ECLIPSIS OF JUPITER'S MOONS, EMERSIONS, LTO.

The above is a perpendicular view of the orbits of Jupiter's satellites. His oroad shadow is projected in a direction opposite the Sun. At C, the second satellite is suffer. Ing 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 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 8 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.

192. 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 cchpseg Explain the process?

of Jupiter's moons?

This progress may be demonstrated as follows:-16m. 26s.=956s. If the radius of tho 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 192,697378 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!

439

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 trace 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 73,484 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

494. How does Jupiter appear from his nearest satellite? 495. Situation of Saturn? flow distinguished? How trace? His rate of motion in the heavens? 496. Distance 'rom 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 Eun 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, encir cling 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 ag tin 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 asually appear? What further discoveries? 500. What the general apparent figure of the rings? Why elliptical? What periodic variat on of expansion? Of inclina'ion Y When nearly invisible?

next fifteen it appears to drop below the apparent center of the planet; and while shifting from one extreme to the other, the rings become almost invisible, appearing only as a faint line of light running from the planet in opposite directions. The rings vary also in their inclination, sometimes dipping to the right, and at others to the left.

TELESCOPIC PHASES OF THE RINGS OF SATURN.

The above is a good representation of the various inclinations and degrees of expan sion of the rings of Saturn, during his periodic journey of 30 years

501. The rings of the planet are always directed more or less toward the Earth, and sometimes exactly toward us; so that we never see them perpendicularly, but always either exactly edgewise, or obliquely, as shown in the last figure. Were either pole of the planet exactly toward us, we should then have a perpendicular view of the rings, as shown in the adjoining cut.

502. The various phases of Saturn's rings are explained by the facts that his axis remains parallel to itself (see following cut), with an uniform inclination to the plane of his orbit, which is very near the ecliptic; and as the rings revolve over his equator, and at right angles with his axis, they also remain parallel to themselves. The revolution of the planet about the Earth every 30 years, must therefore bring first one side of the ings to view, and then the other-causing all the variations of expansion, position, and inclination which the rings present.

01. How are the rings situated with respect to the Earth? How would they appear either pole of Saturn were toward us? 502. How are the various phases of Saturn & rings accounted for?

Here observe, first, that the axis of Saturn, like those of all the other planets, remaind permanent, or parallel with itself; and as the rings are in the plane of his equator, and at right angles with his axis, they also must remain parallel to themselves, whatever position the pianet may occupy in its orbit.

This being the case, it is obvious that while the planet is passing from A to E, the Sun will shine upon the under or south side of the rings; and while he passes from E to A again, upon the upper or north side; and as it requires about 30 years for the planet to traverse these two semicircles, it is plain that the alternate day and night on the rings will be 15 years each.

A and E are the equinoctial, and C and G the solstitial points in the orbit of Saturn. At A and E the rings are edgewise toward the Sun, and also toward the Earth, provided Saturn is in opposition to the Sun. To an observer on the Earth, the rings will seem to expand from A to C, and to contract from C to E. So, also, from E to G, and from G to A. Again: from A to E the front of the rings will appear above the planet's center, and from E to A below it.

The rings of Saturn were invisible, as rings, from the 22d of April, 1848, to the 19th of January, 1949. He came to his equinox September 7, 1848; from which time to February, 1856, his rings continued to expand. From that time to June, 1863. they contracted, until he reached his other equinox at E. and the rings became invisible. From June 1863, to September, 1870, they will again expand; and from September, 1970, to March, 1877, they will contract, when he will be at the equinox passed September 7, 1848, or 29 years before.

The writer has often seen the rings of Saturn in different stages of expansion, and contraction, and once when they were almost directly edgewise toward the Earth. At that time (January, 1849), they appeared as a bright line of light, as represented at A and E, in the first cut on the preceding page.

503. The dimensions of the rings of Saturn may be stated in round numbers as follows:

Distance from the body of the planet to the first ring.

Width of interior ring

Miles.

19,000

Space between the interior and exterior rings

608. State the distances and dimensions of his rings, beginning at the body of the planet, And assing outward? What additional statistics from Herschel?