CHAPTER VII.

On the most noted Systems invented to explain the Planetary Motions.

ART. 194. THE preceding Chapters contain that part of astronomy relating to the sun and fixed stars, most of which would be obvious even to such observers as did not examine the heavens with much minuteness and attention: but it is time to direct our consideration to the moon, and those other bodies which, though the least remarkable objects in the heavens to a careless and cursory observer, have occupied much the greater share of the industry and ingenuity of astronomers, and have led to the noblest inventions and discoveries which have been enrolled in the annals of science. The bodies we now allude to are the planets, or wandering stars, as their name imports; and indeed their appearances, with respect to an inhabitant of the earth, deviate so far from all our ideas of regularity and order, as to render the epithet wandering strictly applicable, and lead us to infer, did not the language of experience contradict the inference, that the reduction of their motions to any fixed and regular rules, would bid defiance to the utmost efforts of human sagacity. But almost insuperable obstacles vanish when approached with constant assiduity and skill; and the history of astronomy furnishes us with indubitable evidence of the astonishing success which has uniformly followed in the train of persevering diligence and ingenuity.

195. The names of the planets, with their astronomical characters, are thus given: mercury, renus q, mars 8, jupiter 4, and saturn hi these were known to the ancient astronomers.

The

indefatigable attention and care of Dr. Herschel have added another to the number of planets, called by him georgium sidus, or the georgian planet: by some astronomers it is called Herschel, in honour of the discoverer; though among almost all foreigners, it has acquired the name of uranius, which it is likely to retain. Its astronomical character

is *.

196. In our enquiry into the natures, distances, magnitudes, and motions, of these bodies, we shall first state the most usual and striking appearances they exhibit; then describe the most celebrated schemes or systems which have been invented to account for these appearances; and assign the reasons which have induced the modern astronomers to reject all these systems, except what is now distinguished by the title Newtonian, and those which are manifestly included in it, or might lead to its invention.

197. With respect to the moon, we naturally conclude that she is constantly at nearly the same dis tance from the earth; for, since there is no reason to suppose her magnitude varies, as her apparent diameter at the same altitude never varies considerably (being at the greatest 33', and at the least 29' 30"), we may fairly infer that her distance from us is not subject to very great changes. As to her shape, we readily conceive it to be nearly spherical; for, although her appearance to a common observer is that of a flat circle, called the disc, yet, the assistance of telescopes, in union with just analogical reasoning, would induce us at once to reject the opinion of her being flat, should our hasty observations lead us to adopt it. If an opake globe be at a moderate distance from the eye, the picture of it upon the retina

* Since this was written, it is just reported, that another new primary planet has been discovered, viz. by M. Piazzi an Italian astronomer, on Jan. 1, 1801, or the first day of the 19th century. It appears only as a star of the 8th magnitude, and moves in an orbit between mars and jupiter, at 23 times the earth's distance from the sun.

is a circle properly diversified with light and shades so as to excite in the mind the idea of a globe; if á globe be at a great distance from the eye, the differ ence between the lights and shades, which form the picture of a globe, will be imperceptible, and then the body will appear like a circular plane. A luminous globe, though but at a moderate distance from the eye, appears like a luminous plane, as will be evident to any one who makes the experiment with a globe of iron heated red-hot. So that whether the moon be luminous or opake, her distance from us causes her to appear flat, though she be in reality nearly globular.

198. The moon comes to the meridian later every day than the preceding, and the differences of the times of her culminating are very unequal; she also has very different altitudes when on the meridian. She, besides this, is very mutable in her figure, constantly putting on different phases. Sometimes she increases, from little more than an arc of a circle, to a full circular form; then she wanes and diminishes as it were in old age; sometimes she is bent into horns, then she appears like a half circle; then she becomes gibbous or hunchbacked, and next assumes a full round face; after this she gradually declines and loses all her lustre. Sometimes she enlightens us the whole night; at other times she does not appear till late at night, or early in the morning. Some times she keeps in the southern regions of the heavens; at others she rises higher and higher, and visits the northern hemisphere. Changes like these occurring so frequently, the whole of them being gone through in less than thirty days, naturally at tracted attention; and thus the periods of these mutâtions soon became known; though Endymion, the first among the Greeks who carefully studied her motions, by his regular attention, gave rise to the fable that he had fallen in love with her.

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199. The planet Venus, when viewed through a telescope, appears to undergo changes in her phases, similar to those of the moon: sometimes her full disc appears, at other times she appears gibbous, at others horned. Her apparent diameter also, as measured at the same altitude, is subject to considerable variation, being at some periods five or six times greater than at others. She sometimes is found to come to the meridian with the sun, then to precede that luminary in coming to the meridian, and so have an apparent motion from east to west, until her time of arriving at the meridian precedes that of the sun by about 3h 10m, when she seems to be stationary for a short time; then her coming to the meridian precedes that of the sun by less and less degrees, until they are again on the meridian together. From this period she culminates later than the sun, and continues her apparent motion from west to east in a retrograde order, till her culminating is about 3h 10m after that of the sun; at this period she again appears stationary for a short time, after which her apparent motion is from east to west, as it was previous to her being stationary in the former instance. Her declination too is found to vary much, being sometimes more than 27° north, and occupying all the intermediate degrees, till it becomes as great toward the south. All the time when venus appears easterly of the sun she sets after him, and is called the evening star; when she is westward of that luminary, she sets before him in the evening, but shines before he rises in the morning, and is then called the morning star.

200. Observations can but seldom be made on the planet Mercury; but it has been observed suffi ciently to determine that his motions, appearances, and mutations, are very similar to those of venus, with the exceptions that they are performed in a far less space of time, and that the distance of mercury

from the sun is never greater than about 1h 50m in time, or nearly 274°.

201. Mars is found sometimes to come to the meridian with the sun, sometimes to precede him, at others to follow him, and at some to be directly op posite to that luminary, so as to rise when he sets, and set when he rises. When mars is thus opposite to the sun, his apparent diameter is the greatest: being about five times as large as at those periods when he comes to the meridian with the sun.

The appa

rent motion of this planet, like that of mercury and venus, is sometimes direct, or from east to west; at others retrograde, or from west to east; and sometimes the appearance is stationary. Mars also has different phases, appearing, when viewed through a telescope, either full and round, or gibbous, but never horned.

202. The motions of Jupiter and Saturn, compared with those of the sun, are found to be at times direct, at others retrograde, and at others stationary, like those of the other planets. These two bodies are also sometimes in opposition to the sun, as above mentioned respecting mars; and they, as well as the other planets, are sometimes to the north of the ecliptic, and sometimes to the south, but very seldom upon it.

203. To explain and account for these apparent motions of the moon and planets, together with those of the sun, various hypotheses or systems have been invented, some complex and confused, others on simple and general principles. "Systems,' as Dr. Adam Smith ingeniously observes, "in many "respects resemble machines. A machine is a little

system created to perform, as well as to connect "together in reality, those different movements and "effects which the artist has occasion for. A system "is an imaginary machine invented to connect to"gether in the fancy those different movements and