From the motion of the spots, which is sometimes straight and sometimes curved, we learn that the sun’s axis is not perpendicular to the plane of his ecliptic, but inclined to it, or the plane of the earth’s annual orbit, so as to form an angle of about 83 degrees. Christopher Scheiner, a most diligent observer of these spot’s in the sun’s disk, published a treatise concerning them in A.D. 1626. These spots are sometimes seen to increase to a very large size, and to continue for a considerable time. In the year 1779, there was a spot on the sun’s disk which was large enough to be seen with the naked eye: it was divided into two parts, and must have been 50,000 miles in diameter: this, and other phenomena of the same kind, may be accounted for from some natural change of the atmosphere. For if some of the fluids which enter into its composition be of a shining brilliancy, while others are merely transparent, then any temporary cause removing the lucid fluid, will permit us to see the body of the sun through the transparent ones. Dr. Herschell supposes that the spots in the sun are mountains on its surface, which, considering the great attraction exerted by this luminary upon bodies placed at its surface, and the slow revolution it has about its axis, he thinks may be more than 300 miles in height, and yet not be rendered unstable by the centrifugal force.
[There appears to be a discrepancy between this last statement—“Dr. Herschell supposes that the spots in the sun are mountains on his surface;”—and the statement made a few paragraphs preceding—“that what are called maculæ, or spots on the sun, Dr. Herschell thought to be real openings in his atmosphere, through which the opake body of the sun becomes visible.” These statements must have been made at different periods of his observations on the sun, which continued about fifteen years. The last statement was, doubtless, Dr. Herschell’s mature opinion.
As this seems to be a settled question among philosophers; and as it has induced the enlightened world to regard the sun as a habitable globe, it will not be out of place to enlarge a little on this point.
The spots on the sun’s surface has led to the conclusion above, and also to a determination of the motion of the sun around his own axis. They appear to have been observed, for the first time, in A.D. 1610, by Fabricius and Harriot; the first in Germany, the second in England. It is uncertain which noticed them first; but it is certain the discovery was original with both.
After the observations of these two fortunate persons were known, the attention of the scientific was directed to this phenomenon. Scheiner supposed the spots to be planets which revolved very near the sun. In process of unwearied observations, it was ascertained that these spots changed their positions. Sometimes two would blend together, and thus run into each other. Sometimes one large one would divide into two or three smaller ones. They were observed to dilate, and contract; and to have umbræ, or shades attending them.
From these phenomena Galileo and others supposed the solar spots were schoria floating on the burning liquid matter, of which they supposed the sun composed. M. de la Hire, and La Lande supposed them to be eminences which occasionally rose above the rolling tides of fire, as islands rise above the sea. All these theories were on the supposition that the sun was an igneous body, in a high state of combustion, by which means he dispenses heat and light to the surrounding planets.
Dr. Wilson, Professor of practical astronomy in the University of Glasgow, was the first to conjecture that these spots were depressions rather than elevations. This was about the year 1769. The Doctor rendered this conjecture very probable, by his close and lucid observations and illustrations.
These spots attracted the attention of the celebrated Dr. Herschell in 1779, who continued to observe them closely until 1794, and by means of his immensely large and powerful telescopes, he clearly established Dr. Wilson’s conjectures, that these spots are openings in the luminous surface of the sun, through which his opake body appears.
Dr. Herschell regards the real body of the sun to be an opake nucleus, fit for the habitation of intellectual creatures: that he has an atmosphere suited in density and height to his own magnitude: that in the higher regions of this atmosphere there are two sets of clouds surrounding the sun, which are permanently and essentially luminous, being phosphoric in their nature. The lower set of these clouds, which are next the sun, are less bright, and more dense than the upper set. They are designed to serve as a curtain to the sun’s body, to prevent a too great intensity of light at his real surface; the higher set of clouds, which are visible to us, being the principal source, or rather agent, of light.
It is plain from the foregoing theory, that we never see the real body of the sun, except when we see the spots on his surface: that what we commonly call the sun, are only those bright, luminous phosphorescent clouds, which permanently surround his body, and which give light outwards to the planets, and also inwards to his own inhabitants.
It will be obvious also to any one, that the inhabitants of the sun cannot see any heavenly body, as the stars, and planets; because they are inclosed by those clouds, which are impenetrable to vision. They may catch a glimpse of a passing star through these openings as we do of the sun’s body.
It is highly probable (see our paper on light, attached to our author’s chapter on the same,) that these luminous phosphoric clouds do not actually emit light, or heat; but only excite them at the surfaces of the different planets. That is: it is very probable there is a matter of light or a luminiferous ether, diffused through all existing matter, as caloric is, which is excited by these clouds, and thus becomes visible, which is light, as latent caloric is excited, and becomes sensible, by becoming free. Indeed it is very probable that the matter of heat and light is the same, and that heat and light are only different modifications of the action of the same substance, excited in a different, or higher degree.]
The sun has two apparent motions, namely, the diurnal and annual. By the former he appears to move round the earth in twenty-four hours: and by the latter he appears to traverse that circle in the heavens, called the ecliptic, in the course of a year. These motions, are, however, only apparent: the sun does not travel round the earth in twenty four hours: he does not change his place in the heavens at different seasons of the year. His apparent motions are occasioned by the earth’s real motions. The sun’s apparent diurnal motion is occasioned by the earth’s real rotation about its axis: and the sun’s apparent annual motion is caused by the earth’s real motion in her orbit, through the whole of which she travels in a little less than 365 days, and 6 hours.
The fixed stars appear every twenty-four hours to make an entire revolution about the earth. The sun makes the same apparent circuit; but the apparent diurnal motion of the sun is evidently slower than that of the fixed stars. This appearance is occasioned by the daily rotation of the earth on its axis; for while it is turning once on its axis it advances in its orbit a whole degree; therefore it must make more than a complete rotation before it can come into the same position with the sun that it had the preceding day. In the same way, as when both hands of a watch set off together at any hour, as twelve o’clock, the minute hand must travel more than the whole circle before it can overtake the hour hand: hence the difference between solar and sidereal days, which it is important to understand in explaining the equation of time.
Though the sun appears to us merely as a circular disk, yet he is a spheroid, higher under his equator than about his poles. The deception arises from this; that all the parts of his surface are equally luminous, and consequently there is nothing which can suggest to us, at the great distance he is from the earth, that the central parts are more prominent than the sides, although in reality, they are nearer by half a million of miles.
This luminous body is supposed to be 886,473 English miles in diameter, about 2,700,000 in circumference, in solid bulk 24,000,000 times as big as the moon, and 1,384,462 times as big as the earth, and its superficies in square miles, about 2,236,603,000,000. This magnitude of the sun may appear exaggerated; for our eyes can discover nothing so large as the earth which we inhabit; and as to this alone we compare the sun, so we are tempted to believe the testimony of sense rather than our reason. But what confirms this prodigious size, is his visible magnitude, notwithstanding the vastly remote point which he occupies in space. And, concerning this subject, no doubt can remain, if we admit the calculations of astronomers, which are made on principles indubitably correct.
The sun does not appear large; but this is owing to his distance from the earth, which is 95,513,794 miles: this is so prodigious, that a cannon-ball, which is known to move at the rate of eight miles in a minute, would be something more than twenty-two years in going from the earth to the sun. If a spectator were placed as near to any of the fixed stars as we are to our sun, he would see our sun as small as we see a common star, divested of its circumvolving planets; and in numbering the stars he would reckon it one of them. But the earth’s orbit being an ellipse, the sun is not always at an equal distance from it. When he is in his apogee, that is, furthest from the earth, the sun is full two millions of miles further from us than when he is in his perigee, or nearest the earth: nevertheless, we feel greater heat than when he is in our winter. The difference of temperature between summer and winter does not depend chiefly upon our nearness to the sun, but upon the following causes. 1. In summer, the solar rays strike upon the earth more perpendicularly than in winter, and therefore they act with greater force than when they strike it obliquely. 2. The rays of the sun coming more perpendicularly in summer than in winter, have less of the atmosphere to pass through. 3. In the summer, the sun continues a longer time above the horizon than below it; and consequently there is time for the earth to accumulate a greater portion of heat than in the days of winter. We know, in the longest days, that the sun to us is above the horizon 16 hours; whereas, in the shortest days, it is not more than 8 hours visible.[114]
The miraculous suspension of the natural powers of the heavenly bodies, as recorded in the book of Joshua, shows that they are upheld, controlled, and directed in their operations, by a Being who is infinitely wise and powerful. To account for this miracle, and to ascertain the manner in which it was wrought, has employed the pens of the ablest divines and astronomers, especially of the last two centuries. For the elucidation of this important fact, I shall transcribe the view which Dr. Adam Clarke has given of it, which he considers to be strictly philosophical, consonant to the Pythagorean, Copernican, or Newtonian system, which is the system of the universe, laid down in the writings of Moses.
He assumes, as a thoroughly demonstrated truth, that the sun is in the centre of the system, moving only round his own axis, and the common centre of the gravity of the planetary system, while all the planets revolve round him; and that his influence is the cause of the diurnal and annual revolutions of the earth.
“Joshua’s address is in a poetic form in the original, and makes the two following hemistichs: