Part 16

Aside from the periodic disappearance of the ring, resulting from the passage of the Sun through its plane, the ring may also disappear from other reasons. Just before or just after the time of the passage of the Sun through the plane of the ring, the Earth and the Sun may occupy such positions, that while the one is north of the plane of the ring, the other is south of it, or vice versa, in which event the ring becomes invisible, because its dark and non-illuminated surface is presented to us. The ring may also become invisible to us when the Earth passes through its plane.

Since the distance from Saturn to the Sun is to the distance of the Earth from this last body as 9.54 is to 1; and since the circumference of a circle increases in the same proportion as its radius, it follows that the diameter of the Earth's orbit projected on the orbit of Saturn would occupy only part of the latter, or about 12° 2', this being 6° 1' on either side of the nodes of the rings. To describe such an arc on its orbit, it takes Saturn almost 360 days on an average, or almost a complete year; the Earth describing therefore almost a whole revolution around the Sun during the time it takes Saturn to advance 12° 2' on its orbit. Then, when Saturn occupies a position comprised within an arc 6° 1' from either side of the nodes of its ring, the Earth, by its motion, is liable to encounter the plane of the ring, when therefore it will only present its thin edge to us, and becomes invisible. At least one such encounter is unavoidable within the time during which Saturn occupies either of these positions on its orbit; while three frequently happen, and two are possible.

The natural impression received by looking at the rings, while seeing the ponderous globe of Saturn enclosed in its interior, is that this gigantic, but very delicate structure, in order to avoid destruction, must be endowed with a swift movement of rotation on an axis perpendicular to its plane, and that the centrifugal force thence arising counterbalances the powerful attraction of the planet, and thus keeps the system in equilibrium.

Theoretically, the rotation of the rings is admitted by every astronomer, as being an essential condition to the existence of the system, which otherwise, it is thought, would fall upon the planet. Although the rotation of the rings seems so probable that it is theoretically considered as certain, yet its existence has not been satisfactorily demonstrated by direct observation, which alone can establish it on a firm basis as a matter of scientific knowledge.

The determination of the period of rotation of the rings, which is supposed to be 10h. 32m. 15s., rests only on the observations of W. Herschel, made in 1790, from the apparent displacement of irregularities on the ring; but his results have been contradicted by other observations, and even by those of Herschel himself, made in later years.

Although the system of rings is very nearly concentric with the globe of Saturn, yet the coincidence is not considered as mathematically exact. It seems to have been satisfactorily demonstrated by direct observations that the centre of gravity of the system oscillates around that of the planet, thus describing a minute orbit. This peculiarity is in accordance with theory, which has shown it to be essential to the stability of the system.

Besides its system of rings, which makes Saturn the most remarkable planet of the solar system, this globe is attended by eight satellites, moving in orbits whose planes very nearly coincide with the plane of the rings, except that of the most distant one, which has an inclination of about 12° 14'. In the order of their distance from the planets, the satellites of Saturn are as follows: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion and Iapetus. The three first satellites are nearer to Saturn than the Moon is to the Earth; while Iapetus, the farthest, is 9½ times the distance of our satellite from us. All the satellites, with the exception of the farthest, move more rapidly around Saturn than the Moon moves around the Earth; while Iapetus, on the contrary, takes almost three times as long to make one revolution.

The period of revolution of the four inner satellites is accomplished in less than three days, that of Mimas being only a little more than 22 hours. From such swiftness of motion, it is easily understood how short must be the intervals between the different phases of these satellites. Mimas, for instance, passes from New Moon to First Quarter in less than 6 hours.

The distance of the nearest satellite from the planet's surface is 84,000 miles, and its distance from the outer ring only 36,000 miles. It is difficult to determine the diameter of objects so faint and distant as are some of these satellites, but the diameter of Titan, the largest of all, is pretty well known, and estimated to be ¹⁄₁₆ the diameter of the planet, or more than half the diameter of our globe.

Iapetus is subject to considerable variations in brilliancy, and as the maxima and minima always occur when this satellite occupies the same parts of its orbit, it was conjectured by W. Herschel that, like our Moon, it turns once upon its axis during each of its revolutions about the planet. It has been shown by my observations, that Iapetus attains its maximum brightness a little before it reaches its greatest western elongation, and its minimum on the opposite side.

As the planes of the orbits of the satellites are inclined to the planet's orbit, it follows that their transits, occultations and eclipses, are only possible when Saturn is near its equinoxes. Passages of the satellites and their shadows across the disk, although rare, have been observed, and they somewhat resemble the phenomena exhibited by the satellites of Jupiter in transit. When the Earth is very near the plane of the rings, the satellites, except the farthest, appear to be in a straight line nearly coincident with the plane of the rings, and are seen occasionally moving along the thin edge of the rings, appearing as luminous beads moving on a thread of light.

Owing to the considerable inclination of the axis of rotation of Saturn to its orbit, the seasons of this planet must have greater extremes of temperature than those of the Earth. As the year of Saturn consists of 25,217 Saturnian days, each season, on the average, is composed of 6,304 Saturnian days.

To an observer on Saturn, the immense arches formed by its rings would appear as objects of great magnificence, spanning the sky like soft colorless rainbows. Moreover, the eight moons, several of which are always visible, would be of the highest interest, with their swift motions and rapid phases. Mimas, traveling in its orbit at the rate of 16 of arc per minute of time, moves over a space equal to the apparent diameter of our Moon in two minutes, or at the rate of 16° an hour.

Owing to the globular form of Saturn, the rings would be invisible in latitudes situated above 65° from its equator, and their apparent form and breadth would naturally vary with the latitude. At 63° only a very small portion of the outer ring would be visible above the equatorial horizon, where it would appear as a small segment of a circle. At 62° the principal division would just graze the horizon. At 46° the outer portion of the dusky ring would become visible, while at 35° its inner edge would appear above the horizon. From 65° of latitude down to the equator, the arches of the rings would be seen more and more elevated above the equatorial horizon, but at the same time that they are seen higher up, their apparent breadth gradually diminishes, owing to the effect of foreshortening, and at the equator itself the system would only present its thin edge to view.

During the summer seasons of either hemisphere of Saturn, the surface of the rings turned towards such hemisphere, being fully illuminated by the Sun, is visible from these regions. In the day time its light must be feeble and similar to the light reflected by our Moon during sunshine; but at night the system would display all its beauty, and the different rings, with their divisions and their various reflective powers, must present a magnificent sight.

During the nights of the long winter seasons on Saturn, on the contrary, the surface of the rings turned towards the hemisphere undergoing winter, receives no light from the Sun, and is invisible, or very nearly so, except towards morning and evening, when it may be faintly illuminated by the secondary light which it receives from the illuminated globe of Saturn. Although dark and invisible, the rings may make their form apparent at night by the absence of stars from the region which they occupy in the sky. Again, in other seasons, the days present very curious phenomena. In consequence of the diurnal rotation of the planet, the Sun seems to move in circular arcs, which, owing to the inclination of Saturn's axis, are more or less elevated above its horizon, according to the position of the planet in its orbit. As such arcs described by the Sun in the sky of Saturn are liable to encounter the rings, the Sun in passing behind them becomes eclipsed. It must be a magnificent spectacle to witness the gradual disappearance of the fiery globe behind the outer ring, and its early reappearance, but for a moment only, through the narrow gap of the principal division; to see it vanish again behind the middle ring, to reappear a little later through the semi-transparent dusky ring, but very faint and red colored at first; and then, gradually brighten up, and finally emerge in all its beauty from the inner edge of the dusky ring.

It is in latitude 23° that the rings produce the most prolonged eclipses of the Sun. During a period equivalent to ten of our terrestrial years, such eclipses continually succeed each other with but very short periods of interruption; and even during a long series of rotations of Saturn, the Sun remains completely invisible in those regions where the apparent arcs which it describes coincide with the arcs of the rings. In neighboring latitudes, the eclipses of the Sun, although still frequent, would have a shorter and shorter duration as the observer should travel north or south. These eclipses of the Sun must produce a partial darkness of the regions involved in the shadow of the rings, which may be compared to the darkness produced on our globe by a total eclipse of the Sun. The frequent recurrence of these eclipses, and their comparatively long duration in some regions, must still further reduce the duration of the short Saturnian days.

The globe of Saturn, as already shown, casts a shadow on the rings, which, according to the position of the planet in its orbit, either extends across their whole breadth, or covers only a part of their surface. The shadow on the rings rising in the east after sunset, ascends to the culminating point of their arcs in the sky, in 2h. 34m., and as rapidly descends on the western horizon, to disappear with sunrise. This shadow, when projected on the rings in the sky, must be hardly distinguishable from the dark background of the heavens, except from the absence of stars in the regions which it occupies. It must appear as a large dark gap, separating the rings into two parts, and constantly moving from east to west. Possibly the refraction of the solar rays, in passing through Saturn's atmosphere, may cast some colored light on the rings, similar to that observed on the Moon during its eclipses.

An observer on the rings would behold phenomena still more curious, a long day of 14¾ years being followed by a long night of 14¾ years. The long days of Saturn's rings are, however, diversified by numerous eclipses of the Sun, which regularly occur every 10¼ hours; the phenomenon being due to the interposition of the globe of Saturn between the rings and the Sun. These eclipses produce partial obscurations of their surface, lasting from 1½ to 2 hours at a time. Although the surface of the rings never receives direct sunlight during their long nights, yet they are not plunged all the time in total darkness, as they receive some reflected light from that part of the globe of Saturn which is illuminated by the Sun. To the supposed observer on the rings, during every 10¼ hours, the immense globe would exhibit continually changing phases. At first he would see a point of light rapidly ascending from the horizon, and appearing under the form of a half crescent of considerable radius; 5⅛ hours later, the crescent having gradually increased, would appear as a half circle, covering ⅛ of the visible heavens, its surface being more than 20,000 times as large as the surface of the Moon. Upon this brilliantly illuminated semi-circle would be projected the shadows of the rings, appearing as black belts separated by a narrow luminous band.

It is very difficult for one to conceive how such a delicate structure, as the system of rings appears to be, can keep together in equilibrium and avoid destruction from the powerful attraction of the planet on one side and the disturbing influence of the satellites on the other. To explain it, several hypotheses have been advanced. The rings were first supposed to be solid, and upon this supposition Laplace determined the necessary conditions for their equilibrium; the most important of which require that the cross section of the rings should be an ellipse of irregular curvature, and having its major axis directed towards the centre of the planet, and also that the system should rotate upon an axis perpendicular to the plane of the rings. This theory was superseded by another, which supposed the rings to be fluid. This one was soon rejected for a third, assuming the system to be composed of vapors or gases; and more recently, all these theories were considered untenable, and replaced by a fourth, which supposes the system of rings to be made up of a congregation of innumerable small, independent bodies, revolving around Saturn in concentric zones. Naturally, such a divergence of opinion can only result from our comparative ignorance of the subject, and sufficiently indicates our inability to explain the phenomena; and it must be admitted that, so far, nothing is certainly known about this strange system. We shall probably remain in the same uncertainty until the rotation of the rings is ascertained by direct observations. It is pretty certain, however, that none of these theories account for the observed phenomena in their details, although a partial explanation may be obtained by borrowing something from each hypothesis.

It has been conjectured, and a theory has been advanced, that the breadth of the whole ring system is gradually increasing inwards, and that it will come in contact with the planet in about 2,150 years; but the question seems to have been settled in the negative by the elaborate measurements of the English observers. It is likely that the increase is only in the defining power of the instruments.

COMETS

PLATE XI

Among the celestial phenomena, none are more interesting than those mysterious apparitions from the depths which unexpectedly display their strange forms in our familiar constellations, through which they wander for a time, until they disappear like phantoms.

A comet, with its luminous diffused head, whence proceeds a long vapory appendage gradually fading away in the sky, presents an extraordinary aspect, which may well astonish and deeply impress the observer. Although these visitors from infinite space do not now inspire dread, as in by-gone times, yet, owing to the mystery in which the phenomenon is still involved, the apparition of a large comet, even in our days, never fails to create a profound sensation, and in some cases that unconscious fear which results from the unknown.

The effect of such a spectacle largely depends upon its rarity; but since the telescope has been applied to the sounding of the heavens, it has been found that the appearance of comets is by no means an unusual occurrence. If so few comets, comparatively, are seen, it is because most of them are telescopic objects, and are therefore invisible to the naked eye. Most of the telescopic comets are not only too faint to be perceived by the unaided eye, but are insignificant objects, even when observed through the largest telescopes.

It was Kepler's opinion that comets are as numerous in the sky as fishes are in the ocean. Undoubtedly the number of these bodies must be great, considering that we can only see them when they come into the neighborhood of the Earth, and that many even here remain invisible, or at least pass unperceived. That many of them have passed unperceived heretofore, is proved by the fact that the number of those observed becomes greater every year, with the increase of the number of instruments used in their search. The number of comets observed with the naked eye during historic times is nearly 600, and that of telescopic comets, which, of course, all belong to the last few centuries, is more than 200, so that we have a total number of about 800 comets of which records have been kept. From theoretical considerations, Lambert and Arago estimated their entire number at several millions, but such speculations have generally no real value, since they cannot be established on a firm basis.

Comets remain visible for more or less time, according to their size and the nature and position of their orbits, but in general, the large ones can be followed with the telescope for several months after they have become invisible to the naked eye. The comet of 1861, for example, remained telescopically visible for a year, and that of 1811, for 17 months after disappearing from ordinary sight.