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peared to move daily from east to west, were conceived to be at no grea. distance from it, and to be only designed for its use or ornament. Severa reafons, however, occurred, which rendered this opinion improbable ; is needless to mention them, because we have now a sufficient proof o the figure of the earth, from the voyages of many navigators, who have actually failed round it; as for instance that of Magellan's ship, which was the first that circumnavigated the globe, failing west from a port in Europe in .519, and returning to the same, after a voyage of 1124 days, without altering its direction, except to the north or south, as compelled by the winds, or intervening land.
The spherical figure of the earth being fully proved, the hypothesis of its motion was evidently rendered much more probable. For while it was considered as a plane, mankind had an obscure notion of its being supported, like a scaffolding, on pillars, though they could not tell what supported there. But the figure of a globe is much better adapted to motion; and a very strong, and, in reality, unanswerable, argument for that motion was derived from considering, that, if the earth did not move round the sun, not only the sun, but all the stars and planets, must move round the earth. Now, as astronomers, by reckonings founded on the surest observačions, have been able to ascertain pretty nearly the distances of the heavenly bodies from the earth and from each other, in the same manner as every person acquainted with the first elements of mathematics can measure the height of a steeple, or any object placed on it,-it appeared, that, if we conceived the heavenly bodies to move round the earth, we must suppose them endowed with a motion or velocity so immense as to exceed all conception : whereas all the appearances in nature may be as well explained by imagining the earth to move round the sun in the space of a year, and to turn on its own axis once in 24 hours.
To form a conception of these two motions of the earth, we may ime. gine a ball moving on a billiard table or bowling-green : the ball proceeds forwards upon the green or table, not by Niding along like a plane upon wood, or a llate upon ice, but by turning round its own axis, which is an imaginary line drawn through the centre or middle of the ball, and ending on its surface in two points called its poles. We muft, however, remember that these two motions in the earth are perfectly diftinct, and not imagine that the number of revolutions caused by the rotatory motion is in proportion to the space palled through by the progreisive, as is the cafe with the ball on the table or the bowling.green. The earth, therefore, in the space of 14 hours, moves from weit to east, while the inhabitants on the surface of it, like men on the deck of a ship, who are insensible of their own motion, and think that the banks move from them in a con. trary direction, will conceive that the fun and fiars move from east to west in the same time of 24 hours in which they, along with the earth, move from west to eaft. This daily or diurnal motion of the earth being once clearly conceived, will enable us easily to form a notion of its annual or yearly motion round the fun. For as that Juminary seems to have a diurnal motion round our earth, which is really occasioned by the daily motion of the earth round its own axis, fo, in the course of a year, he seems to have an annual motion in the heavens, and to rise and set in different points of them, which is really occafioned by the annual motiot of the earth in its orbit or path round the fun, which it completes in the fpace of a year. Now, as io the first of these motions we owe the difference of day and night, so tò the second we are indebted for the differcace in the length of the days and nights, and in the seasons of the year.
The Planets.] It will easily be conceived that what has been said with regard to the motion of the earth, is equally applicable to all the other planets. Of these, beside the seven already mentioned, which move round the sun, there are fourteen others which move round four of these, in the same manner as they do round the fun; and of these our earth has one, called the moon ; Jupiter has four; Saturn has seven (two * of, these having been lately discovered by Dr. Herschel); and the Georgium Sidus two, as that excellent astronomer has shown. These are called moons, from their resemblance to our moon; and sometimes secondary planets, because they seem to be attendants of the Earth, Jupiter, Saturn, and the Georgium Sidus, about which they move, and which are called primary
It will be necessary for the understanding of the following table, to explain what is meant by the inclination of the ares of the planets to their orbits, and the mean distunces of the planets. We have already said that the annual motion of the earth occalioned the diversity of seasons. But this would not happen, were the axis of the earth exactly parallel to or in a line with the axis of its orbit : decause then the same parts of the earth would be turned towards the fun in every diurnal revolution ; which would deprive mankind of the grateful vicitlitudes of the seasons, arising from the difference in length of the days and nights. This, therefore, is not the case :—the axis of the earth is inclined to the plane of the earth's orbit, which we may conceive by supposing a spindle put through a ball, with one end of it touching the ground; if we move the ball directly forwards, while one end of the spindle continues to touch the ground, and the other points towards some quarter of the heavens, we may form an idea of the inclination of the earth's axis to its orbit, from the inclination of the spindle to the ground. The fame observation applies to some of the other planets, as may be seen from the table.
In order to understand what is meant by the mean distances of the planets from the sun, we must observe that the orbit, or path which a planet describes, were it to be marked out, would not be quite round or circular, but in the faape of a figure called an elliptis, which, though resembling a circle, is longer than broad. Hence the same planet is not always at the same distance from the fun; and the mean distance of it is that which is exactly betwixt its greatest and least distance. Here follows the table :
See the 8th vol. of the Philosophical Transactions
A TABLE OF THE DIAMETERS, PERIODS, &c. OF THE SE
VERAL PLANETS IN THE SOLAR SYSTEM.
from the sun,
period rotation motion English
of its of axis to round the
in its planets,
cions of the
equa. transit of Ve.
tor. nus, in 1761.
H.D. H. M. Sun 890,000
25 6 0
3,818 30 O' 0" Mercury 3,000 36,841,468 0 87 23 unknown (109,699 unknw unknown Venus 7,906
69,891,486 O 224 1724 8 0 80,295 1,043 15 0 0 Earth 7,970 95,173,0001 1 0 0 1 0 0 68,243 1,042 23 29 Moon 2,180 ditto
1 0) 0129 12 44 22,290 942 10 0 Mars 5,400 145,014,148 1 321 171 1 0 40 55,287 556 0 0 o Jupiter 94,000 494,990,976 11 314 18 0 956 29,083 25,9201 Satum 78,000 907,956,130 29 167 6 0 10 16 22,101 22,400 28 0 Georgium
Sidus 34,217|1,815,912,26083 121 0 unknown | 15,000 unknw unknown
The Georgian planet (or Georgium Sidus) was discovered by Dr. Herschel with his telescope of great size and power, forty feet in length, and four and a half in diameter, in the year 1781. For this discovery he obtained from the Royal Society the honorary recompense of fir Godfrey Copley's medal. Though it was not till then known as a planet, yet there are many reasons to suppose it had been seen before, but had been considered as a fixed star. But, from the steadiness of its light, from its diameter being increased by high magnifying powers, and from the change he had observed in its situation, Dr. Herschel first concluded that it was a comet; but in a little time, he, with others, determined that it was a planet, from its vicinity to the ecliptic, the direction of its motion being stationary in the time, and in such circumstances, as correspond with similar appearances in other planets.-When the moon is absent, it may be seen by the naked eye; and the discovery of two satellites attending it seems to confer upon it a dignity, and to raise it into a more conspicuous situation among the great bodies of our solar system. As the distances of the planets, when marked in miles, are a burden to the memory, astronomers often express their mean distances in a shorter way, by supposing the distance fron the earth to the sun to be divided into ten parts. Mercury may then be estimated at four of such parts from the fun, Venus at seven, the Earth at ten, Mars at fifteen, Jupiter at fiftytwo, Saturn at ninety-five, and the Georgium Sidus at one hundred and ninety.
Comets.] The reader having obtained an idea of the planets from the table, and the previous observations neceffary for understanding it, muft next turn his attention to the comets, which, as they revolve round our sun, are a part of the folar system. Thefe, descending from the far distant parts of the system with great rapidity, surprise us with their_fingular appearance of a train or tail, which accompanies them; become visible to us in the lower parts of their orbits, and, after a short stay, go off again to vaft distances, and disappear. Though some of the ancients had more just notions of them, yet the opinion having prevailed that they were only meteors generated in the air, like to those we see in it
very night, and in a few moments vanishing, no care was taken to observe or record their phænomena accurately, till of late. Hence this part of astronomy is very imperfect. The general doctrine is that they are folid compact bodies, like other planets, and regulated by the fame laws of gravity, so as to describe equal areas in proportional times. by radii drawn to the common centre. They move about the fun in very eccentric ellipses, and are of much greater density than the earth; for some of them are heated in every period
to such a degree as would vitrify or difiipate any substance known to us. Sir Isaac Newton computed the heat of the comet that appeared in the year 1680, when nearest the sun, to be 2000 times hotter than red-hot iron, and that, being thus heated, it must retain its heat till it comes round again, although its period should be more than 20,000 years; and it is computed to be only 575. The number of comets is very much greater than that of the planets which move in the vicinity of the sun. From the reports of historians, as well as from the observations of late years, it has been ascertained that more than 450 have been seen previous to the year 1771; and when the attention of astronomers was called to this object by the expectation of the return of the comet of 1759, no fewer than seven were observed in the course of seven years. From this circumstance, and the probability that all the comets recorded in ancient authors were of confiderable apparent magnitude, while the smaller were overlooked, it is reasonable to conclude that the number of comets considerably exceeds any estimation that might be made from the observations we now poffefs. But the number of those, whose orbits are settled with sufficient accuracy to ascertain their identity when they may appear again, is no more than 59, reckoning as late as the year 1771. The orbits of most of these are inclined to the plane of the ecliptic in large angles, and the greater number of them approached nearer to the sun than to the earth. Their motions in the heavens are not all in the order of the signs, or direct, like those of the planets; but the number whose motion is retrograde is nearly equal to that of those whose motion is direct. All which have been observed, however, have moved through the ethereal regions and the orbits of the planets, without suffering the least fensible resistance in their motions; which sufficiently proves that the planets do not move in folid orbs. Of all the comets, the periods of three only are known with any degree of certainty, being found to return at intervals of 75, 129, and 575 years; and of these, that which appeared in 1680 is the most remarkable. This comet, at its greatest diftance, is about 11 thousand 200 millions of miles from the sun, while its least distance from the centre of the sun is about 1.90 thousand miles ; being less than one third part of the sun's femidiameter from his surface. In that part of its orbit which is nearest the sun, it flies with the amazing velocity of 880,000 miles in an hour; and the sun, as seen from it, appears 100 degrees in breadth, consequently 40,000 times as large as he appears to us. The aftonishing distance that this comet runs out into empty space naturally fuggests to our imagination the valt distance between our fun and the nearest of the fixed liars, of whose attractions all the comets must keep clear, to return periodically and go round the fun, Dr. Halley, to whom every part of astronomy, but this in a particular manner, is highly indebted, has joined his labours to those of fir Isaac Newton on this subject. Our earth was out of the way, when this comet last passed near her orbit : but it requires a more perfect knowledge of the motion of the comet, to be able to judge if it will always pass by us with so little effect; for it may be here observed that the comet, in one part of its orbit, approaches very near to the orbit of our earth ; so that, in some revolutions, it may approach near enough to have very confderable, if not fatal, effe&ts upon it. See Newton, Halley, Gregory, Keil, Maclaurin, Derham, Ferguson, and Whiston.
THE FIXED STARS.] Having thus briefly surveyed the solar system, which, though great in itself, is small in comparison with the immenfity ef the universe, we next proceed to the contemplation of thole other vaft bodies, called the fixed stars, which being of infinite use in the practice of geography, claim a particular notice in this work. These fixed fiars are distinguished by the naked eye from the planets, by being less bright and luminous, and by continually exhibiting that appearance which we call the twinkling of the stars. This arises from their being so extremely small, that the interposition of the least body, of which there are many constantly floating in the air, deprives us of the light of them; when the interpoted body changes its place, we again see the star; and this fucceision being perpetual, occasions the twinkling. But a more remarkable property of the fixed stars, and that from which they have obtained their name, is their never changing their situation, with regard to each other; as the planets, from what we have already faid, muft evidently be always changing theirs. The stars which are nearest to us seem largest, and are therefore called fiars of the first magnitude. Those of the second magnitude appear less, being at a greater distance; and so proceeding on to the sixth magnitude, which includes all the fixed stars that are vible without a telescope. As to their number, though, in a clear winter's night without moonshine, they seem to be innumerable (which is owing to their firong sparkling, and our looking at them in a confused manner), yet when the whole firmament is divided, as it has been by the ancients, into signs and contellations, the number that can at any time be seen with the naked eye, is not above a thousand. Since the invention of telescopes, indeed, the number of the fixed stars has been juftly considered as immense; because the greater perfection we arrive at in our glasses, the more stars always appear to us.' (Mr. Flamsteed, late royal astronomer at Greenwich, has given us a catalogue of about 3000 ftars. These are called telescopic ftars, from their being invisible without the assistance of the telescope. Dr. Herschel, to whose ingenuity and affiduity the altronomical world is so much indebted, has evinced what great discoveries may be made by improvements in the inttruments of observation. In speaking here of his discoveries, I shall use the words of M. de la Lande ;
In passing rapidly over the heavens with his new telescope, the universe “ increased under his eye; 44,000 stars, seen in the space of a few de. «
grees, seemed to indicate that there were seventy-five millions in the “ heavens.” But what are all these, when compared to those that fill the whole expanse, the boundless fields of æther? Indeed the immensity of the universe must contain such numbers as would exceed the utmost stretch of the human imagination ; for who can say how far the universe extends, or point out those limits where the Creator “ stayed his rapid wheels," or where he “fixed the golden compafles ?”
The immense distance of the fixed ftars from our earth, and from each other, is, of all considerations, the most proper for raising our ideas of the works of God. For, notwithstanding the great extent of the earth's orbit or path (which is at least 190 millions of miles in diameter) round the fun, the distance of a fixed star is not sensibly affected by it; so that the ftar does not appear to be any nearer to us when the earth is in that part