Saturn Introduction

A short introduction to the planet Saturn

The Planet Saturn 

Saturn is the sixth of the major planets in the order of dis tance from the sun, around which it revolves in 29 years at a mean distance of about 880 millions of miles.  In mass and size it stands next to Jupiter.  To show the disparity in the masses of the planets we may refer to the table already given, showing that although Saturn is not one -third the mass of Jupiter, it has about three times the mass of the six planets, which are smaller than itself put together.  Its surroundings are such as to make it the most magnificent object in the solar system.  While no other planet is known to have more than four satellites, Saturn has less than eight.  It is also surrounded by a pair of rings, the interior diameter of which is about 100,000 miles.  The aspect of these rings is subject to great variations, for reasons which will soon appear.  The great distance of the planet renders the study of its details difficult unless the highest telescopic power is applied.  The whole combination of Saturn, his rings, and his satellites is often called the Saturnian System.

The planet Saturn generally shines with the brilliancy of a moderate first-magnitude star, and with a dingy, reddish light, as if seen through a smoky atmosphere.  Its apparent bright ness is, however, different at different times : during the years 1876-1879 it was fainter than the average, owing to its ring being seen nearly edgewise. 
In subsequent years opposition will occur about thirteen days later every year, so that by adding this amount to the date for each -year the oppositions can be found until the end of the century without an error of more than a few days.

The physical constitution of Saturn seems to bear a great resemblance to that of Jupiter ; but, being twice as far away, it cannot be so well studied.  The farther an object is from the sun, the less brightly it is illuminated ; and the farther from the earth, the smaller it looks, so that there is a double difficulty in getting the finest views of the more distant plan ets.  When examined under favorable circumstances, the sur : face of Saturn is seen to be diversified with very faint mark ings; and if high telescopic powers are used, two or more very faint streaks or belts may be seen parallel to its equator, the strongest ones lying on, or very near, the equator.  As in the case of Jupiter, these belts change their aspect from time to time, but they are so faint that the changes cannot be easily followed.  It is therefore, in general, difficult to say with certainty whether we do or do not see the same face of Saturn on different nights ; and, consequently, it is only on extraordinary occasions that the time of rotation can be de termined.

The first occasion on which a well-defined spot was known to remain long enough on Saturn to determine the period of its rotation was in the time of Sir W. Herschel, who, from observations extending over several weeks, found the time of rotation to be 10 hours 16 " minutes.* No further opportu nity for determining this period seems to have offered itself until 1876, when an appearanqe altogether new suddenly show r ed itself on the globe of this planet.  On the evening of December 7th, 1876,Professor Hall, who had been engaged in measures of the satellites of Saturn with the great Wash ington telescope, saw a brilliant white spot near the equator of the planet.  It seemed as if an immense eruption of white hot matter had suddenly burst up from the interior.  The spot gradually spread itself out in the direction which would be east on the planet, so as to assume the form of a long light streak, of which the brightest point was near the following end.  It continued visible until January, when it became faint and ill-defined, and the planet was lost in the rays of the sun. Immediately upon the discovery of this remarkable phenom enon, messages were sent to other observers in various parts of the country, and on the 10th it was seen by several observers, who noted the time at which it crossed the centre of the disk in consequence of the rotation of the planet.  From all the observations of this kind, Professor Hall found the period of Saturn to be 10 hours 14 minutes, taking the brightest part of the streak, which, as we have said, was near one end.  Had the middle of the streak been taken, the time would have been less, because the bright matter seemed to be carried along in the direction of the planet's rotation.  Attributing .this to a wind, the velocity of the latter would have been be tween 50 and 100 miles an hour.

Saturns Rings

The most extraordinary feature of Saturn is the magnificent system of rings by which he is surrounded.  To the early telescopists, who could not command sufficient optical power to see exactly what it was, this feature was a source of great

* It is very curious that nearly all modern writers give about 10 hours 29 min utes as the time of rotation of Saturn which Herschel finally deduced.  I can find no such result in Herschel's papers.  A suspicious coincidence is that this period agrees with that assigned for the time of rotation of the ring.

Perplexity and difference of opinion.  To Galileo it made the planet appear triform a large globe with two small ones af fixed to it, one on each side.  After he had observed it for a year or two, he was greatly perplexed to find that the append ages had entirely disappeared, leaving Saturn a single round globe, like the other planets.  His chagrin was heightened by the fear, not unnatural under the circumstances, that the curi ous form he had before seen might be due to some optical il lusion connected with his telescope.  It is said (I do not know on what authority) that his annoyance at the supposed decep tion into which he had fallen wa^ so great that he never again looked at Saturn.

A very few years sufficed to show other observers, who had command of more powerful telescopes, that the singularity of form was no illusion, but that it varied from time to time.  We give several pictures from Huyghens's Systema Saturniurrij showing how it was represented by various observers during the first forty years of the telescope.  If the reader will com pare these with the picture of Saturn and his rings as they actually are, he will see how near many of the observers came to a representation of the proper apparent form, though none divined to what sort of an appendage the appearance was due.

The man who at last solved the riddle was Huyghens, of whose long telescopes we have already spoken.  Examining Saturn in March and April, 1655, he saw that instead of the appendages presenting the appearance of curved handles, as in previous years, a long narrow arm extended straight out on each side of the planet.  The spring following, this arm had disappeared, and the planet appeared perfectly round as Gal ileo had seen it in 1612.  In October, 1655, the handles had reappeared, much as he had seen them a year and a half be fore.  To his remarkably acute mathematical and mechanical mind this mode of disappearance of the handles sufficed to suggest the cause which led to their apparent form.  Waiting for entire confirmation by future observations, he communica ted his theory to his fellow-astronomers in the following : Specimens of drawings of Saturn by various observers before the rings were recognized as such: I. Form as given by Galileo in 1610 ; II.  Drawing by Scheinev, in 1614, "showing ears to Saturn;" III.  Drawing by Ricciolus, in 1640 and 1643; IV.,V.,

VI. , and VII.  Are by Hevelius, and show the changes due to the different angles under which the rings were seen ; VIII.  And IX.  Are by Ricciolus, between 1648 and 1650, when the ring was seen at the greatest angle ; X. is by a Jesuit who passed under the pseudonym of EustacMus cfe Divinis; XI.  Is by Fontana; XII.  By Gaesendi and Blaucauus, and XIII.  By Ricciolus.

Combination of letters, printed without explanation at the end of a little pamphlet on his discovery of the satellite of Saturn :

aaaaaaa ccccc d eeeee g h iiiiiii llll mm nnnnnnnnn oooo pp q rr s ttttt uuuuu,

which, properly arranged, read:

* * Annulo cingitur, tenui, piano, nusquam cokcerente, ad eclipticam inclinato" (It is girdled by a thin plane ring, nowhere touching, inclined to the ecliptic).

This description is remarkably complete and accurate ; and enabled Hnyghens to give a satisfactory explanation of the various phases which the ring had assumed as seen from the earth.  Owing to the extreme thinness and flatness of the ob ject, it was completely invisible in the telescopes of that time when its edge was presented towards the observer or towards the sun. This happens twice in each revolution of Saturn, in much the same way that the earth's equator is twice directed towards the sun in the course of the year.  The ring is in clined to the plane of the planet's orbit by 27, corresponding to the angle of 23J between the earth's equator and the ecliptic.  The general aspect from the earth is very near the same as from the sun. As the planet revolves around the sun, the axis and plane of the ring preserve the same absolute direction in space, just as the axis of the earth and the plane of the equator do.

When the planet is in one part of its orbit, an observer at the sun or on the earth will see the upper or northern side of the ring at an inclination of 27.  This is the greatest angle at which the ring can ever be seen, the position occurring when the planet is in 262 of longitude, in the constellation Sagittarius.  When the planet has moved through a quarter of a revolution, the edge of the ring is turned towards the sun, and, owing to its extreme thinness, it is visible only in the most powerful telescopes as an exceedingly fine line of light, stretching out on each side of the planet.  In this position the planet is in longitude 352, in the constellation Pisces.  When the planet has moved 90 farther, an observer on the sun or earth again sees the ring at an angle of 27 ; but now it is the lower or southern side which is visible.  The planet is now in longitude 82, between the constellations Taurus and Gemini.  When it has moved 90 farther, to longitude 172, in the con stellation Leo, the edge of the ring is again turned towards the earth and sun.

Thus there are a pair of opposite points of the orbit of Sat urn in which the rings are turned edgewise to us, and another pair half-way between the first in which the ring is seen at its maximum inclination of about 27.  Since the planet per forms a revolution in 29 years, these phases occur at average intervals of about seven years and four months. 
Owing to the motion of the earth, the times when the edge of the ring is turned towards it do not accurately correspond to those when it is turned towards the sun, and the points of Saturn's orbit in which this may occur range over a space of several degrees.  The most interesting times for viewing the rings with powerful telescopes are on those rare occasions when the sun shines on one side of the ring, while the dark side is directed towards the earth.  On these occasions the plane of the ring, if extended out far enough, would pass be tween the sun and the earth.  This will be the case between February 9th and March 1st, 1878 ; but, unfortunately, at that time the earth and Saturn are on opposite sides of the sun, so that the planet is nearly lost in the sun's rays, and can be ob served only low down in the west just after sunset.  In 1891 the position of Saturn will be almost equally unfavorable for the observation in question, as it can be made only in the early mornings of the latter part of October of that year, just after Saturn has risen.  In fact, a good opportunity will not occur till 1907.  In northern latitudes the finest telescopic views of Saturn and his ring may be obtained between 1881 and 1889, because during that interval Saturn passes his perihelion, and also the point of greatest northern declination, while the ring is opened out to its widest extent.  In fact, these three most favorable conditions all fall nearly together during the years 1881-'85.


After Huyghens, the next step forward in discoveries on Saturn's ring was made by an English observer, named Ball, otherwise unknown in astronomy, who found that there were really two rings, divided by a narrow dark line.  The breadth of the rings is very unequal, the inner ring being several times broader than the outer one.  A moderate sized telescope is sufficient to show this division near the extreme points of the ring if the atmosphere is steady ; but it requires both a large telescope and tine seeing to trace it all the way across that part of the ring which is between the observer and the ball of the planet.  Other divisions, especially in the outer ring, have at times been suspected by various observers, but if they real ly existed, they must have been only temporary, forming and closing up again.

In December, 1850, the astronomical world was surprised by the announcement that Professor Bond, of Cambridge, had discovered a third ring to Saturn.  It lay between the rings already known and the planet, being joined to the inner edge of the inner ring.  It had the appearance of a ring of crape, being so dark and obscure that it might easily have been overlooked in smaller telescopes.  It was seen in England by Messrs.  Lassell and Dawes before it was formally announced by the Bonds.  Something of the kind had been seen by Dr. Galle, at Berlin, as far back as 1838 ; but the paper on the subject by Encke, the director of the observatory, did not de scribe the appearance very clearly.  Indeed, on examining the descriptions of observers in the early part of the eighteenth century, some reason is found for suspecting that they saw this dusky ring ; but none of the descriptions are sufficiently definite to establish the fact, though it is strange if an object so plain as this ring now is should have been overlooked by all the older observers.

The question whether changes of various sorts are going qn in the rings of Saturn is one which is still unsettled.  There is some reason to believe that the supposed additional divis ions noticed in the rings from time to time are only errors of vision, due partly to the shading which is known to exist on various parts of the ring.  By reference to the diagram of Saturn, it will be seen that the outer ring has a shaded line extending around it about two-thirds of the way from its in ner to its outer edge.  This line, however, is not fine and sharp, like the known division, but seems to shade off gradual ly towards each edge.  As observers who have supposed them selves to see a division in this ring saw it where this shaded line is, and do not speak of the latter as anything distinct from the former, there is reason to believe that they mistook this permanent shading for a new division.  The inner ring is brightest near its outer edge, and shades off gradually towards its inner edge.  Here the dusky ring joins itself to it, and ex tends about half-way in to the planet.

As seen with the great Washington equatorial in the au tumn of 1874, there was no great or sudden contrast be tween the inner or dark edge of the bright ring and the out er edge of the dusky ring.  There was some suspicion that the one shaded into the other by insensible gradations.  No one could for a moment suppose, as some observers have, that there was a separation between these two rings.  All these considerations give rise to the question whether the dusky ring may not be growing at the expense of the inner bright ring.

A most startling theory of changes in the rings of Saturn was propounded by Struve, in 1851.  This was nothing less than that the inner edge of the ring was gradually approach ing the planet in consequence of the whole ring spreading in wards, and the central opening thus becoming smaller.  The data on which this theory was founded were the descriptions and drawings of the rings by the astronomers of the seven teenth century, especially Iluyghens, and the measures ex ecuted by later astronomers up to the time at which Struve wrote.  The rate at which the space between the ring and the planet was diminishing seemed to be about 1".3 per century.  The following are the numbers used by Struve, which are de duced from the descriptions by the ancient observers, and the measures by the modern ones :

If these estimates and measures were certainly accurate, they would place the fact of a progressive approach of the rings to the ball beyond doubt, an approach which, if it con tinued at the same rate, would bring the inner edge of the ring into contact with the planet about the year 2150.  But in measuring such an object as the inner edge of the ring of Saturn, which, as we have just said, seems to fade gradually into the obscure ring, different observers will always obtain different results, and the differences among the four observ ers commencing with W. Struve are no greater than are often seen in measuring an object of such uncertain outline.  Hence, considering the great improbability of so stupendous a cosmi cal change going on with so much rapidity, Struve's theory has always been viewed with doubt by other astronomers.

At the same time, it is impossible to reconcile the descrip tions by the early observers with the obvious aspect of the ring as seen now without supposing some change of the kind.  The most casual observer who now looks at Saturn will see that the breadth of the two bright rings together is at least half as great again, if not twice as great, as that of the dark space between the inner edge of the bright ring and the plan et.  But Huyghens describes the dark space as about equal to the breadth of the ring, or a little greater.  Supposing the ring the same then as now, could this error have arisen from the imperfection of his telescope ?  No ; because the effect of the imperfection would have been directly the opposite.  The old telescopes all represented planets and other bright objects too large, and therefore would show dark spaces too small, owing .to the irradiation produced by their imperfect glasses, A strong confirmation of Struve's view is found in old pictures by those observers who could not clearly make out the ring.  In nearly all cases the dark spaces were more conspicuous than the edges of the ring.  But if we now look at Saturn through a very bad atmosphere, though the elliptical outline of the ring may be clearly made out, the dark space will be almost obliterated by the encroachment of the light of the planet and ring upon it* The question is, therefore, one of those the complete solution of which must be left to future observers.

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