Part 16

After the discovery of Jupiter’s fifth satellite, astronomers seem to have become possessed with that dull spirit of orderliness such as is sometimes exhibited by city councils in substituting numbers for historic and beautiful names in designating streets. No more of Jupiter’s satellites were given names such as might be appropriate for members of this Jovian family; but all were given numbers--the first four in order of their distance from Jupiter, the others in order of their discovery. Io, Europa, Ganymede, and Callisto are now designated, respectively, I, II, III, and IV, while V, VI, VII, and VIII have never had any designation other than these numbers.

The fifth satellite, discovered in 1892, is the nearest to Jupiter, and the smallest of all his satellites. Its diameter is probably not more than one hundred and twenty miles, but its exact size can be estimated only by the amount of light it reflects. It is too small to show a measurable disc, and cannot even be seen when it makes a transit across the planet. It would seem then a mere speck, if we could see it at all. It makes one revolution about Jupiter in less than twelve hours (eleven hours and fifty-seven minutes), and is only a little more than twenty-two thousand miles from the surface of the planet at the equator. It appears to us as a star of about the thirteenth magnitude, and cannot be seen except with a large telescope. Owing to the great curvature of the planet, and to the satellite’s being so near him, it cannot be seen from the surface of Jupiter beyond sixty-five degrees of latitude. It moves faster than any other satellite in the solar system, going at the rate of sixteen and a half miles a second. It does not make a revolution in as short a time as Phobos, the little satellite of Mars, does, but it has a much longer distance to travel and goes at a faster rate. The fact that Jupiter rotates in ten hours and the satellite makes a revolution around him in twelve hours results in the satellite’s taking five of Jupiter’s days to cross from the eastern horizon to the western. It would go through all its phases four times during that period if it were not that, being so near the planet, his huge form cuts off the sunlight from the little satellite for nearly one-fifth of the time, and it is never seen “full.”

This satellite is very difficult for us to see on account of its diminutive size and its nearness to the shining disc of Jupiter; yet it was discovered by means of the telescope, and not by photography, as so many small bodies are discovered nowadays, and by a man who thus far has not been able to see the fine line markings on Mars, which some other astronomers think they can see--a fact that is very interesting as showing the difference between observers even of great keenness of vision. From this satellite Jupiter would seem an enormous body, nearly eighty-five times larger than our sun appears to us, and, no doubt, a splendid object. But the little satellite pays rather dearly for the view by suffering numerous and long-continued eclipses.

The sixth and seventh satellites are also very minute bodies, measuring probably less than one hundred miles in diameter. They circle about Jupiter at a distance nearly thirty times more remote than our moon is from us. They are about seven million miles from the planet, and probably not more than barely visible from it. It takes them two hundred and sixty-five days to make one revolution, which is more than five hundred times as long as the period of Jupiter’s nearest satellite. These two satellites are so nearly of one size and revolve so nearly in the same time and at the same distance from Jupiter that they are thought to have had a common origin. Just what their relation is has not yet been determined.

The eighth satellite, discovered in January, 1908, is certainly no larger, and is perhaps still more tiny, than the sixth and the seventh, though it is a little brighter than either one of them. It is about three times farther away from Jupiter than the seventh satellite, and with eyes such as ours would not be visible from Jupiter. It shows to us as about a seventeenth-magnitude star, which is almost at the limit of our vision with even the largest telescope. It seems to revolve about Jupiter in a direction exactly opposite to that of the other satellites--a retrograde motion that appears in the solar system in only two or three other cases and has not yet been entirely accounted for.

Jupiter’s satellites have played an important part in astronomical discoveries and investigations. It was through observation of their transits that it was discovered that light occupied time in passing through space. When Jupiter was near us in his orbit, the eclipses occurred too soon for their calculated time; when he was farther away, they occurred too late. It was found that these irregularities were due to the fact that light is not transmitted through space instantaneously, and further investigation showed that it travels at the rate of 186,400 miles a second. The eclipses of Jupiter’s moons are carefully computed and recorded in the Nautical Almanac, and it is through observations of them that chronometers are corrected at sea.

Ganymede and Callisto have been found to keep always the same face toward the planet, as our moon keeps always the same face toward us; and it is thought that all of Jupiter’s satellites probably do this.

The symbol of Jupiter is ♃, a hieroglyph for the eagle, which was the bird of Jove.

XIV

SATURN

Among the four planets that we commonly see, the easiest, perhaps, to keep track of is Saturn. Its peculiar aspect is very distinctly marked. It appears as a large, pale, yellow star shining with a soft, misty light that sometimes barely escapes being dull. It is always as bright as a first-magnitude star, but not always as bright as Sirius, and never as brilliant as Mars, Jupiter, or Venus when they are at their brightest. The general effect of it is as a large rather than a brilliant star.

The only time it loses these very marked characteristics is when it is drawing in toward the sun, and thus nearing conjunction. At such times we see it each evening lower in the rosy glow of the setting sun, and more and more obscured and changed in color by the surrounding atmosphere. Then it sometimes seems as red as Mercury, and sometimes even twinkles a little in a sort of farewell gaiety as it backs away from us into the rays of the dazzling sun and finally disappears for a time from the evening sky. Proximity to the sun and entanglement in the atmosphere of the horizon has this effect more or less on all the planets, as we know, but it always seems unexpected in Saturn, because it is so out of keeping with his ordinarily large, pale, placid face, which suggests softness and gentleness rather than vivacity.

But there is no mistaking the planet even under this aspect if we but stop to think where he is. And it is through knowing where he is that it is so easy to keep track of Saturn. For nearly two years and a half, on an average, he remains in the same constellation, passing slowly over about one degree a month, or a little more than twelve degrees in a year, occupying almost thirty years in making one circuit through the constellations of the zodiac. One has, therefore, ample time to get well acquainted with him before he has wandered far from the position in which one first found him.

For nearly six months each year Saturn shines as an evening star, and, returning each year as he does with such slight changes of position, he comes to have something of the stability of a fixed star. Having seen him one year, we can count on his returning the next only about thirteen days behind time, and but little farther from his original position than twice the distance between the pointers in the Big Dipper.

The one degree a month which he travels along the ecliptic is toward the east, except for a little more than two months before opposition, and the same length of time afterward, when he has the slight apparent retrograde motion due to our overtaking and passing him, which has been explained. With Saturn this motion is so slight--only four degrees--that it does not put him much out of position, and it is, in fact, not much noticed except by close observers. He has all the time been going steadily on toward the east (for the retrograde motion is only an apparent motion), and the annual change of twelve degrees in position is always in this direction.

My first acquaintance with Saturn was when he was traveling through Pisces and Aries, where there are no first-magnitude stars to mark the path of the wandering bodies in the heavens. It was then that I was most impressed with the fixity and reliability of his return. Every autumn then for five years we watched Antares passing toward the west, followed by the little “milk dipper” in Sagittarius; and then Fomalhaut, crossing the sky in the same direction, though below the constellations of the zodiac; and then turned our eyes toward the east, knowing that the next bright body to come peeping over the tops of the trees would be Saturn. And when the first frosts began to strip the leaves from the trees we found the compensation that nature always gives when she destroys one beauty: we could see earlier in the evening, through the bare branches, that lovely yellowish disc, with its suggestion of aloofness and grandeur that is peculiar to it. For the face of Saturn, while never what we would call cold, has little in it of that bright, warm, friendly aspect which is at times so characteristic of Venus, Mars, and Jupiter.

AROUND ONE CIRCUIT OF THE SKIES WITH SATURN

Saturn is now (the autumn of 1912) in the first part of his path through Taurus, and he will be in that constellation during all of 1913 and the greater part of 1914.

From 1912 to 1920 he will be a beautiful object in the winter sky, threading his way slowly through that splendid galaxy of stars that blazes across the glittering sky peculiar to the cold winter nights. He will pass between the Pleiades and Aldebaran, and will be in opposition in that region on November 23, 1912. Farther east in the constellation he will be in opposition in the first week of December, 1913. Almost on the border line between Taurus and Gemini he will be in opposition during the third week in December, 1914; and, as this is very near the perihelion point in Saturn’s orbit, the planet will then be at his brightest.

In 1915 he will not be in opposition at all; but sometime within the first two or three days of 1916 he will reach that position, and will then be well on in his journey across Gemini. For these four years--from 1912 to 1916--he will be visible during the entire night, at the times of his opposition, and in his best condition. The rings that surround him will then be placed so that we will get a broad expanse of light from them, as well as from the planet itself, which greatly increases its brightness.

Saturn will then continue to move across Gemini, passing in the early part of 1917 under Castor and Pollux, and very near to Neptune--a meeting which, unfortunately, cannot be seen with the naked eye. During this year (1917) he will begin his journey through the smallest of all the constellations of the zodiac, Cancer, passing near the lovely cluster of stars we call the Bee-hive, and will reach Leo early in 1919, where he will remain until about the end of 1921. While in this region he will be visible during the winter and all of the spring and the early summer. All three of these constellations--Gemini, Cancer, and Leo--while seen in the winter, are particularly lovely in the spring. Gemini, in the beautiful evenings of May, hangs with its two splendid stars in the northwest above the setting sun; and with the soft face of Saturn near them, these stars will be more than ever charming in the two seasons that the planet remains with them.

In 1917 Saturn will be in opposition in the region of Gemini, about the middle of January. In 1918 opposition will occur about the last of January, and Saturn will then be in Cancer. The next year he will be in opposition sometime during the second week in February, and will then be situated between the Bee-hive, in Cancer, and the brilliant first-magnitude star Regulus, in Leo. The next two oppositions will be in Leo, about thirteen days later each year. Saturn will then pass during the first half of 1922 into Virgo, which is the largest of all the constellations, and he will remain there until three oppositions have taken place, about thirteen days later each year.

About a year after passing Spica, the white, sparkling, first-magnitude star in Virgo, Saturn will enter Libra, crossing that constellation near the lower part of the square in it. From there he will go through Scorpio and Sagittarius, passing above Antares and the “milk dipper,” and in about 1932 will have reached that comparatively starless region which includes a part of Sagittarius and all of Capricornus, Aquarius, Pisces, and Aries. For the next nine and a half years he will give distinction to this part of the heavens, and thus complete his circuit of twenty-nine and a half years, and, with never resting, never changing movement, will start on a new round, with a new generation of eyes following his fair face along the great circle of the ecliptic.

Saturn is brightest when he is in Taurus, not far from Gemini, as he will be in 1914, and again when he is in Scorpio, as he will be between fourteen and fifteen years later. The recurring times at which we can get an evening view of him at his greatest brightness thus alternate between midwinter and midsummer. He is least bright when he is in the last half of Leo and when he is in that part of Aquarius above Fomalhaut. Between these positions he gradually waxes and wanes in brightness, changes that are largely due to the position of his rings.

DISTANCE AND SIZE

Saturn is almost twice as far from the sun as Jupiter, and between nine and ten times farther than we are. His mean distance from the sun is eight hundred and eighty-seven million miles; but his distance varies nearly one hundred million miles between perihelion and aphelion. His orbit is only a trifle more eccentric than that of Jupiter, but the variation in miles is so much greater because the orbit is so much larger.

His average distance from the earth at opposition is seven hundred and ninety-four million miles, but at the most favorable opposition it may be fifty million miles nearer than that. At conjunction his average distance is nine hundred and eighty million miles; but his greatest possible distance at such times may be as much as one billion miles. When he is in this situation it takes light a little more than an hour and a half to pass from him to us. At his nearest we receive light from him in about an hour and six minutes. At his average distance from the sun, light requires about an hour and twenty minutes to go from one to the other.

While Saturn is next to Jupiter in size among the planets, he is not as large as Jupiter by two-thirds, but his mass is almost three times greater than that of all the other planets put together except Jupiter. It is ninety-five times greater than that of the earth. In diameter Saturn is 72,772 miles; but it is more flattened at the poles than any other planet, and in consequence there is a difference of nearly seven thousand miles between its polar and its equatorial diameters.