out its true nature. The seeming fact pointed out by Kirkwood was that, when these bodies are arranged in the order of their mean motions, there are found to be gaps in the series at those points where the mean motion is commensurable with that of Jupiter; that is to say, there seem to be no mean daily motions near the values 598", 748" and 898", which are respectively 2, 21 and 3 times that of Jupiter. Such mean motions are nearly commensurable with that of Jupiter, and it is shown in celestial mechanics that when they exist the perturbations of the planet by Jupiter will be very large. It was therefore supposed that if the commensurability should be exact the orbit of the planet would be unstable. But it is now known that such is not the case, and that the only effect of even an exact commensurability would be a libration of long period in the mean motion of the planetoid. The gaps cannot therefore be ac-counted for on what seemed to be the plausible supposition that the bodies required to fill these gaps originally existed but were thrown out of their orbits by the action of Jupiter. The fact can now be more precisely stated by saying that we have not so much a broken series as a tendency to an accumulation of orbits between the points of commensurability. The law in question can be most readily shown in a graphical form. In fig. 2 the horizontal line represents distances from the sun,limits of the groups shown in the figure. Eros is so near the sun, and its orbit is so eccentric, that at perihelion it is only about o.16 outside the orbit of the earth. On those rare occasions when the earth is passing the perihelion point of the orbit at nearly the same time with Eros itself, the parallax
parallax
A few of the minor planets are of such special interest
... . 3.0 &6 3.4 313 312 3.1 3.0 2.9 2.8 2.7 '~0 FIG. 2. 21-5 21-4 2.3 2.2 2 1 2.0 increasing toward the left, of which certain equidistant numerical values are given below the line. Points on the line corresponding to each oor of the distances are then taken, and at each point a perpendicular line of dots is drawn
Continuing the question beyond these large collections, it will be seen that between the values 3.22 and 3'33 there are no orbits at all. Then between 3.3 and 3.5 there are nine orbits. The space between 3.5 and 3.9 is thus far a complete blank; then there are three orbits between 3.90 and 3.95, not shown in the diagram. A group of great interest
Several planetoids of much interest are situated without thenear that of the earth. With most of the others little more can be done than to compute their elements with a view of subsequently identifying the object when desired. Unless followed up at several oppositions after discovery, the planet is liable to be quite lost. Of those discovered before 1890 about fifteen have not again been found, so that if discovered, as they doubt-less will be, identification will be difficult. The system of nomenclature of these bodies is not free from difficulty. When discoveries began to go on at a rapid rate, the system was introduced of assigning to each a number, in the order of its discovery, and using as its symbol its number enclosed in a circle. Thus Ceres was designated by the symbol(); Pallas by , &c., in regular order. This system has been continued to the present time. When photography was applied to the search it was frequently doubtful whether the planet of which the image was detected on the plates was or was not previously known. This led to the use of capital letters in alphabetical order as a temporary designation. When the alphabet was exhausted a second letter was added. Thus there are planetoids temporarily designated as A, B, &c., and AB, AC, &c. The practice of applying a name to be selected by the discoverer has also been continued to the present time. Originally the names were selected from those of the gods or goddesses of classical mythology, but these have been so far exhausted that the name is now left to the discretion of the person selecting it. At present it is customary to use both the number and the name, the former being necessary to the ready finding of the planetoid in a list
End of Article: PLANETS, MINOR If you wish, you can link directly to this article.
<a href="http://jcsm.org/StudyCenter/Encyclopedia/PIG_POL/PLANETS_MINOR.html"> PLANETS, MINOR </a> |
|
|
(Previous) PLANET (Gr. ssXavirrns, a wanderer) |
(Next) PLANK |
Jesus Christ Saves Ministries, P.O. Box 70696, Pasadena, CA 91117JCSM is a 501(c)(3), non-profit organization. Copyright © 1997-present. |
Free & Cheap Cell
Phones |
Cheap Long Distance
Phone Service Carriers |
Talk America Local Phone Service
|
Ztel & MCI - Unlimited Long Distance
Compare
Cell Phone Plans & Companies |
International Calling Cards & Prepaid Phone Cards |
Voice Over IP Broadband Internet Phone
Service | Wireless
Phone Plans & Cheap Cell Phones
|
_____________________________________________________________________________
