Part 15

From the great variations in the distance of Saturn from the Earth, necessarily result corresponding changes in the brightness and apparent diameter of this body. When it is farthest from us, its angular diameter measures but 14"; while, when it is nearest, it measures 20".

The orbit of Saturn is inclined 2°30' to the ecliptic, and its eccentricity, which equals 0,056, is over three times that of the Earth's orbit.

This planet revolves around the Sun in a period of 29 years and 5½ months, or 10,759 terrestrial days, which constitutes its sidereal year. The extension of the immense curve forming the orbit of this planet, is no less than 5,505,000,000 miles, which is traversed by the planet with a mean velocity of a little less than 6 miles per second, or three times less than the motion of our globe in space.

The real dimensions of the globe of Saturn are not yet known with accuracy, and the equatorial diameter has been variously estimated by observers, at from 71,000 to 79,000 miles. If we adopt the mean of these numbers, 75,000 miles, the circumference of the Saturnian equator would measure 235,620 miles, or 9½ times the circumference of our globe; the surface of Saturn would be 86 times, and its volume over 810 times that of the Earth.

However great the volume of Saturn, its mass is proportionally small, being only 90 times greater than that of our globe; the mean density of the materials composing this planet being less than that of cork, and only 0.68 the density of water. The force of gravitation at the surface of Saturn is greater, by a little over ⅑, than it is at the surface of the Earth; a body falling in a vacuum at its surface, would travel 17.59 feet during the first second.

From observations of markings seen on the surface of Saturn, and from the study of their apparent displacements on the disk, William Herschel found that the planet rotated upon its axis in 10h. 16m. 0.24s. Since Herschel's determination, new researches have been made, and lately, Professor Hall, noticing a bright spot, followed it for nearly a month, observing its transits across the central meridian of the disk. From these observations he has obtained for the rotation period 10h. 14m. 23.8s., a result which agrees very closely with that obtained 82 years earlier by Herschel, considering the fact that the markings from which the period of rotation is ascertained are not fixed on the planet, but are always more or less endowed with proper motion. The velocity of rotation at the equator is 21,538 miles per hour, or nearly 6 miles per second.

The axis of rotation of Saturn is inclined 64° 18' to the plane of the orbit, so that its equator makes an angle of 25° 42' with the same plane. The seasons of this planet therefore present greater extremes of temperature than those of the Earth, but not quite so great variations as the seasons of Mars.

The globe of Saturn is not a perfect sphere, but its figure is that of an oblong spheroid, flattened at the poles. The polar compression of Saturn is greater than that of any other planet, surpassing even that of Jupiter. Though not yet determined with a great degree of accuracy, the compression is known to be between ²⁄₁₈ and ⅒ of the equatorial diameter; that is, a flattening of about 3,894 miles, at each pole, the polar diameter being 7,788 miles shorter than the equatorial.

The internal condition of the planet Saturn, whether solid, liquid or gaseous, cannot be discovered from the examination of its surface, as its globe is enwrapped in a dense opaque layer of vapors and cloud-like forms, through which the sight fails to penetrate. The appearance of this vapory envelope is like that of cumulus clouds, and one of its characteristics is to arrange itself into alternate bright and dark parallel belts, broader than those seen on Jupiter, and also more regular and dark. These belts, which are parallel to the equator of the planet, vary in curvature with the inclination of its axis of rotation to the line of sight.

The belts of Saturn, like those of Jupiter, are not permanent, but keep changing more or less rapidly. Sometimes they have been observed to be quite numerous; while at other times they are few. Occasionally conspicuous white or dark spots are seen on the surface, although the phenomenon is quite rare. It is from the observation of such spots that Saturn's period of rotation has been determined, as stated above. The equatorial zone of Saturn always appears more white and brilliant than the other parts, as it also appears more mottled and cloud-like. In late years the globe has been characterized, and much adorned, by a pale pinkish tint on its equatorial belt, resembling that of Jupiter, but somewhat fainter. On either side of the equatorial belt there is a narrower band, upon which the mottled appearance is visible. Below these, one or two dark belts, separated by narrow white bands, are usually seen; but, of late, the bands have been less numerous, being replaced in high latitudes by a dark segment, which forms a polar cap to Saturn. The globe of Saturn does not anywhere appear perfectly white, and when compared with its ring, it looks of a smoky yellowish tint, which becomes an ashy gray on its shaded parts. It usually appears darker near the limb than in its central portions; although on some occasions I have seen portions of the limb appear brighter, as if some white spots were traversing it.

Some observers have seen the limb deformed and flattened at different places, and W. Herschel even thought such a deformation to be a permanent feature of this globe, which he termed diamond-shaped, or "square shouldered." But this was evidently an illusion, since the planet's limb usually appears perfectly elliptical, although it occasionally appears as if flattened at some points, especially where it comes in apparent contact with the shadow cast by the globe on the ring, as observed by myself many times. But with some attention, it is generally found that this deformation is apparent rather than real, and is caused by the passage of some large dark spots over the limb, which is thus rendered indistinguishable from the dark background upon which it is projected.

What distinguishes Saturn from all known planets, or heavenly bodies, and makes it unique in our universe, is the marvelous broad flat ring which encircles its equator at a considerable distance from it. With a low magnifying power this flat ring appears single, but when carefully examined with higher powers, it is found to consist of several distinct concentric rings and zones, all lying nearly in the same plane with the planet's equator.

Observed on November 30, 1874, at 5h. 30m. P.M.]

At first sight only two concentric rings are recognized, the outer and the middle, or intermediary, which are separated by a wide and continuous black line, called the principal division. This line, and indeed all the features of the surface of the rings are better seen, and appear more prominent on that part of the ring on either side called the ansa, or handle. Besides these two conspicuous rings, a third, of very dark bluish or purplish color, lies between this middle ring, to which it is contiguous, and the planet. This inner ring, which is quite wide, is called the gauze or dusky ring. Closer examination shows that the outer ring is itself divided by a narrow, faint, grayish line called the pencil line, which, from its extreme faintness, is only visible on the ansæ. Moreover, the middle ring is composed of three concentric zones, or belts, which, although not apparently divided by any interval of space, are distinguished by the different shadings of the materials composing them. The outer zone of this compound middle ring is, by far, the brightest of all the system of rings and belts, especially close to its external border, where, on favorable occasions, I have seen it appear on the ansæ as if mottled over, and covered throughout with strongly luminous cloud-like masses. On the ansæ of the double outer ring, similar cloudy forms have also been seen at different times. The second zone of the middle ring is darker than the first, the innermost being darker still. All the characteristic points which have thus been described, are shown in Plate X.

Although suspected in 1838, the dusky ring was not recognized before 1850, when G. P. Bond discovered it with the 15-inch refractor of the Cambridge Observatory. It was also independently discovered the same year in England by Dawes and Lassell. The dusky ring differs widely in appearance and in constitution from the other rings, inasmuch as these last are opaque, and either white or grayish, while the former is very dark, and yet so transparent that the limb of the planet is plainly seen through its substance. On particularly favorable occasions, the appearance of this ring resembles that of the fine particles of dust floating in a ray of light traversing a dark chamber. Whatever may be the material of which this ring is composed, it must be quite rarefied, especially towards its inner border, which appears as if composed of distinct and minute particles of matter feebly reflecting the solar light. That the inner part of the dusky ring is composed of separate particles, is proved by the fact that the part of the ring which is seen in front of the globe of Saturn has its inner border abruptly deflected and curved inward on entering upon the disk, causing it to appear considerably narrower than it must be in reality, a peculiarity which is shown in the Plate. This phenomenon may be attributed to an effect of irradiation, due to the strong light reflected by the central parts of the ball, which so reduces the apparent diameter of the individual particles that they become invisible to us, especially those near the inner border, which are more scattered and less numerous than elsewhere.

The dusky ring, which was described by Bond, Lassell and other astronomers as being equally transparent throughout all its width, has not been found so by me in later years. The limb of the planet, seen by these observers through the whole width of the dusky ring in 1850, could not be traced through its outer half by myself in 1872 and 1874, and this with the very same instrument used by Bond in his observations of 1848 and 1850. Moreover, I have plainly seen that its transparency was not everywhere equal, but greatest on the inner border, from which it gradually decreases, until it becomes opaque, as proved by the gradual loss of distinctness of the limb, which vanishes at about the middle of the dusky ring. These facts, which have been well ascertained, prove that the particles composing this ring are not permanently located, and are undergoing changes of relative position. It will be shown that the surface of the other rings is also subject to changes, which are sometimes very rapid.

The globe of Saturn is not self-luminous, but opaque. It shines by the solar light, as is proved by the shadow it casts opposite the Sun upon the ring. Although receiving its light from the Sun, Saturn does not exhibit any traces of phases, like the other planets nearer to the Sun, owing to its great distance from the Earth. When near its quadratures, however, the limb opposite to the Sun appears much darker, and shows traces of twilight. As far as can be ascertained, the rings, with the exception of the inner one, are opaque, as proved by the strong shadow which they cast on the globe of Saturn.

The shadows cast by the planet on the ring, and by the ring on the planet, are very interesting phenomena, inasmuch as they enable the astronomer to recognize the form of the surface which receives them. The shadow cast by the ring on the ball is not quite so interesting as the other, although it has served to prove that the surface of this globe is not smooth, as is likewise suggested by its mottled appearance. I have sometimes found, as have also other observers, that the outline of this shadow upon the ball was irregular and indented, an observation which proves either that the surface of the ball is irregular, or that the border of the ring casting the shadow was jagged. The shadow of the globe on the rings has much more interest, as it enables us to get at some knowledge of the form of the surface of the rings, which otherwise is very difficult to discover, owing to the oblique position in which we always see them.

In general, the shadow of the ball on the middle ring has its outline concave towards the planet; while on the outer ring it is usually slanting, and at a greater distance from the limb than on the middle, and dusky rings. This form of the shadow evidently proves that the middle ring stands at a higher level than the two others, especially towards its outer margin. The system seems to increase gradually in thickness from the inner border of the dusky ring to the vicinity of the outer margin of the middle ring, after which it rapidly diminishes on this border, while the surface of the outer ring is almost level.

But this surface is by no means fixed, as its form sometimes changes, as proved by my observations and those of others. As may be noticed on Plate X., the outline of the shadow of the planet on the rings is strongly deviated towards the planet, near the outer margin of the middle ring; the notch indicating an abrupt change of level, and a rise of the surface at that point. Some observers have endeavored to explain these deviations by the phenomena of irradiation, from which it would follow that the maximum effect of deviation should be observed where the ring is the brightest, which does not accord with observation; as the deepest depression in the shadow is not to be found usually at the brightest part, which is towards the outer border of the middle ring, but occurs near its centre. From these observations it is undoubtedly established that the surface of the rings is far from being flat throughout, and is, besides, not permanent, but changes, as would, for instance, the surface of a large mass of clouds seen from the top of a high mountain. In general, the system is thickest not very far from the outer border of the intermediary ring.

Some interesting phenomena which I had occasion to observe before and after the passage of the Sun through the plane of the rings, on February 6th, 1878, conclusively show that the surface of this system cannot be of a uniform level, but must be thicker towards the outer border of the middle ring, thence gradually sloping towards the planet. Many of my observations irresistibly lead to this conclusion. As it would, however, be out of place to have them recorded here in detail, I will simply give one of the most characteristic among them.

From December 18th, 1877, when the Sun was about 41' above the plane of the rings, to February 6th, 1878, the day of its passage through their plane, the illuminated surface of this system gradually decreased in breadth with the lowering of the Sun, until it was lost sight of, February 5th, on the eve of the passage of the Sun through their plane. The phenomenon in question consisted in the gradual invasion of their illuminated surface by what appeared to be a black shadow, apparently cast by the front part of the outer portion of the middle ring the nearest to the Sun. On January 25th, when the elevation of the Sun above the plane of the rings was reduced to 15', the shadow thus cast had extended so far on their surface that it reached the shadow cast by the globe on the opposite part of the ring in the east, and accordingly the remaining portion of the illuminated surface of the eastern ansa then appeared entirely disconnected from the ball, by a large dark gap, corresponding in breadth to that of the globe's shadow on the rings. On February 4th, when the Sun was only 5' above the plane of the rings, their illuminated and only visible surface was reduced to a mere thread of light, which on the 5th appeared broken into separate points. It is evident that the phenomenon was not caused by the obliquity of the ring as seen from our globe, since the elevation of the Earth above the plane of the rings--which on December 18th was 3° 20'--was still 1° 20' on the 4th of February. In ordinary circumstances, when the Sun is a little more elevated, and the rings seen at this last angle, they appear quite broad and conspicuous, and even the dark open space separating the dusky ring from the planet is perfectly visible on the ansæ, where the Earth's elevation above their plane is reduced to 40'. It is also evident that the phenomenon was not to be attributed to the reduction of the light which they received from the Sun, although the illumination in February might be expected to be comparatively feeble, since the Sun then shone upon the rings so obliquely; yet (on the supposition that their surface is flat) they should have been illuminated throughout, and if not very brightly, sufficiently so, at least, to make them visible and as bright as was the narrow thread of light observed on the 4th of February. The phenomenon actually observed may be explained most readily by assuming, as other phenomena also indicate, that the surface of the ring is not flat, but more elevated towards, or in the vicinity of its outer border, from which place it slopes inwardly towards the planet. On this assumption, it is evident that the elevated part of the ring the nearest to the Sun would cast a shadow, which, with the increasing obliquity of the Sun, would gradually cover the whole surface comprised within the elevated part, and thus become invisible to us. Several observations made by Bond and other observers undoubtedly show the same phenomenon, and do not seem to be intelligible on any other supposition. From my observations made in 1881 it would appear, however, that the opposite surfaces of the rings do not exactly correspond in form, but this may not be a permanent feature, as the surface of this system is subject to changes, as already shown.

The dimensions of the rings are great, the diameter of the outer one being no less than 172,982 miles, the distance from the centre of the globe to the outer border of the system being, therefore, 86,491 miles. The breadth of the outer ring is 9,941 miles; that of the principal division, 2,131 miles; that of the middle ring, 19,902 miles, and that of the dusky ring, 8,772 miles. The breadth of all the rings taken together is, therefore, 40,746 miles. The interval between the surface of Saturn and the inner border of the dusky ring is 7,843 miles.

The thickness of the system of rings has been variously estimated by astronomers, on account of the great difficulties attending its determination. While Sir John Herschel estimated it at more than 250 miles, G. P. Bond reduces it to 40 miles. Both of these numbers are evidently too small, as so slight a thickness cannot explain the observed phenomenon of the shadow cast by a portion of the ring on its own surface, when the Sun is very low in its horizon, as shown above.

The plane of the system of rings is inclined 27° to the planet's orbit, and is parallel, or at least very nearly so, with the equator of the planet, passing, therefore, through its centre, and dividing its globe into northern and southern hemispheres. Seen from the Earth, a portion of the ring always appears projected in front of the planet, thus concealing a small part of its globe, while the opposite portion passes behind the globe, which hides it from sight.

As the plane of the ring is not affected by the motion of the planet around the Sun, but always remains parallel to itself, it follows that as Saturn advances in its orbit the rings must successively present themselves to us under various angles of inclination, appearing, therefore, more or less elliptical, and presenting two maxima and two minima of inclination in the course of one of its revolutions. As the revolution of Saturn is accomplished in 29½ years, the maxima and the minima must recur every 14 years and 9 months; the maxima being separated from the minima by an interval of 7 years and 4½ months.

When Saturn arrives at the two opposite points of its orbit, where the major axis of its ring is at right angles to the line joining its centre to that of the Sun, the ring, which is then viewed at an inclination of 27°, the greatest angle at which it can ever be seen, has reached its maximum opening, the smaller diameter of its ellipse being then about half that of the larger. At this moment the outer ring projects north and south beyond the globe, which is then completely enclosed in its ellipse. The maximum opening of the northern surface of the ring takes place, at present, when Saturn arrives in longitude 262°, in the constellation Sagittarius, and that of the southern surface when it arrives in longitude 82° in the constellation Taurus. When, on the contrary, Saturn reaches the two opposite points of its orbit, where the plane of its ring is parallel to the line joining its centre to that of the Sun, the opening vanishes, as only the thin edge of the ring is then presented to the Sun and receives its light, the rest being in darkness. At this moment the ring disappears, except in the largest telescopes, where it is seen as an exceedingly thin thread of light; and the Saturnian globe, having apparently lost its ring, appears solitary in the sky, like the other planets. The disappearance of the ring from this cause occurs now when Saturn arrives at 90° from either of the positions of maximum inclination, that is, in longitude 352° in the constellation Pisces, and in longitude 172° in the constellation Leo.

When the planet is in any other position than one of these last two, either the northern or the southern surface of the ring is illuminated by the Sun, while the opposite surface is in the night, and does not receive any direct sunlight. At the time of the passage of the plane of the ring through the Sun's centre, a change takes place in the illumination of the ring. If it is the northern surface which has received the rays of the Sun during the previous half of the Saturnian year, at the moment the plane has passed the centre of the Sun, the southern surface, after having been buried in darkness for 14¾ years, sees the dawn of its long day of the same length. Such a phenomenon will not occur until 1892, when the passage of the Sun from the northern to the southern side of the ring will close in twilight the day commenced in 1878.