The spot first observed by Cassini, in 1665, which has both disappeared and reappeared in the same form and position for the space of 43 years, could not possibly be occasioned by any atmospherical variations, but seems evidently to be connected with the surface of the planet. The form of the belt, according to some astronomers, may be accounted for by supposing that the atmosphere reflects more light than the body of the planet, and that the clouds which float in it, being thrown into parallel strata by the rapidity of its diurnal motion, form regular insterstices, through which are seen its opaque body, or any of the permanent spots which may come within the range of the opening.

MOONS OF JUPITER.

They

TELESCOPIC VIEWS OF THE MOONS OF

JUPITER.

485. Jupiter is attended by four satellites or moons are easily seen with a common spyglass, appearing like small stars near the primary. (See adjoining cut.) By watching them for a few evenings, they will be seen to change their places, and to occupy different positions. At times, only one or two may be seen, as the others are either between the observer and the planet, or beyond the primary, or eclipsed by his shadow."

486. The size of these satellites is about the same as our moon, except the second, which is a trifle less. The first is about the distance of our moon; and the others, respectively, about two, three, and five times as far off.

4th.

COMPARATIVE DISTANCES OF JUPITER'S MOONS

3d.

2d. 1st.

487. Their periods of revolution are from 1 day 18 hours to 17 days, according to their distances. This rapid motion is necessary, in order to counterbalance the powerful centripetal force of the planet, and to keep the satellites from falling to his surface.

485. How many moons has Jupiter? How seen? Why not all seen at once? 486. Their size? Distances? 487. Periods of Jupiter's satellites? Why so rapid?

The magnitudes, distances, and periods of the moons of Jupiter are as follows:

488. The orbits of Jupiter's moons are all in or near the plane of his equator; and as his orbit nearly coincides with the ecliptic, and his equator with his orbit, it follows that, like our own moon, his satellites revolve near the plane of the ecliptic. On this account, they are sometimes between us and the planet, and sometimes beyond him, and seem to oscillate, like a pendulum, from their greatest elongation on one side to their greatest elongation on the other.

489. Their direction is from west to east, or in the direction their primary revolves, both upon his axis and in his orbit. From the fact that their elongations east and west of Jupiter are nearly the same at every revolution, it is concluded that their orbits are but slightly elliptical. They are supposed to revolve on their respective axis, like our own satellite, the moon, once during every periodic revolution.

490. As these orbits lie near the plane of the ecliptic, they have to pass through his broad shadow when in opposition to the Sun, and be totally eclipsed at every revolution. To this there is but one exception. As the fourth satellite departs about 3° from the plane of Jupiter's orbit, and is quite distant, it sometimes passes above or below the shadow, and escapes eclipse. But such escapes are not frequent.

These moons are not only often eclipsed, but they often eclipse Jupiter, by throwing their own dark shadows upon his disc. They may be seen like dark round spots traversing it from side to side, causing, wherever that shadow falls, an eclipse of the Sun. Altogether, about forty of these eclipses occur in the system of Jupiter every month.

491. The immersions and emersions of Jupiter's moons have reference to the phenomena of their being eclipsed. Their entrance into the shadow is the immersion; and their coming out of it the emersion.

488. How are their orbits situated? How satellites appear to move? 489. Direction of secondaries? Form of orbits? How ascertained? What motion on axis? 490. What said of eclipses? Of fourth satellite? Of solar eclipses upon Jupiter? Number of solar and lunar? 491. What are the immersions and emersions of Jupiter's moons? Are the immersions and emersions always visible from the Earth? Why not? Illustrate.

A

ECLIPSES OF JUPITER'S MOONS, EMERSIONS, ETC.

E

The above is a perpendicular view of the orbits of Jupiter's satellites. His broad shadow is projected in a direction opposite the Sun. At C, the second satellite is suffering an immersion, and will soon be totally eclipsed; while at D, the first is in the act of emersion, and will soon appear with its wonted brightness. The other satellites are seen to cast their shadows off into space, and are ready in turn to eclipse the Sun, or cut off a portion of his beams from the face of the primary.

If the Earth were at A in the cut, the immersion, represented at C, would be invisible; and if at B, the emersion at D could not be seen. So, also, if the Earth were exactly at F, neither could be seen; as Jupiter and all his attendants would be directly beyond the Sun, and would be hid from our view.

492. The system of Jupiter may be regarded as a miniature representation of the solar system, and as furnishing triumphant evidence of the truth of the Copernican theory. It may also be regarded as a great natural clock, keeping absolute time for the whole world; as the immersions and emersions of his satellites furnish a uniform standard, and, like a vast chronometer hung up in the heavens, enable the mariner to determine his longitude upon the trackless deep.

By long and careful observations upon these satellites, astronomers have been able to construct tables, showing the exact time when each immersion and emersion will take place, at Greenwich Observatory, near London. Now suppose the tables fixed the time for a certain satellite to be eclipsed at 12 o'clock at Greenwich, but we find it to occur at 9 o'clock, for instance, by our local time: this would show that our time was three hours behind the time at Greenwich; or, in other words, that we were three hours, or 45°, west of Greenwich. If our time was ahead of Greenwich time, it would show that we were east of that meridian, to the amount of 15° for every hour of variation. But this method of finding the longitude is less used than the "lunar method" (Art. 407), on account of the greater difficulty of making the necessary observations.

493. By observations upon the eclipses of Jupiter's moons, as compared with the tables fixing the time of their occurrence, it was discovered that light had a progressive motion, at the rate of about 200,000 miles per second.

This discovery may be illustrated by again referring to the preceding cut. In the year 1675, it was observed by Roemer, a Danish astronomer, that when the Earth was nearest to Jupiter, as at E, the eclipses of his satellites took place 8 minutes 13 seconds sooner than the mean time of the tables; but when the earth was farthest from Jupiter, as at F, the eclipses took place 3 minutes and 13 seconds later than the tables predicted, the entire difference being 16 minutes and 26 seconds. This difference of time he ascribed to the progressive motion of light, which he concluded required 16 minutes and 26 seconds to cross the earth's orbit from E to F.

492. How may the system of Jupiter be regarded? What use of it made in navigation? Illustrate method? Is it much used? 493. What discovery by observing the eclipses of Jupiter's moons? Explain the process?

This progress may be demonstrated as follows:-16m. 26s.-986s. If the radius of the Earth's orbit be 95,000,000 of miles, the diameter must be twice that, or 190,000,000. Divide 190,000,000 miles by 986 seconds, and we have 192,69738 miles as the progress of light in each second. At this rate, light would pass nearly eight times around the globe at every tick of the clock, or nearly 500 times every minute!

494. Jupiter, when seen from his nearest satellite, appears a thousand times larger than our Moon does to us, exhibiting on a scale of inconceivable magnificence, the varying forms of a crescent, a half moon, a gibbous phase, and a full moon, every 42 hours.

SATURN.

495. SATURN is situated between the orbits of Jupiter and Uranus, and is distinctly visible to the naked eye. It may be easily distinguished from the fixed stars by its pale, feeble, and steady light. It resembles the star Fomalhaut, both in color and size, differing from it only in the steadiness and uniformity of its light.

From the slowness of its motion in its orbit, the pupil throughout the period of his whole life, may tracé its apparent course among the stars, without any danger of mistake. Having once found when it enters a particular constellation, he may easily remember where he is to look for it in any subsequent year; because, at a mean rate, it is just 2 years in passing over a single sign or constellation.

Saturn's mean daily motion among the stars is only about 2', the thirtieth part of a degree.

496. The mean distance of Saturn from the Sun is nearly double that of Jupiter, being about 909,000,000 of miles. His diameter is about 82,000 miles; his volume, therefore, is eleven hundred times greater than the Earth's. Moving in his orbit at the rate of 22,000 miles an hour, he requires 29 years to complete his circuit around the Sun: but his diurnal rotation on his axis is accomplished in 10 hours. His year, therefore, is nearly thirty times as long as ours, while his day is shorter by more than one-half. His year contains about 25,150 of its own days, which are equal to 10,759 of our days.

497. The surface of Saturn, like that of Jupiter, is diversified with belts and dark spots. Dr. Herschel sometimes perceived five belts on his surface; three of which were dark and two bright. The dark belts have a yellowish tinge, and generally cover a broader zone of the planet than those of Jupiter.

To the inhabitants of Saturn, the Sun appears 90 times less than he appears at the Earth; and they receive from him only one ninetieth part as much light and heat. But

495. Situation of Saturn?

494. How does Jupiter appear from his nearest satellite? How distinguished? How trace? His rate of motion in the heavens ? 496. Distance from the Sun? Diameter? Volume? Rate of motion in orbit? Periodic time? Diur nal revolution? Days in his year? 497. Appearance of his surface? Belts? The

Sun as seen from Saturn? Light and heat of that planet? Estimated strength of the

It is computed that even the ninetieth part of the Sun's light exceeds the illuminating power of 8000 full moons, which would be abundantly sufficient for all the purposes of life.

498. The telescopic appearance of Saturn is unparalleled. It is even more interesting than Jupiter, with all his moons and belts. That which eminently distinguishes this planet from every other in the system, is a magnificent zone or ring, encircling it with perpetual light.

The adjoining out is an excellent representation of Saturn as seen through a telescope. The oblateness of the planet is easily perceptible, and his shadow can be seen upon the rings back of the planet. The shadow of the rings may also be seen running across his disc. The writer has often seen the opening between the body of the planet and the interior ring as distinctly as it appears to the student in the cut. Under very powerful telescopes, these rings are found to be again subdivided into an in

definite number of concentric circles, one within the other, though this is considered doubtful by Sir John Herschel.

499. The light of the ring is more brilliant than the planet itself. It turns around its center of motion in the same time that Saturn turns on its axis. When viewed with a good telescope, it is usually found to consist of two concentric rings, divided by a dark band.

It has been ascertained, however, that these rings are again subdivided; the third division was distinctly seen by Prof. Encke, on the 25th of April, 1837, and also by Mr. Lassel, on the 7th of September, 1843, at his observatory near Liverpool, England. Six different rings were seen at Rome, in Italy, on the night of the 29th of May, 1838. And more recent observations by Professor Bond, of Cambridge, have led to the conclusion that, in all probability, these wonderful rings are fluid! It is well known that under the most powerful instruments they seem to be almost indefinitely subdivided.

500. As our view of the rings of Saturn is generally an oblique one, they usually appear elliptical, and never circular. The ellipse seems to contract for about 7 years, till it almost entirely disappears, when it begins to expand again, and continues to enlarge for 7 years, when it reaches its maximum of expansion, and again begins to contract. For fifteen years, the

part of the rings toward us seems to be thrown up, while for the

solar radiance?

498. Telescopic appearance of Saturn? For what distinguished? 499. Comparative light of his rings? Time of rotation around the planet? How does it usually appear? What further discoveries? 500. What the general apparent figure of the rings? Why elliptical? What periodic variation of expansion? Of inclination? When nearly invisible?