SATURN, in astronomy, one of the planets of our solar system, revolving at the distance of more than 900 millions of miles from the sun. See ASTRONOMY, no 31, 104-109, 191, and 269.
Dr Herschel, who has so much signalized himself by his discoveries in the celestial regions, has not omitted to make his observations on this planet, which he considers as one of the most engaging objects that astronomy offers to our view. His attention was first drawn to it in the year 1774, when he saw its ring resembling in appearance a narrow line, extending on both sides not much less than the diameter of the planet's disc. The observation was taken with a five and an half feet reflector.
sector on the 17th of March; and on the 3d of April, the same year, when the planet appeared totally deprived of this noble appendage, by reason of the edge of the ring being then turned directly towards the earth, and invisible on account of its thinness or incapacity to reflect the light to such a distance. During the succeeding year, the ring appeared gradually opened, and at last assumed the shape of an ellipse. "It should be noticed (says he), that the black disk or belt upon the ring of Saturn is not in the middle of its breadth. Nor is the ring subdivided by many such lines, as has been represented in several treatises of astronomy; but that there is one single, dark, considerably broad line, belt, or zone, upon the ring, which I have always permanently found in the place where my figure represents it."
This zone, which is on the northern part of the ring, does not change its shape or colour like the belts of Jupiter, so that it is probably owing to some permanent projection. It cannot, however, be the shadow of a chain of mountains, as it is visible all round the ring; and there could be no shades visible at the ends of the axis, on account of the direction of the sun's illumination, which would be in the line of the chain; and the supposed arrangement will hold good against the supposition of caverns or concavities. It is likewise evident, that this dark zone is contained between two concentric circles, as all the phenomena answer to the projection of such a zone. The Doctor gives a figure, representing the planet as it appeared to him on the 10th of May 1780; whence we see that the zone is continued all the way round, with a gradual decrease towards the middle, answering to the appearance of a narrow circular plane projected into an ellipse. See Philosoph. Trans. for 1790, p. 3, &c.
It hath been conjectured, that this appearance is owing to a division of the ring, or rather that there are two rings about the planet; "but (says Dr Herschel) if one ring, of a breadth so considerable as that of Saturn, is justly to be esteemed the most wonderful arch that by the laws of gravity can be held together, how improbable must it appear to suppose it subdivided into narrow slips of rings, which by this separation will be deprived of a sufficient depth, and thus lose the only dimension which can keep them from falling upon the planet? It is true, indeed, that it may revolve with such velocity as greatly to assist its strength, and that in the subdivisions, of course, the different velocities for each division may be equally supposed to keep them up."
As to the substance of the ring, the Doctor supposes it to be no less solid than that of Saturn himself. Thus in the two figures given with the Doctor's Dissertation in the Philosophical Transactions above referred to, the shadow of the planet is delineated upon the ring as it actually appeared, according to the situation of the sun; and in like manner we will see the shadow of the ring upon the planet; and if we deduce the quantity of matter contained in the planet from the power by which the satellites are preserved in their orbits, the ring must also be taken into account. It is indeed evident that the ring exerts a very considerable force upon these bodies, since we find them affected with many irregularities in their motions, which we cannot properly ascribe to any other cause than the quantity of matter contained in the ring; or, at least, it ought to be allowed to have a proper share in producing them.
The ring seems to be endowed with a greater reflective power than the body of the planet; and the Doctor gives instances of his seeing part of the ring brighter than Saturn himself, as well as of his seeing it plainly through a telescope which would scarcely afford light enough for the planet. The most remarkable property of this wonderful ring, however, is its extreme thinness. "When we were nearly in the plane of the ring (says our author), I have repeatedly seen the first, second, and third satellites, nay even the sixth and seventh, pass before and behind the ring in such a manner that they served as excellent micrometers to estimate its thickness. It may be proper to mention a few instances, especially as they will serve to solve some phenomena that have been remarked by other astronomers, though they have not been accounted for in a manner consistently with other known facts. July 13th 1789, at 19h 41' 9", sidereal time, the first satellite seemed to hang upon the following arm, declining a little towards the north, and I saw it gradually advance upon it towards the body of Saturn; but the ring was not so thick as the lucid point. July 23d, at 19h 41' 8", the second satellite was a very little preceding the ring; but the ring appeared to be less than half the thickness of the satellite. July 27th, at 20h 15' 12", the second satellite was about the middle, upon the following arm of the ring, and towards the south; and the sixth satellite on the farther end towards the north; but the arm was thinner than either of them. Aug. 29th, at 22h 12' 55", the third satellite was upon the ring, near the end of the preceding arm, when the latter seemed not to be the fourth, or at most the third part of the diameter of the satellite; which, in the situation it was, I took to be less than one single second in diameter. At the same time, I also saw the seventh satellite following the third, at a little distance, in the shape of a bead upon a thread, projecting on both sides of the same arm. Hence also we are sure that the arm appeared thinner than the seventh satellite, which is considerably smaller than the sixth, which again is less than the first. August 31st, at 20h 48' 26", the preceding arm was loaded about the middle with the third satellite. October 15th, at 0h 43' 44", I saw the sixth satellite, without obstruction, about the middle of the preceding arm, though the ring was but barely visible with my 40 feet reflector, even while the planet was in the meridian. However, we were then a little inclined to the plane of the ring, and the third satellite, when it came near its conjunction with the first, was so situated, that it must have partly covered it a few minutes after I lost it behind my house. In all these observations, the ring did not in the least interfere with my view of the satellites. October 16th, I followed the sixth and seventh satellites up to the very disk of the planet; and the ring, which was extremely faint, did not in the least obstruct my seeing them gradually approach the disk, where the seventh vanished at 21h 46' 44", and the sixth at 22h 36' 44". There is, however, some suspicion, that by a refraction through some very rare atmosphere on the two planes of the ring, the satellites might be lifted up and depressed so as to become visible on both sides of the ring, even though the latter should be equal in thickness to the diameter of the smallest satellite, which may amount to 1000 miles. As for the arguments of its incredible thinness, which some
some astronomers have brought from the short time of its being invisible when the earth passes through its plane, we cannot set much value upon them; for they must have supposed the edge of the ring, as they have also represented it in their figures, to be square; but there is the greatest reason to suppose it either spherical or spheroidal; in which case evidently the ring cannot disappear for any long time. Nay, I may venture to say, that the ring cannot possibly disappear, on account of its thinness; since, either from the edge or the sides, even if it were square on the corners, it must always expose to our sight some part which is illuminated by the rays of the sun; and that this is plainly the case we may conclude from its being visible in my telescopes during the time when others of less light had lost it; and when evidently we were turned towards the unenlightened side, so that we must either see the rounding side of the unenlightened edge, or else the reflection of the light of Saturn upon the side of the darkened ring, as we see the reflected light of the earth on the darkened part of the new moon. I will not, however, take upon me to decide which of the two may be the case, especially as there are other very strong reasons which induce us to think that the edge of the ring is of such a nature as not to reflect much light."
Several astronomers have supposed that the ring of Saturn is full of mountains and inequalities, like the moon; and of this opinion Dr Herschel himself was for a considerable time, till happening to observe one of these lucid points with attention for a considerable time, he saw it leave the ring altogether, and show itself as a satellite never before observed. With regard to the ring itself, he concludes his observations in these words: "Upon the whole, therefore, I cannot say that I had any one instance that could induce me to believe that the ring was not of one uniform thickness; that is, equally thick at equal distances from the centre, and of an equal diameter throughout the whole of its construction. The idea of protuberant points upon the ring of Saturn, indeed, is of itself sufficient to render their existence inadmissible, when we consider the enormous size which such points ought to be of to render them visible at the distance we are from that planet."
With regard to the satellites, the Doctor informs us, that he was long convinced of the existence of a sixth; and had he been more at leisure at the time of his discovering those of the Georgium Sidus, he would probably have completed the discovery of the satellites of Saturn also. The sixth was first observed distinctly on the 28th of August 1789, and the seventh on the 17th of September the same year. These satellites, however, do not occupy the place which we should have previously supposed them, being, in fact, the innermost of the whole. The seventh is next the body of the planet itself, and is very small. It revolves at the distance of from the centre of Saturn, and seems to move exactly in the plane of the ring; but the Doctor observes, that it is exceedingly difficult to make a sufficient number of observations on it to determine the revolution exactly. He computes its periodical time at . The sixth satellite is next to the seventh, and revolves at the distance of from the centre of its primary in . Its light is considerably stronger, but not equal to that of the first satellite of former astronomers, which lies immediately beyond it.
The planet Saturn is now observed to have belts or fasciae upon its disk as distinctly as Jupiter. Dr Herschel, on the 9th of April 1775, observed a northern belt on his body, inclined a little to the line of the ring. On the 1st of May 1776, there was another belt observed, inclined about to the same line, but more to the south; and on the following side came up to the place where the ring crosses the body of the planet. — On the 8th of April two belts were observed, and these continued with variations, and sometimes the appearance of a third belt, till the 8th of September, when the account of the observations was discontinued. The Doctor remarks, that he generally observed these belts in equatorial situations, though sometimes it was otherwise. Two conclusions, he says, may be drawn from the observations he made this year. "The first, which relates to the changes in the appearance of the belts, is, that Saturn has probably a very considerable atmosphere, in which these changes take place, just as the alterations in the belts of Jupiter have been shewn with great probability to be in his atmosphere. This has also been confirmed by other observations. Thus, in occultations of Saturn's satellites, I have found them to hang to the disk for a long while before they would vanish. And though we ought to make some allowance for the encroachment of light, whereby a satellite is seen to reach up to the disk sooner than it actually does, yet without a considerable refraction it could hardly be kept so long in view after the apparent contact. The time of hanging upon the disk in the seventh satellite has actually amounted to 20 minutes. Now, as its quick motion during that interval carries it through an arch of near six degrees, we find that this would denote a refraction of about two seconds, provided the encroaching of light had no share in producing the effect. By an observation of the sixth satellite, the refraction of Saturn's atmosphere amounts to nearly the same quantity; for this satellite remained about 14 or 15 minutes longer in view than it should have done; and as it moves about degrees in that time, and its orbit is larger than that of the seventh, the difference is inconsiderable. The next inference we may draw from the appearance of the belts on Saturn is, that this planet turns upon an axis which is perpendicular to his ring. The arrangement of the belts, during the course of 14 years that I have observed them, has always followed the direction of the ring, which is what I have called being equatorial. Thus, as the ring opened, the belts began to advance towards the south, and to show an incurvature answering to the projection of an equatorial line, or to a parallel of the same. When the ring closed up, they returned towards the north, and are now, while the ring passes over the centre, exactly ranging with the shadow of it, on the body, generally one on each side, with a white belt close to it. When I say that the belts have always been equatorial, I pass over trifling exceptions, which certainly were owing to local causes. The step from equatorial belts to a rotation on an axis is so easy, and, in the case of Jupiter, so well ascertained, that I shall not hesitate to take the same consequence for granted here. But if there could remain a doubt, the observations of June 19th, 20th, and 21st, 1780, where the same spot upon one of the belts was seen in three different situations, would remove it completely."
Another evidence that Saturn, as well as the other planets, revolves upon its axis, is drawn from its flattened shape, like that of Mars, Jupiter, and Saturn. On the 31st of May 1781, the disc seemed to deviate as much from a true circle as that of Jupiter, though by the interference of the ring this could not be so well determined as after an interval of eight years. On the 18th of August 1787, the difference between the equatorial and polar diameters was measured, the mean of three observations of the former being , of the latter . From these observations, it appears that the polar diameter of Saturn is to his equatorial diameter nearly as 10 to 11; and that his axis is perpendicular to the plane of the ring.
In a subsequent paper, the Doctor gives up his reasoning against fixed lucid points in the ring, in consequence of having frequently observed them in such situations as could not by any means be accounted for by the satellites. He even attempts to invalidate his own arguments above-mentioned concerning the vast magnitude of the mountains necessary to make them visible at this distance. "As observations (says he) carefully made should always take the lead of theories, I shall not be concerned if such lucid spots as I am now going to admit, should seem to contradict what has been said in my last paper concerning the idea of inequalities or protuberant points. We may, however, remark, that a lucid and apparently protuberant point may exist without any great inequality in the ring. A vivid light, for instance, will seem to project greatly beyond the limits of the body on which it is placed. If, therefore, the luminous places on the ring should be such as proceed from very bright reflecting regions, or, which is more probable, owe their existence to the more fluctuating causes of inherent fires acting with great violence, we need not imagine the ring of Saturn to be very uneven or distorted, in order to present us with such appearances. In this sense of the word, then, we may still oppose the idea of protuberant points, such as would denote immense mountains of elevated surface.
"On comparing together several observations, a few trials shew that the brightest and best observed spot agrees to a revolution of ; and calculating its distance from the centre of Saturn, on a supposition of its being a satellite, we find it , which brings it upon the ring. It is therefore certain, that unless we should imagine the ring to be sufficiently fluid to allow a satellite to revolve in it, or suppose a notch, groove, or division in the ring, to suffer the satellite to pass along, we ought to admit a revolution of the ring itself. The density of the ring, indeed, may be supposed to be very inconsiderable by those who imagine its light to be rather the effect of some shining fluid, like an aurora borealis, than a reflection from some permanent substance; but its disappearance, in general, and in my telescopes its faintness, when turned edgewise, are in no manner favourable to this idea.—When we add also, that this ring casts a deep shadow upon the planet, is very sharply defined both in its outer and inner edge, and in brightness exceeds the planet itself, it seems to be almost proved that its consistency cannot be less than the body of Saturn, and that consequently no degree of fluidity can be admitted sufficient to permit a revolving body to keep in motion for any length of time. A groove might afford a pas-
sage, especially as on a former occasion we have already considered the idea of a divided ring. A circumstance also which seems rather to favour this idea, is, that in some observations a bright spot has been seen to project equally on both sides, as the satellites have been observed to do when they passed the ring. But, on the other hand, we ought to consider, that the spot has often been observed very near the end of the arms of Saturn's ring, and that the calculated distance is consequently a little too small for such appearances, and ought to be 19 or 20 seconds at least. We should also attend to the size of the spot, which seems to be variable: for it is hardly to be imagined that a satellite, brighter than the sixth, and which could be seen with the moon nearly at full, should so often escape our notice in its frequent revolutions, unless it varied much in its apparent brightness. To this we must add another argument drawn from the number of lucid spots, which will not agree with the motion of one satellite only; whereas, by admitting a revolution of the ring itself in , and supposing all the spots to adhere to the ring, and to share in the same periodical return, provided they last long enough to be seen many times, we shall be able to give an easy solution of all the remaining phenomena. See Phil. Trans. 1790, p. 427.