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.
| Years. | Passage of the Perihelion. | Nearest distance from the Sun in English Miles. | Direction of 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. |
But 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 distance, is about 11,200 millions of miles from the sun, while its least distance from the centre of the sun is about 490,000 miles; being less than one third part of the sun’s semi-diameter 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 astonishing distance that this comet runs out into empty space, naturally suggests to our imagination the vast distance between our sun and the nearest of the fixed stars, of whose attractions all the comets must keep clear, to return periodically and go round the sun. How wonderful that, though this body travelled almost two thousand times faster than a cannon ball, yet it drew after it a tail of fire, or of phosphoric gas, eight millions of miles in length! How amazing to consider, that this stupendous body, traversing the immensity of the creation with such rapidity, and at the same time wheeling about in that line which its great Creator prescribed to it, should move with such inconceivable velocity, and at the same time with such exact regularity! How spacious must the universe be, that, gives such bodies as these full play, without suffering the least disorder or confusion by it! With what a glorious exhibition must those beings be entertained, who can look into this great theatre of nature, and see myriads of these tremendous objects wandering through those immeasurable depths of æther, and running their appointed courses! Our eyes may hereafter be strong enough to command this magnificent prospect, and our understandings able to find out the several uses of these immense parts of the universe. In the mean time, they are most suitable objects for our imagination to contemplate, that we may form more extensive notions of infinite wisdom and power, and learn to think humbly of ourselves, and of all the little works of human invention.[135]
The Fixed Stars are objects of peculiar interest, and are so denominated, because they are observed always to preserve the same distance from each other; and are distinguished from the planets by their twinkling, which seems to depend on the atmosphere; for we are assured, that where the air is exceedingly pure and dry, the stars appear with a light altogether free from scintillation. All the heavenly bodies, the sun, moon, and stars, appear to move round the earth, in circles parallel to the equinoctial, once in the compass of twenty-four hours; though these apparent motions are almost entirety to be accounted for by the real motions of the earth: but by far the greater number of them never change their relative situations, each (so long as an observer continues in the same place) rising and setting at the same interval of time, and at the same points of the horizon;—these are called the fixed stars.
The fixed stars, as appears from several considerations, are placed at immense distances from us. Mr. Exley, in a friendly communication, says, “It should be noticed, that the distances of the fixed stars have never yet been discovered; not indeed so much for want of a method, as for want of a base line sufficiently large for this admeasurement. The diameter of the earth’s orbit is about one hundred and ninety millions of miles; and the fixed stars, viewed from the opposite ends of this extensive base line or diameter, have no sensible parallax, but all appear in the very same situations, and of the same magnitudes; and as this is the greatest line to the extremities of which we can have access, it is very probable we shall ever remain in ignorance of the true distances of the fixed stars. One thing, however, is fully ascertained by the observations which have been made to find the parallax of the stars, which is, that they are so immensely distant from our planetary regions, that the whole solar system, consisting of the sun and planets, with their satellites, and the comets, would, if viewed from the nearest fixed star, appear as crowded into one single point of space, which is also known from other observations. How astonishingly extensive is the view of the universe which such observations furnish!”
Our earth is at so great a distance from the sun, that if seen from thence, it would appear no bigger than a point, although its diameter is 7,954 miles. Yet that distance is so small, compared with the earth’s remote situation from the fixed stars, that if the orbit in which the earth moves round the sun were the circumference of a globe, that globe, seen from the nearest star, would likewise appear no bigger than a point, although, it is at least 190,000,000 miles in diameter. For the earth in going round the sun is 190,000,000 miles nearer to some of the stars at one time of the year than at another, and yet their apparent magnitudes, situations, and distances from one another still remain the same; and being viewed through a telescope which magnifies above 200 times, they still appear as mere points: which proves them to be at least 400,000 times further from us than we are from the sun.
It is not to be imagined, that all the stars are placed in one concave surface, so as to be equally distant from us; but that they are scattered at immense distances from one another through unlimited space. So that there may be as great a distance between any two neighboring stars, as between our sun and those which are nearest to him. Therefore an observer, who is nearest any fixed star, will look on it alone as a real sun; and consider the rest as so many shining points, placed apparently at equal distances from him in the firmament. The star nearest to us, or the largest in appearance, is Sirius, or the Dog Star, and astronomers have calculated from indubitable principles, that its distance from us is considerably more than two millions of millions of miles! The apparent magnitude of Sirius has been computed at 27,000 times less than the sun, and, therefore, supposing their magnitudes equal, is 27,000 times more distant. If so, as our earth is ninety-five millions of miles from the sun, that multiplied by twenty-seven thousand, will give two millions of millions, and an addition of 565 thousand millions, for the distance of this star from the sun.[136] Our earth, in moving round the sun, is 195,000,000 miles nearer to this star in one part of its orbit, than in the opposite one; and yet the magnitude of the star appears not to be in the least altered or affected by it. A cannon-ball flying from thence at the rate of 400 miles in an hour, would not reach us in 732,000 years! The distance of the star γ Draconis appears, by Dr. Bradley’s observations, to be at least 400,000 times that of the sun, and the distance of the nearest fixed star not less than 80,000 diameters of the earth’s annual orbit; that is, the distance of the earth from the former is = to 400,000 × 95,000,000 = 38,000,000,000,000, and the latter not less than 7,600,000,000,000. As these distances are much too great to be comprehended by the human imagination, we shall, perhaps, obtain a better idea of them by comparing them with the velocity of some moving body, by which they may, in some way, be estimated. The swiftest motion we know of is that of light, which passes from the sun to the earth in about eight minutes, or, at the rate of 200,000 miles nearly in a second of time: and yet even light would be more than six years in traversing the first space, and a year and a quarter nearly, in passing from the nearest fixed star to the earth. Again, a cannon ball, moving with its initial or greatest velocity of about ten miles in a minute, would be more than seven millions of years in passing from the star γ Draconis to the earth. The celebrated M. Huygens carried his thoughts so far upon this subject, as to believe that there might be stars at such inconceivable distances from our earth, that their light, though it is known to travel at the rate of 12,000,000 miles in a minute, has not yet reached us, since the creation of the world!
“How distant some of the nocturnal suns!
So distant, says the sage, ‘twere not absurd
To doubt, if beams, set out at nature’s birth,