Chapter V: Fourth Day (4)

_Mars_ is the first of the four superior planets in order from the sun, his orbit being immediately beyond that of the earth. He was called by the Greeks Αρης, the supposed god of war, which, says Parkhurst, comes from the Hebrew עריץ _violent_, _destructive_. Costard remarks, “This planet, I suppose, was called ערע _Ara_, ערץ _Aretz_, Αρης, and, in another dialect, מערץ _Ma-aretz_, or _Mars_, in a softer pronunciation, from his _strong glowing brightness_.” He is distinguished from the other planets by the red and fiery appearance, of his disk: whether his ruddy troubled color arises from a natural disposition to reflect the red rays of light best, or from a thick atmosphere attending it, is rather uncertain; but it is generally attributed to the density of his atmosphere.

“In larger circuit rolls the orb of Mars,
Guiltless of stern debate, and wasteful wars,
As some have erring taught: he journies on,
Impell’d and nourish’d by the attractive sun;
Like us, his seasons and his days he owes
To the vast bounty which from Phœbus flows.”

His figure, like that of the earth, is an oblate spheroid. His mean distance from the sun is 145,000,000 miles, and he travels round that common centre of gravity in about 687 of our days, or 1 year, 321 days, 22 hours, 18 minutes, 27 seconds, which is nearly equal to two of our years; and therefore his velocity in his orbit is at the rate of 55,000 miles an hour. He has likewise a rotation upon his axis, which is performed in 1 day, 39 minutes, 22 seconds. This was discovered by means of spots seen on his surface. Dr. Hook, in 1665, observed several spots, which, having a motion, he concluded that the planet revolved upon its axis. In 1666, M. Cassini saw several spots in the two hemispheres of Mars, which, by continuing his observations very diligently, he found to move from east to west, and to return in the space of 24 hours, 40 minutes, to their former situation. Whence both the motion and period, or natural day of this planet, were determined. In 1781, Dr. Herschell observed the spots of Mars very minutely, from the motion of which he has found his rotation upon his axis to be performed in 24 hours, 39 minutes, 21-2/3 seconds; and he says that there cannot be more than two seconds of uncertainty in this result. The different seasons will take place on this planet very much like what they are known to do upon our earth, with this difference, that the seasons there will be almost as long again as with us, on account of the time he takes in moving round the sun being nearly twice as long as our year. The diameter of Mars being 4,135 miles, he is about 2/11, or less than a fifth, and more than a sixth part as large as the earth; and if any moon attend him, she must be very small, for it has not yet been discovered by the best telescopes of our most eminent astronomers; if without a moon, walking his round in perpetual solitude, he must consequently want that division of time, which, from the moon’s revolution round the earth, is called a month.

From the greater distance of Mars in his orbit than our earth is, the inhabitants there will scarcely see Mercury, unless it be when he appears on the sun’s face, and passes over him like a dark spot, in the same manner as he sometimes does to us. Venus will to them appear somewhat similar to the appearances of Mercury to our earth, the apparent distance from the sun being nearly the same to them as Mercury is to us. Our earth to them, also, will be an inferior planet, or within his orbit, being nearer to the sun, in a way similar to what Venus appears to us, and will alternately be a morning or evening star; and our moon, which will always be seen to accompany her, when in a position to have the benefit of the sun’s light, will not be seen at a greater distance, than about a semi-diameter of the sun or moon from it.

This planet being half as far again from the sun as our earth is, his light and heat are not half so much as our own. When in opposition to the sun, he is found to be five times nearer to us than when in conjunction; and, therefore, he appears so much bigger and brighter at one time than another. In 1719, his apparent magnitude and brightness were so much increased, that, by the uninformed, he was taken for a new star.

The telescopic appearance of Mars is very variable. This planet exhibits larger and more remarkable spots than any of the others. The belts and cloudy appearances are found to change their shape and arrangement frequently. The predominant brightness of the polar regions leads to the supposition that those parts of his surface, like the poles of the earth, are intensely frozen, or always covered with snow; and Dr. Herschell imagines that the changes in brightness are connected with the summer and winter seasons on that planet. The phases of Mars were first discovered by Galileo. Having his light from the sun, and revolving round it, he has an increase and decrease like the moon. At his quadratures, he appears gibbous, but never horned, like Venus, Mercury, and the Moon; which shows, that his orbit includes that of the earth, and that it is from the sun that he receives his light.

Between the orbit of Mars and that of Jupiter, the smaller planetary bodies, lately discovered, revolve. _Ceres_ was discovered on the 1st of January, 1801, by M. Piazzi, astronomer at Palermo, in the island of Sicily. When viewed through a good telescope, it is of a ruddy color, appears to be of the size of a star of the eighth magnitude, and surrounded with a dense atmosphere. Her mean distance from the sun is 260,000,000 miles; and her revolution is performed in 4 years, 7 months, 10 days. Dr. Herschell and Schroëter differ very much as to the magnitude of this planet; the former says the diameter is only 160 miles, but the latter makes it more than ten times greater, or 1,624 miles. _Pallas_ was discovered on the 28th of March, 1802, by Dr. Olbers, of Bremen. Its mean distance from the sun 270,000,000 miles; its diameter 80 miles; and it performs its revolution in about 4 years, 280 days. _Juno_ was discovered on the 1st of September, 1804, by M. Harding, of Lilienthal. Its mean distance from the sun is 290,000,000 miles; and its diameter is 119 miles, and the time of revolution round the sun 5 years, 181 days. _Vesta_ was discovered by Dr. Olbers, on the 29th of March, 1807. It is nearer to Mars than either of the other newly discovered planets; and the revolution through its orbit is performed in less time. The size of this planet is not known. Its light is more intense, pure, and white, than any of the other three.

A century and half ago it was conjectured, says a very intelligent author, that there must be a planet between the orbits of Jupiter and Mars, on account of the distance subsisting between those two planets. The discovery of Ceres confirmed this happy conjecture; but the opinion which it seemed to establish respecting the harmony of the solar system, appeared to be completely overturned by the discovery of Pallas and Juno. Dr. Olbers, willing to find a theory that should account for the facts newly ascertained, imagined that these small celestial bodies were merely the fragments of a larger planet, which had burst asunder by some internal convulsion, and that several more might yet be discovered between the orbits of Mars and Jupiter. He therefore concluded, that though the orbits of all these fragments might be differently inclined to the ecliptic, yet, as they must have all diverged from the same point, they ought to have two common points of re-union, or two nodes in opposite regions of the heavens, through which all the planetary fragments must sooner or later pass. One of these nodes Dr. Olbers found to be in Virgo, and the other in the Whale; and it was actually in the latter of these regions that M. Harding discovered the planet Juno. With the intention, therefore, of detecting other fragments of the supposed planet, Dr. Olbers examined, thrice every year, all the little stars in the opposite constellation of the Virgin and the Whale, till his labors were crowned with success, by the discovery of a new planet in the constellation of Virgo, to which he gave the name of Vesta.

The existence of four planets between the orbits of Mars and Jupiter, (continues the same author,) revolving round the sun at nearly the same distances, and differing from all the other planets in their diminutive size, and in the form and position of their orbits, is acknowledged to be one of the most singular phenomena in the history of astronomy. The discordance of these phenomena with the regularity of the planetary distances, and with the general harmony of the system, naturally suggests the opinion, that the inequalities in this part of the system were produced by some great convulsion, and that the four planets, as we have already hinted, are the fragments of a large celestial body, which once existed between Mars and Jupiter. To suppose them independent planets, as they must necessarily be if they did not originally form one, their diminutive size, the great eccentricity and inclination of their orbits, and their numerous intersections, when projected on the plane of the ecliptic, are phenomena absolutely inexplicable on every principle of science, and subversive of that harmony and order which before the discovery of these bodies, seemed to pervade the planetary system. Admitting, however, the hypothesis that these planetary bodies, are the remains of a larger body, which circulated round the sun, nearly in the orbit of the greatest fragment, the system resumes its order, and we discover a regular procession in the distances of the planets, and a general harmony in the form and position of their orbits. But, independently of analogical reasoning, the elements of the new planets furnish several direct arguments, drawn from the eccentricity and inclination of their orbits, and from the position of their perihelia and nodes; and all concurring to show, that the four new planets have diverged from one point, and have, therefore, been originally combined in a larger body.

_Jupiter_ is the largest of all the planetary bodies, and, next to Venus the brightest. He was called by the Greeks Ζευς, which is from ζεω, _to be hot_, or, says Parkhurst, immediately from the Hebrew זי _to shine_, compounded, perhaps, with יש _substance_, q.d. _the shining substance_; a name very justly given to this planet, on account of his strong and clear light.

----“In distant skies
Revolves the mighty magnitude of Jove,
With kingly state, the rival of the sun.”

His mean distance from the sun is 490,000,000 miles, and his diameter is 89,170 miles, or more than 11 times that of the earth, and therefore his magnitude is 1,400 times greater than our earth; of course, as the surface of a globe increases according to the square of its diameter, our earth will, to the inhabitants of Jupiter, appear 121 times less than this noble planet appears to us. His revolution round the sun, from east to west, is performed in 11 years, 315 days, 14 hours, 39 minutes, 2 seconds, which is nearly twelve of our years; and his motion in his orbit is 29,000 miles an hour. He performs his diurnal rotation upon his axis in 9 hours, 55 minutes, 33 seconds, by which motion his equatorial parts are carried round at the amazing rate of 26,000 miles an hour, which is about twenty-five times the velocity of the like parts of our earth. He has, of course, a rapid succession of days, as the poet observes,

“In ample compass Jove conducts his sphere,
And later finishes his tedious year;
Yet swiftly on his axle turn’d, regains
The frequent aid of day to warm his plains.”

The axis of Jupiter is nearly perpendicular to his orbit, so that he has no sensible change of seasons. This is not the work of chance, as Dr. O. Gregory observes, but wisely ordered by the Divine Architect; for if the axis of this planet were inclined any considerable number of degrees, so many degrees round each pole would be almost six years together in darkness. And as each degree of a great circle on this planet contains more than 700 miles, it is natural to conceive, that vast tracts of land would be rendered uninhabitable by any considerable inclination of his axis.

The appearance of this planet, through a telescope, opens a vast field for interesting inquiry. His surface is not equally bright, but variegated with certain bands, or belts, of a dusky appearance: they run parallel to each other, and are continued round the body of the planet. They are not regular or constant in their appearance: sometimes only one is seen; at other times six or eight. The breadth of them is likewise variable; one belt is sometimes becoming narrow, while another, in its neighborhood, grows broader as if one had flowed into the other: in these cases, an oblique belt has been observed to be between them, as if for the purpose of establishing a communication. Sometimes, one or more spots are formed between the belts, which increase till the whole is united in a large dusky belt. There are also bright spots to be discovered on Jupiter’s surface; these are rather more permanent than the belts, and re-appear after unequal intervals of time. The remarkable spot, by whose motion the rotation of Jupiter upon his own axis was first ascertained, disappeared in the year 1694, and was not seen again till 1708, when it re-appeared exactly in the same place, and has been occasionally seen ever since.

Jupiter is enlightened by four moons, or satellites, each of them larger than that with which we are supplied, and which revolve at different distances from that planet. In the solar system the moons, or satellites, revolve round their respective primary planets as centres, in the same manner as the primary planets revolve round the sun. By means of Jupiter’s satellites, a method has been obtained for demonstrating that the motion of light is progressive, and not instantaneous, as was formerly supposed; which discovery is important to the interests of science. M. Huygens, in his Treatise on Light, concludes from these eclipses, that light transmits itself about 600,000 times faster than sound.

Distances and Revolutions of Jupiter’s Satellites.

Revolution.
Distance. _d._ _h._ _m._
1st Satellite 250,000 1 18 36
2d 401,000 3 13 15
3d 648,000 7 3 59
4th 1,128,000 16 18 30

They are thus referred to by Mallet:

“About him round _four_ planetary moons,
On earth with wonder all night long beheld,
Moon above moon, his fair attendants dance.”

To a spectator placed on the surface of Jupiter, each of the satellites would put on the phases of the moon; but as the distance of any of them from Jupiter is but small, when compared with the distance of that planet from the sun, the satellites are therefore illuminated by the sun very nearly in the same manner with the primary itself; hence they appear to us always round, having constantly the greatest part of their enlightened half turned towards the earth: and indeed they are so small, that were they to put on the phases of the moon, these phases could scarcely be discerned through the best telescopes. When the satellites pass through their inferior semicircles, they may cast a shadow upon their primary, and thus cause an eclipse of the sun to his inhabitants; and in some situations this shadow may be observed going before or following the satellite. On the other hand, in passing through their superior semicircles, the satellites may be eclipsed in the same manner as our moon by passing through the shadow of Jupiter: and this is actually the case with the first, second, and third; but the fourth, by reason of the extent of its orbit, passes sometimes above or below the shadow, as is the case with our moon.

These satellites were first discovered on the 7th of January, 1610, by the celebrated Galileo, who called them _Medician Stars_, in honor of the family of the Medici, dukes of Tuscany, his patrons. These satellites, revolving about Jupiter at different distances, from west to east, when viewed through a telescope, make a beautiful appearance. As our moon revolves round the earth, enlightening the nights, by reflecting the light she receives from the sun; so these satellites, revolving round Jupiter, may also be supposed to enlighten the nights of that planet.

_Saturn_ is a very conspicuous planet, though he shines with a pale and feeble light, very unlike that of Jupiter and the other planets. He was called by the Greeks φαινων. “From the account given by Diodorus Siculus,” says Costard, “it seems as if the Chaldeans called this planet by some name not widely different from this of the Greeks. In the language of Chaldea, the verb פנא _phana_, or פנה _phanah_, signifies _convertere se_, _divertere se_, _declinare_. And whatever _vanishes_, or _disappears_, very properly _declines_, or _turns aside_, from our view. This planet, therefore, was most probably called פן _phen_, or פין _phain_, and, with a Greek termination, φαινων, on account of his _withdrawing_ himself, by reason of his distance. And this conjecture is yet further confirmed from his name in another dialect, or among another people. For from סתר _sater, latuit, abscondit se_, with the paragogic ן _nun_ which is not unusual in the formation of Eastern words, comes the word סתרן _Saturn_, and with the Latin termination _us, Saturnus_.”

His mean distance from the sun is 900,000,000 miles, consequently his motion in his orbit is proportionably slow; and his annual revolution round the sun, from west to east, being so much longer likewise than that of the other planets, he takes 29 years, 164 days, 7 hours, 21 minutes, 50 seconds, which is almost _thirty_ of our years, to accomplish it, in his orbit travelling with a velocity of 22,000 miles an hour. His diameter is 79,000 miles; and his magnitude is about 1,000 times that of the earth. The time of rotation upon his axis is 10 hours, 17 minutes.

“Still further off, scarce warm’d by Phœbus’ ray,
Through his wide orbit, Saturn wheels away;
How great the change, could we be wafted there!
How slow the seasons! and how long the year!”

There is a singular and curious appendage to Saturn, namely, a thin, broad, opake ring, encompassing the body of the planet, without touching it; like the horizon of an artificial globe; it appears to be suspended round the planet, and to keep its place without any immediate connection with it. The distance of this prodigious circle from the body of the planet is usually stated to be about 21,000 miles.

The dimensions of the ring, or of the two rings with the space between them, Dr. Herschell has given as follows:

Miles.
Inner diameter of the smaller ring 146,345
Outside diameter of ditto 184,393
Inner diameter of the larger ring 190,248
Outside diameter of ditto 204,883
Breadth of the inner ring 20,000
Breadth of the outer ring 7,200
Breadth of the vacant space, or dark zone 2,839

It puts on different appearances to us, sometimes being seen quite open, or as a wide oval, and at others, only as a single line. When our eye is in the plane of the ring, or looking at it directly on the edge, it is invisible to us; and it is in this situation twice in each revolution of the planet; that is, once in about fifteen years: at these times, he appears quite round, for nine or ten months together. The ring was invisible to us on the 15th of June, 1803, and, since that time, gradually increased in light and breadth for about seven years: and, after which, has again decreased, till, as before, after an interval of fifteen years, in the present year 1818, the ring is again edgewise to us, and invisible. With telescopes of great magnifying power, two belts or stripes have been discovered on Saturn; they appear parallel to the ring, and are supposed to be permanent. Of what component materials this ring is composed, or by what means it is suspended, we as yet remain ignorant: but of its use, it is supposed to supply light and heat to the planet, agreeably to the observation of a poet who has evinced an extensive acquaintance with philosophy.

“Muse! raise thy voice, mysterious truth to sing,
How o’er the copious orb a lucid ring,
Opake and broad, is seen its arch to spread,
Round the big globe at stated periods led;
Perhaps (its use unknown) with gather’d heat
To aid the regions of that gelid seat,
The want of nearer Phœbus to supply,
And warm with reflex beams his summer sky;
Else might the high-plac’d world, expos’d to frost,
Lie waste, in one eternal winter lost.”

Besides the ring, Saturn is also furnished with seven attendant moons, or satellites, which move around him at different distances, in a way similar to those of Jupiter.

Distances and Revolutions of Saturn’s Satellites.

Revolution.
Distance. _d._ _h._ _m._ _s._
1st Satellite 172,000 1 21 18 26
2d 217,000 2 17 44 51
3d 315,000 4 12 25 11
4th 705,000 15 22 41 14
5th 2,126,000 79 7 53 42
6th 137,000 1 8 53 9
7th 107,000 0 22 37 30

The sixth and seventh satellites were discovered by Dr. Herschell in 1787 and 1788: they are nearer to Saturn than any of the other five; but, to prevent confusion, they have been called the 6th and 7th. The 5th satellite has been observed by Dr. Herschell to turn once round its axis, exactly in the time in which it revolves round Saturn: in this respect it resembles our moon. Their distance from us is so far, as not to be easily visible, even with a good telescope, unless the air be exceedingly clear.

It was for ages that astronomical science limited the solar system to six planets, and Saturn was considered as its utmost extent. Vitruvius, speaking of the planet Saturn, says, that star “is near the extremity of the world, and touches the frozen regions of heaven.” He did not understand the extent of our planetary system.

It is to the indefatigable application of Dr. Herschell that we are indebted for the discovery of a new planet, which is the fourth of the superior ones then known, and, being at twice the distance of Saturn from the sun, has quadrupled the bounds formerly assigned to the solar system. This planet was discovered on the 13th of March, 1781, and is called by different names: the discoverer bestowed upon it that of _Georgium Sidus_, in honor of our present venerable and beloved sovereign; by the French it is called _Herschell_, and by the Italians, _Uranus_. This important discovery is very deservedly noticed by the Poet Laureat, in his Ode entitled “Carmen Seculare for the year 1800.”

“Mathesis with uplifted eye,
Tracing the wonders of the sky,
Beholds new constellations rise,
New systems crown the argent skies;
Views with new lustre round the glowing pole,
Wide his stupendous orb the _Georgian Planet_ roll.”

On the 11th January, 1787, Dr. Herschell discovered the second and fourth satellites which attend his own planet the Georgium Sidus; and in the following years, previously to 1791, he observed four others revolving round the same body. Though this celebrated astronomer was the first who discovered the Georgium Sidus to be one of the planets of the solar system, yet no doubt can be entertained of its having been before observed and considered as a fixed star. Flamsteed in 1690, Mayer in 1756, and Monnier in 1769, determined the places of three stars which cannot now be found. And M. La Place, according to his theory of Jupiter and Saturn, has found that the Georgium Sidus was _exactly_ in those three points at those very times. These truly singular occurrences leave no doubt of the identity of these three stars with the new planet. The lines which Mallet applied to Saturn are now, with a little alteration, more applicable to the Georgium Sidus, or Herschell planet.

“Last, outmost Herschell walks his frontier round,
The boundary of worlds; with his pale moons,
Faint-glimmering through the darkness night has thrown,
Deep-dy’d and dead, o’er this chill globe forlorn:
An endless desert, where extreme of cold
Eternal sits, as in his native seat,
On wintry hills of never-thawing ice;
Such Herschell’s earth.”

His mean distance from the sun is about 1,800,000,000 miles, and he performs his revolution from west to east round the sun in 83 years, 294 days, 8 hours, 39 minutes; and in his orbit he moves with a velocity of 15,846 miles an hour. His diameter is 4½ times larger than that of the earth, being more than 35,000 miles; and his magnitude is 80½ times larger than that of the earth. The orbit in which he revolves is nineteen times further from the sun than the earth’s orbit; consequently he has 361 times less light and heat from the sun than we have. Notwithstanding this, his proportion of light is considerable; for having been calculated, it is found to be equal to the effect of 284 of our full moons. When the sky is very serene and clear, and the moon absent, this planet may be perceived with the naked eye, unassisted by a telescope: and it appears as a star of the fifth magnitude, with a blueish white light, and a brilliancy between that of Venus and the Moon.

The want of light arising from the great distance of this planet from the sun, is supplied by six satellites, which revolve at different distances round their primary.

Distances and Revolutions of the Satellites of the Georgium Sidus.

Revolution.
Distance. _d._ _h._ _m._ _s._
1st Satellite 226,450 5 21 25 0
2d 293,053 8 17 0 0
3d 342,784 10 23 0 0
4th 392,514 13 11 0 0
5th 785,028 38 1 49 0
6th 1,570,057 107 7 35 10

All these satellites, it has been said, perform their revolutions in their orbits contrary to the order of the signs; that is, their real motion is retrograde, but probably, as suggested by Dr. Hutton, this is an optical illusion.[130] As the indefatigable Dr. Herschell has already discovered six satellites belonging to this planet, does not its immense distance from the sun leave some ground for conjecture, that there may remain some undiscovered, and that his attendants are as numerous, if not more so, than those of Saturn?

Characters used for the Sun, Moon, and Planets.

☉ The Sun
☽ The Moon
☿ Mercury
♀ Venus
⊕ The Earth
♂ Mars
♃ Jupiter
♄ Saturn
♅ Herschell, or Georgium Sidus.

The mark which characterises the planet Herschell is the initial of the discoverer’s name, intersected by a cross bar to represent a cross, by which to denote that the discovery of the planet took place after the birth of Christ.

Astronomy produces calculations concerning the magnitudes, distances, and revolutions of the planets, and their respective satellites, which, to the uninformed, appear absurd, chimerical, and presumptuous; while, probably, they laugh at such notions as were received among men, when even the wisest of them were weak enough to believe, that the earth was an immense plain, situated in the centre of the universe; that the vault of heaven was of crystal; and that the sun was no other than a plate of red hot iron, about as large as the Peloponnessus. The following thoughts, communicated by my much esteemed friend Thomas Exley, A. M. may assist such persons to entertain more favorable sentiments of the science of astronomy, and also serve to enlarge their views of the Supreme Being.

“Many persons who have not had the advantages of proper instruction in mathematical science, cannot be persuaded that it is in the power of man to ascertain the distances of the sun, moon, and planets, and, of course, pay little regard to the assertions of astronomers on this subject. Sometimes, they are bold enough to say the thing is impossible, because no one has ever been to any of those bodies. Let such persons consider, that it is not necessary to go to a remote object in order to measure its distance; for that purpose, it will be sufficient to know the length of a line at the place of the spectator, and the inclination of this line to two others directed from its extremities to the object; for, on the length of this line, and the position of the two others, depends the distance of the object from the ends of that line.

“Thus, if I wish to know the distance of a neighboring tower, or other object beyond a river, or in some other way inaccessible; I measure any convenient line terminating in my station, and by some instrument proper for measuring angles, I ascertain the position of my measured line to the lines connecting its extreme points and the object. On these data depends the distance, and from this line and these angles accurately measured, the exact distance may be with great ease truly found. It is on similar principles that astronomers investigate the distances of the heavenly bodies. They take as the given or measured line, which may be called the base, some line on the earth, the semi-diameter for instance, as being the most convenient. The angle formed, or rather contained by two lines drawn from the sun or planet to the ends of the semi-diameter of the earth, is called the parallax, because it shows the difference of the apparent situation of the object as seen from the extremities of the semi-diameter, that is, it measures the arc of a great circle in the heavens contained between its two apparent places. Hence to ascertain its parallax, or difference of the apparent place when the object is viewed from the other end of the semi-diameter, becomes a problem of great importance in astronomy; for this being truly discovered, the distance of the planet will be obtained with the utmost exactitude. If any other line besides the semi-diameter of the earth, whose length and position are known, be used as a base, and the parallax in respect of this line be found, the same conclusions will follow. The chief difficulty in this affair arises from the smallness of the angle to be measured, which is a consequence of the greatness of the distance in respect of the earth’s semi-diameter. Several ingenious methods have been proposed and employed by astronomers to discover the distances of the sun and planets, but nothing serves this purpose so well as the transits of Venus over the sun’s disk. At certain periods, which can be foretold by astronomers, this planet passes exactly between us and the sun, and is seen as a dark round spot for some hours, moving in a line across the sun’s face or disk. The observer should be furnished with a good chronometer, or pendulum clock with seconds, to note the time of the transit; and good instruments, to take the apparent diameters of the sun and Venus, and her greatest distance from the sun’s limb while passing over his disk: from these observations, and the known phenomena of the motions of the earth and Venus, the parallax may be found. But if two observers, at very distant places of the earth properly chosen, make these observations, the parallax may be obtained with much greater ease and nicety; because the distance of the apparent tracks of Venus across the sun as seen from the two places, and also the difference of the time of the passage, arises from the parallax of Venus and that of the sun. The two last transits, which happened in the years 1761 and 1769, were carefully observed for this purpose; and it is to the results of these observations that the present astronomers are indebted for their more accurate knowledge of the distances of the planets, and the dimensions of the solar system.

“It should be observed, that if the parallax, and consequently the distance of any one of the planets by any means becomes known, the same is easily obtained for each of the other planets, from the relation which has been clearly discovered to subsist between the periodical times of revolution of the planets round the sun, and their distances from that central luminary. Astronomers have most decidedly proved that the square of the time in which any planet revolves is to the square of the time in which any other revolves, as the cube of the distance of the first, is to the cube of the distance of the other; and since all the times are known from observation, if the distance of any one be determined, there is no difficulty at all to find the distances of all the other planets from the sun.

“It has also been matter of great surprise to the unlearned, that astronomers should pretend to tell the magnitudes of the sun and planets. But this is no difficult problem when the distance is known. The _apparent_ diameter is readily found from observation, and on this and the distance depends the _true_ diameter. If the apparent diameters of two objects be equal, the true diameter of the one will be greater as it is more remote; and the apparent diameter of any object will increase as the distance of it from the observer diminishes. From this every one sees, that a knowledge of the distance of the object is an indispensible element for finding its bulk; and, according to the accuracy of the measure of the distance, will be that of the measure of the magnitude, provided the apparent distance be truly taken; and this, in the present improved state of our instruments, presents no obstacle. There can be no doubt but that astronomers are very near the truth in the numbers which they now give us for expressing the distances and magnitudes of the sun and planets.

“The telescope has been of singular use to the astronomer; it has shown him many phenomena of the heavenly bodies, concerning which he would otherwise have been totally ignorant. It is by the assistance of this noble instrument that we have attained to the knowledge of the rotations of the sun and planets, the phases of Venus and Mercury, Saturn’s ring, and many other particulars exceedingly interesting. The telescope has discovered several planets which otherwise would have revolved in their courses unknown and unnoticed by the inhabitants of this globe; it has informed us that several of the planets have moons moving round them, as our moon revolves round the earth; besides, it has presented to our view an innumerable multitude of fixed stars which without this assistance we should never have seen.

“It is no wonder that great efforts have been made to improve this excellent instrument; these efforts have been attended with great success, and what may be further done in this respect we cannot tell; however, there is a limit to the improvements of the telescope, for after it has attained a certain degree of magnifying power, the motes and vapors in the atmosphere would be so magnified as to occupy its whole field of view, and thus render it a useless incumbrance.”

Who can contemplate the power which produced the solar system, at once so magnificent, beautiful, and delightful, without astonishment and admiration? The planets are kept in a regular motion, and retained in an invariable course round the sun, by the power of this luminary’s attraction or gravity. These bodies have a projectile force, being propelled forwards in a right line, which is the nature of all simple motion; but the sun’s attraction combining with their own projectile force, withdraws them from their rectilineal courses, and preserves the most perfect harmony in the system. This wonderful mechanism was originally impressed on the system by its infinitely wise and omnipotent Creator; to which primary impulse it has with undeviating uniformity adhered, having never suffered in its operations from the greatest distance of space, or intervals of time!

Surely no power less than that which at first gave existence and modification to matter, is equal to the government of the world. The solar orb and the planetary bodies could no more subsist in their present form and order, without a Divine, supporting, and directing hand, than they could at the beginning make themselves. What is that general law or force called _gravitation_, without which the whole frame of nature would soon be dissolved? Is it not a power constantly issuing from the Deity, and which if he should suspend but for one moment, the whole creation would sink into ruins? How inconceivably great and operative must that power be, that is present throughout the universe, with all the heavenly orbs to preserve them in their courses; and on this earth, with every creature, and every particle of matter, to preserve its present form!

In addition to the planets and their satellites, there are _Comets_, which revolve round the sun, and, consequently, are a part of the solar system. They have often a long tail, in appearance resembling hair, issuing from that side which is turned away from the sun. Comets are popularly divided into three kinds, namely, bearded, tailed, and hairy: but this arrangement seems to apply rather to the different circumstances of the same comet, than to the phenomena of several. Thus, when a comet is eastward of the sun, and moves with him, it is said to be bearded, because the light precedes it in the manner of a beard: but when it is westward of him, it is said to be tailed, because the train of light follows it in the manner of a tail: and, lastly, when the sun and comet are diametrically opposite, the earth being between them, the train is hid behind the body of the comet, excepting the extremities, which being broader than the body of the comet, appear round it like a border of hair (_coma_), from which circumstance it is said to be hairy, and is denominated a comet.

Without attending to the variety of opinions which philosophers and astronomers have entertained concerning the nature and use of comets, we may affirm, that they have been considered as alarming phenomena, displayed by the Divine Being to warn mankind of the near approach of some dreadful calamity, such as wars, pestilence, and famine. This opinion prevailed during the dark ages between the decline of the Roman empire, and the dawn of the Reformation. To this apprehension some of our modern poets have alluded in strong and descriptive language. Young says,

“Hast thou ne’er seen the comet’s flaming light?
Th’ illustrious stranger passing, terror sheds
On gazing nations, from his fiery train
Of length enormous; takes his ample round
Through depths of ether; coasts unnumber’d worlds
Of more than solar glory; doubles wide
Heaven’s mighty cape; and then revisits earth,
From the long travel of a thousand years.”

Milton uses still greater strength of language when he compares his hero to a comet:

“Incensed with indignation, Satan stood
Unterrified, and like a comet burn’d
That fines the length of Ophiucus huge
In the arctic sky, and from his horrid hair
Shakes pestilence and war.”

Similar ideas are finely expressed by Savage:

“In fancy’s eye encount’ring armies glare,
And sanguine ensigns wave unfurled in air!
Hence the deep vulgar deem impending fate,
A monarch ruined, or unpeopled state.
Thus comets, dreadful visitants! arise,
To _them_ wild omens, science to the _wise_,
These mark the comet to the sun incline,
While deep red flames around its centre shine!
While its fierce rear a winding trail displays,
And lights all ether with a sweeping blaze!
Or when, compell’d, it flies the torrid zone,
And shoots by worlds unnumbered and unknown;
By worlds, whose people, all aghast with fear,
May view that minister of vengeance near.”

Notwithstanding the present improved state of astronomical science, it is evident that the philosophy of comets is very imperfect. Kepler, though in other respects a very great genius, and to whose useful labors astronomy is deeply indebted, indulged in the most extravagant conjectures; he imagined that the planets were large animals, swimming round the sun: and that the comets were animals still more huge and monstrous, which had been generated in the celestial spaces. Jean Bodin, a learned Frenchman of the 16th century, entertained an opinion, if possible, still more absurd and ridiculous. He maintained that the comets are spirits, which having lived on the earth innumerable ages, and being at last arrived on the confines of death, celebrated their last triumph, or are recalled to the firmament like shining stars! Mr. Whiston was of opinion, that comets are so many hells, appointed in their orbits alternately to carry the damned into the confines of the sun, there to be scorched by its violent heat, and then to return with them beyond the orb of Saturn, there to starve them in those cold and dismal regions. Thus

“Born in an age more curious than devout;
More fond to fix the place of heaven or hell,
Than studious this to shun, or that secure.”[131]

James Bernoulli, in his Systema Cometarum, says, that comets are no other than the satellites of some very distant planet, which is itself invisible to us on account of its vast distance, as are also the satellites, unless when they are in that part of their orbits which is nearest the earth. Comets, according to Des Cartes, were formerly fixed stars: but which becoming gradually covered with maculæ, and at length wholly deprived of their light, cannot keep their places, but are carried off by the vortices of the circumjacent stars; and in proportion to their magnitude and solidity, moved in such a manner as to be brought nearer the orb of Saturn; and thus, coming within reach of the sun’s light, are rendered visible.

Aristotle, Seneca, Plutarch, and others, testify, that the Pythagoreans, and the whole Italian sect, maintained, that a comet was a kind of planet or wandering star, which appeared again after a long interval of time. Hippocrates Chius was of the same opinion as Aristotle informs us. Democritus held also the same opinion, as Seneca tells us in his “Natural Questions;” book vii, chap. 3, “For,” says he, “Democritus, the most curious and subtle of all the ancients, suspected that there were many more stars which moved, meaning by this expression the comets; but he neither established their number, or their names, the courses of the five planets not having as yet been discovered.” Again, Seneca assures us, that Apollonius Myndius, one of the most skilful philosophers in the search of natural causes, asserted, that the Chaldeans reckoned comets among the other wandering stars, and that they knew their courses. Apollonius himself maintained, that a comet was a star of its own kind, as the sun and moon are, but that its course was not yet known; that by its motions it mounts very high in the heavens, and only appears when it descends into the lower part of its orbit. And Seneca himself embraces this opinion in the following truly philosophical words: “I cannot believe,” says he, “that a comet is a fire suddenly kindled, but that it ought to be ranked among the eternal works of nature. A comet has its proper place, and is not easily moved from thence; it goes its course, and is not extinguished, but runs off from us. But you will say, if it were a wandering star it would keep in the zodiac. But who can set one boundary to all the stars? Who can restrain the works of the Divinity to a narrow compass? For each of those bodies, which you imagine to be the only ones that have motion, have very different circles; why, therefore, may there not be some that have peculiar ways of their own, wherein they recede far from the rest? But that their courses may be known, it is necessary to have a collection of all the ancient observations about comets; for their appearances are so rare, that their orbits are not yet determined; nor can we as yet find whether they have their periods, or whether they return again in a certain order.”--“The time will come,” continues he, “wherein these things which are now hid from us will be discovered; which observation, and the diligence of after ages, will find out. For it is not one age that is sufficient for so great matters: the time will be when posterity will wonder that we were ignorant of things so plain; one will arise who will demonstrate in what regions of space the comets wander, why they recede so far from the other planets; how great and what sort of bodies they are.”[132] The period, predicted by Seneca, in the first century of the Christian era, is not yet arrived. “After all that has been done and written on the subject of comets,” says a late writer, “we must confess, that our knowledge of these wandering bodies is still very imperfect.” “It appears to me,” says La Lande, “that almost every thing depends on comets. The only thing that I recommend to my correspondents, is to look after and attend to comets: the knowledge of comets is alone wanting to complete the science of astronomy.”

Several ages elapsed before this prediction of Seneca seemed likely to be fulfilled. Tycho Brahé was the first who attempted to restore the comets to their proper rank in creation. Having diligently observed the comet of 1577, and finding that it had no sensible diurnal parallax, he very properly determined its true place to be among the other revolving bodies in the planetary regions, as appears by his book De Cometa, 1577. And Sir Isaac Newton, from his amazing discoveries, gives the following theory of comets: “They are,” says he, “compact, solid, fixed, and durable bodies; in fact, a kind of planets, which move in very oblique and eccentric orbits, every way with the greatest freedom; persevering in their motions even against the course and direction of the planets: and their tail is a very thin and slender vapor, emitted by the head or nucleus of the comet, ignited or heated by the sun.”

Various conjectures have been formed concerning the nature of the tails of comets. Dr. Hamilton, of Dublin, in the second of his Philosophical Essays, urges several objections against the Newtonian hypothesis: he remarks, that, since the tail of a comet, though exceedingly rare, meets with no resistance in its rapid motion round the sun (except so slight a one as can only cause a very small condensation on that side of it which moves foremost, and thereby may make it a little brighter than the other side), it cannot possibly move in a medium denser and heavier than itself, and therefore cannot be raised up from the sun by the superior gravity of such a medium. And since the stars seen through all parts of a comet’s tail appear in their proper places, and with their usual colors, he infers that the rays of light suffer no refraction in passing through the tail; therefore, since bodies reflect and refract light by one and the same power, he concludes that the matter of a comet’s tail has not the power of refracting or reflecting light, and is, of consequence, a lucid or self shining substance. Also from what astronomers say of the splendor of comets’ tails, it is manifest they do not shine with such a dull light as would be reflected to us by the clouds or vapors at so great a distance, but with a brisker though a glimmering light, such as would arise from a very thin, volatile, burning matter. Dr. Halley, speaking of the great streams of light in the remarkable aurora borealis seen in 1716, says, “they so much resembled the long tails of comets, that at first sight they might be taken for such:” and afterwards, “this light seems to have a great affinity to that which the effluvia of electric bodies emit in the dark.” Dr. Hamilton improves upon these hints: and since, as he shows, the tails of comets, the aurora borealis, and the electric fluid, agree remarkably, not only in their appearance, but also in such properties as we can observe of each of them, he concludes that they are substances of the same nature. And, because the electric matter, from its vast subtility and velocity, seems capable of making great excursions from the planetary system, he imagines that the several comets, in their long excursions from the sun in all directions, may overtake this matter; and by attracting it to themselves may come back replete with it, and being again heated by the sun, may disperse it among the planets, and so keep up a circulation of this matter, which there is reason to think is necessary in our system.[133]

Comets, descending from the remote parts of the system with great rapidity become visible to us in the lower parts of their orbits; and after a short stay, go off again to vast distances, and disappear. They move about the sun in very eccentric ellipses; and the velocity with which they seem to move is variable in every part of their path round the sun; when near to which they appear to move with great swiftness, and, when very remote, their motion is slow. They are opake bodies, but of a much greater density than the earth; for some of them are heated in every period to such a degree, as would vitrify or dissipate any substance known to us. Sir Isaac Newton computed the heat of the comet, which appeared in the year 1680, when nearest the sun, to be 2,000 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 the comets is much greater than that of the planets belonging to our system. From the beginning of the Christian era, till now, there have appeared about five hundred. Before that time, we have accounts of about one hundred others. But, when it is considered that there may have been many that have not been seen, from being too near the sun, from appearing in moon-light, from being in the other hemisphere, or from being too small, or from not being recorded, the number is probably much greater. Miss Herschell, by means of the telescope, has, within the last twenty years, discovered several comets. The orbits of about one hundred comets have been calculated with sufficient accuracy for ascertaining their identity on any future appearance. Many of these orbits are inclined to the plane of the ecliptic in large angles, and many of them approach much nearer the sun than the earth does. Their motions are also different from those of the planets, some of them being direct and others retrograde, nearly half the number moving each way. The different motions of the comets, and the various inclinations of their orbits to the plane of the ecliptic, must not be regarded as the work of chance, but as calculated to answer beneficial purposes, or avoid baneful consequences; for if these orbits had been nearly coincident with that of the earth, both bodies might have arrived at the common point of intersection of their orbits at the same time; in which case a derangement of both motions must, at least, have been the necessary result.[134] But, according to all the observations that have been made respecting their present distribution and direction, there is not the least reason to apprehend any such consequence.

The following table contains a list of the last twenty-three of the principal comets that have been observed, with the time of passing their perihelia, and their nearest approach to the sun.

Nearest distance
Passage of the from the Sun in Direction of
Years. Perihelion. English Miles. their Motion.
------ -------------- ---------------- -------------
1790 January 15 71 millions Retrograde.
1790 January 28 101 Direct.
1790 May 21 75 Retrograde.
1792 January 13 122 Retrograde.
1792 December 27 91 Retrograde.
1793 November 4 38 Retrograde.
1793 November 18 142 Direct.
1795 December 15 23 Direct.
1796 April 2 149 Retrograde.
1797 July 9 50 Retrograde.
1798 April 4 46 Direct.
1798 December 31 73 Retrograde.
1799 September 7 79 Retrograde.
1799 December 25 25 Retrograde.
1801 August 8 22 Retrograde.
1802 September 9 103 Direct.
1804 February 13 101 Direct.
1805 November 18 35 Direct.
1805 December 31 84 Direct.
1806 December 28 102 Retrograde.
1807 September 18 61 Direct.
1811 August 20 25 Direct.
1815 April 26 121 Direct.