With a field-gla.s.s, however, you will have no difficulty in seeing all of the moons when they are properly situated. If you miss one or more of them, you may know that it is either between you and the planet, or behind the planet, or buried in the planet"s shadow, or else so close to the planet as to be concealed by its radiance.
It will be best for the observer to take out of the Nautical Almanac the "configurations of Jupiter"s satellites" for the evenings on which he intends to make his observations, recollecting that the position of the whole system, as there given, is reversed, or presented as seen with an astronomical telescope, which inverts objects looked at, as an opera-gla.s.s does not. In order to bring the satellites into the positions in which he will see them, our observer has only to turn the page in the Nautical Almanac showing their configurations upside down.
Of course, since the motions of the satellites, particularly of the inner ones, are very rapid, their positions are continually changing, and their configurations are different every night. If the observer has any doubt about his identification of them, or thinks they may be little stars, he has only to carefully note their position and then look at them again the next evening. He may even notice their motion in the course of a single evening, if he begins early and follows them for three or four hours. It is impossible to describe the peculiar attractions of the scene presented by the great planet and his four little moons on a serene evening to an observer armed with a powerful gla.s.s. Probably much of the impressiveness of the spectacle is owing to the knowledge that those little points of light, shining now in a row and now in a cl.u.s.ter, are actually, at every instant, under the government of their giant neighbor and master, and that as we look upon them, obediently making their circuits about him, never venturing beyond a certain distance away, we behold a type of that gravitational mastery to which our own little planet is subject as it revolves around its still greater ruler, the sun, to whose control even Jupiter in his turn must submit.
The beautiful planet Saturn requires for the observation of its rings magnifying powers far beyond those of the instruments with which our readers are supposed to be armed. It would be well, however, for the observer to trace its slow motion among the stars with the aid of the Nautical Almanac, and he should be able with a good field-gla.s.s to see, under favorable circ.u.mstances, the largest of its eight moons, t.i.tan.
This is equal in brilliancy to an 8.5 magnitude star. Its position with respect to Saturn on any given date can be learned from the Ephemeris.
It may appear somewhat presumptuous to place Ura.n.u.s, a planet which it required the telescope and the eye of a Herschel to discover, in a list of objects for the opera-gla.s.s. But it must not be forgotten that Ura.n.u.s was seen certainly several, and probably many, times before Herschel"s discovery, being simply mistaken, on account of the slowness of its motion, for a fixed star. When near opposition, Ura.n.u.s looks as bright as a sixth-magnitude star, and can be easily detected with the naked eye when its position is known. With an opera-gla.s.s (and still more readily with a field-gla.s.s) this distant planet can be watched as it moves deliberately onward in its gigantic orbit. Its pa.s.sage by neighboring stars is an exceedingly interesting phenomenon, and it is in this way that you may recognize the planet.
On the evening of May 29, 1888, I knew, from the co-ordinates given in the Nautical Almanac, that Ura.n.u.s was to be found a short distance east of Mars, which was then only a few degrees from the well-known star Gamma Virginis. Accordingly, I turned my opera-gla.s.s upon Mars, and at once saw a star in the expected position, which I knew was Ura.n.u.s. But there were other small stars in the field, and, supposing I had not been certain which was Ura.n.u.s, how could I have recognized it? The answer is plain: simply by watching for a night or two to see which star moved.
That star would, of course, be Ura.n.u.s. The accompanying cuts will show the motions of Mars and Ura.n.u.s with respect to neighboring stars at that time, and will serve as an example of the method of distinguishing a planet from the fixed stars by its change of place. In the first cut we have the two planets and three neighboring stars as they appeared on May 29th. These stars were best seen with a field-gla.s.s, although an opera-gla.s.s readily showed them.
[Ill.u.s.tration: MARS AND URa.n.u.s, MAY 29, 1888.]
[Ill.u.s.tration: MARS AND URa.n.u.s, JUNE 1, 1888.]
[Ill.u.s.tration: MARS AND URa.n.u.s, JUNE 6, 1888.]
On June 1st the relative positions of the planets and stars were as shown in the second cut. A glance suffices to show that not only Mars but Ura.n.u.s also has shifted its position with respect to the three immovable stars. This change of place alone would have sufficed to indicate the ident.i.ty of Ura.n.u.s. To make sure, the inexperienced observer had only to continue his observations a few nights longer.
On June 6th Mars and Ura.n.u.s were in conjunction, and their position, as well as that of the same set of three stars, is shown in the third cut.
It will be seen that while Mars had changed its place very much more than Ura.n.u.s, yet that the latter planet had now moved so far from its original position on May 29th, that there could be no possibility that the merest tyro in star-gazing would fail to notice the change. Whenever the observer sees an object which he suspects to be a planet, he can satisfy himself of its ident.i.ty by making a series of little sketches like the above, showing the position of the suspected object on successive evenings, with respect to neighboring stars. The same plan suffices to identify the larger planets, in the case of which no gla.s.s is necessary. The observer can simply make a careful estimate by the naked eye of the supposed planet"s distance and bearing from large stars near it, and compare them with similar observations made on subsequent evenings.
THE SUN.--That spots upon the sun may be seen with no greater optical aid than that of an opera-gla.s.s is perhaps well known to many of my readers, for during the past ten years public attention has been drawn to sun-spots in an especial manner, on account of their supposed connection with meteorology, and in that time there have been many spots upon the solar disk which could not only be seen with an opera-gla.s.s, but even with the una.s.sisted eye. At present (1888) we are near a minimum period of sun-spots, and the number to be seen even with a telescope is comparatively very small, yet only a few days before this page was written there was a spot on the sun large enough to be conspicuous with the aid of a field-gla.s.s. During the time of a spot-maximum the sun is occasionally a wonderful object, no matter how small the power of the instrument used in viewing it may be. Strings of spots of every variety of shape sometimes extend completely across the disk. Our ill.u.s.tration shows the appearance of the sun, as drawn by the author on the 1st of September, 1883. Every one of the spots and spot-groups there represented could be seen with a good field-gla.s.s, and nearly all of them with an opera-gla.s.s.
[Ill.u.s.tration: THE SUN, SEPTEMBER 1, 1883.]
As in all such cases, our interest in the phenomena increases in proportion to our understanding of their significance and their true scale of magnitude. In glancing from side to side of the sun"s disk, the eye ranges over a distance of more than 860,000 miles--not a mere ideal distance, or an expanse of empty s.p.a.ce, but a distance filled by an actual and, so to speak, tangible body, whose diameter is of that stupendous magnitude. One sees at a glance, then, the enormous scale on which these spots are formed. The earth placed beside them would be but a speck, and yet they are mere pits in the surface of the sun, filled perhaps with partially cooled metallic vapors, which have been cast up from the interior, and are settling back again. It is worth anybody"s while to get a glimpse at a sun-spot if he can, for, although he may see it merely as a black dot on the shining disk, yet it represents the play of physical forces whose might and power are there exercised on a scale really beyond human comprehension. The imagination of Milton or Dante would have beheld the mouth of h.e.l.l yawning in a sun-spot.
In order to view the sun it is, of course, necessary to contrive some protection for the eyes. This may be constructed by taking two strips of gla.s.s four or five inches long and an inch wide, and smoking one of them until you can without discomfort look at the sun through it. Then place the two strips together, with the smoked surface inside--taking care to separate them slightly by pieces of cardboard placed between the ends--and fasten the edges together with strips of paper gummed on.
Then, by means of a rubber band, fasten the dark gla.s.s thus prepared over the eye-end of your opera-gla.s.s in such a way that both of the lenses are completely covered by it. It will require a little practice to enable you to get the sun into the field of view and keep it there, and for this purpose you should a.s.sume a posture--sitting, if possible--which will enable you to hold the gla.s.s very steady. Then point the gla.s.s nearly in the direction of the sun, and move it slowly about until the disk comes in sight. It is best to carefully focus your instrument on some distant object before trying to look at the sun with it.
As there is some danger of the shade-gla.s.s being cracked by the heat, especially if the object-gla.s.ses of the instrument are pretty large, it would be well to get the strips of gla.s.s for the shade large enough to cover the object-end of the instrument instead of the eye-end. At a little expense an optician will furnish you with strips of gla.s.s of complementary tints, which, when fastened together, give a very pleasing view of the sun without discoloring the disk. Dark red with dark blue or green answer very well; but the color must be very deep. The same arrangement, of course, will serve for viewing an eclipse of the sun.
A word, finally, about the messenger which brings to us all the knowledge we possess of the contents and marvels of s.p.a.ce--light.
Without the all-pervading luminiferous ether, narrow indeed would be our acquaintance with the physical creation. This is a sympathetic bond by which we may conceive that intelligent creatures throughout the universe are united. Light tells us of the existence of suns and systems so remote that the mind shrinks from the attempt to conceive their distance; and light bears back again to them a similar message in the feeble glimmering of our own sun. And can any one believe that there are no eyes out yonder to receive, and no intelligence to interpret that message?
Sir Humphry Davy has beautifully expressed a similar thought in one of his philosophical romances:
In Jupiter you would see creatures similar to those in Saturn, but with different powers of locomotion; in Mars and Venus you would find races of created forms more a.n.a.logous to those belonging to the Earth; but in every part of the planetary system you would find one character peculiar to all intelligent natures, a sense of receiving impressions from light by various organs of vision, and toward this result you can not but perceive that all the arrangements and motions of the planetary bodies, their satellites and atmospheres, are subservient.
The spiritual natures, therefore, that pa.s.s from system to system in progression toward power and knowledge preserve at least this one invariable character, and their intellectual life may be said to depend more or less upon the influence of light.[G]
[G] See "Consolations in Travel, or, the Last Days of a Philosopher"; Dialogue I.
Light is a result, and an expression, of the energy of cosmical life.
The universe lives while light exists. But when the throbbing energies of all the suns are exhausted, and s.p.a.ce is filled with universal gloom, the light of intelligence must vanish too.
One can not read the wonderful messages of light--one can not study the sun, the moon, and the stars in any manner--without perceiving that the physical universe is enormously greater than he had thought, and that the creation, of which the Earth is an infinitesimal part, is almost infinitely more magnificent in actual magnitude than the imaginary domain which men of old times pictured as the dwelling-place of the all-controlling G.o.ds; without feeling that he has risen to a higher plane, and that his intellectual life has taken a n.o.bler aim and a broader scope.
THE END.