and ages either past or to come. This fact demonstrates the truth of the Copernican theory, and illustrates the order and stability that everywhere reign throughout the planetary regions.

CHAPTER VII.

SATELLITES OF THE EXTERIOR PLANETS.

TELESCOPIO VIEWS OF THE MOONS OF

JUPITER

263. JUPITER is attended by four satellites or moons. They are easily seen with a common spy-glass, appearing like small stars near the primary. (See adjoining cut, and note at 178.) By watching them for a few evenings, they will be seen to change their places, and to occupy dif ferent 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.

264. 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.

COMPARATIVE DISTANCES OF JUPITER'S MOONS.

4th.

3d.

2d.

1st.

262. What said of the calculation of eclipses? What does this demonstrate and illustrate?

263. How many moons has Jupiter? How seen? Why not all seen at once! 264. Their size? Distances? Perids? Why so rapid?

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 satel lites from falling to his surface.

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

265. 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.

266. 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 axes, like our own satellite, the moon, once during every periodic revolution.

267. 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 Jupi ter's orbit, and is quite distant, it sometimes passes above or below the shadow, and escapes eclipse. But such escapes are not frequent.

265. How are their orbits situated? How satellites appear to move? 266. Direction of secondaries? Form of orbits? llow ascertained?

What motion on axes?

267. What said of eclipses? Of fourth satellite? Of solar eclipses upon Jupiter? Number of solar and lunar?

These moons are not only often eclipsed, but they often eclipse Jupiter, by throwing their own dark shadows upon his disk. 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.

268. 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.

ECLIPSES OF JUPITER'S MOONS, EMERSIONS, ETC.

E

1. The above is a perpendicular view of the orbits of Jupiter's satellites. His broau 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 cmersion, and will soon appear with its wonted brightness. The other satellites are Been 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.

2. 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.

269. 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 Coper nican theory. It may also be regarded as a great natu ral 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.

268. What are the immersions and emersions of Jupiter's moons? (Are the immersions and emersions always visible from the earth? Why not? Illustrate.)

269. How may the system of Jupiter be regarded? What use made of in naviga ion? (Illstrate method. Much used?)

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 450, 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 150 for every hour of variation. But this method of finding the longitude is less used than the "lunar method" (Art. 245), on account of the greater difficulty of making the necessary observations.

270. 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.

1. This discovery may be illustrated by again referring to the opposite 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.

2. This progress may be demonstrated as follows:-16m. 26s. 9868. If the radius of the earth's orbit be 95 millions of miles, the diameter must be twice that, or 190 millions. Divide 190,000,000 miles by 986 seconds, and we have 192,69737 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!

SATURN.

SATELLITES OF SATURN.

271. The moons of Saturn are eight in number, and are seen only with telescopes of considerable power. The best time for observing them is when the planet is at his equinoxes, and his rings are nearly invisible.

In January, 1849, the author saw five

of these satellites, as represented in the adjoining cut. The rings appeared only as a line of light, extending each way from the planet, and the satellites were in the direction of the line, at different distances, as here represented.

272. These satellites all revolve eastward with the rings of the planet, in orbits nearly circular, and, with the exception of the eighth, in the plane of the rings. Their mean distances, respectively, from the planet's cen

270. What discovery by observing these eclipses? (Illustrate method. Diagram. Demonstration.)

271. Number of Saturn's moons? How seen? Best time? Shape of orbits? How situated?

272. How revolve?

Periods?

Distances?

ter are from 123,000 to 2,366,000 miles; and their pe riods from 22 hours to 79 days, according to their dis

tances.

The distances and periods of the satellites of Saturn are as follows:

273. The sixth of these satellites is the largest, supposed to be about the size of Mercury; and the remainder grow smaller as they are nearer the primary. They are seldom eclipsed, on account of the great inclination of their orbits to the ecliptic, except twice in thirty years, when the rings are edgewise toward the sun. The eighth satellite, which has been studied more than all the rest, is known to revolve once upon its axis during every periodic revolution; from which it is inferred that they all revolve on their respective axes in the same manner.

1. Let the line A B represent the plane of the planet's orbit, CD his axis, and FF the plane of his rings. The satellites being in the plane of the rings, will revolve around the shadow of the primary, instead of passing through it, and being eclipsed.

2. At the time of his equinoxes, however, when the rings are turned toward the sun (see A and E, cut, page 92), they must be in the center of the shadow on the opposite side; and the

moons, revolving in the plane of the rings, must pass through the shadow at every revolution. The eighth, however, may sometimes escape, on account of his departure from the plane of the rings, as shown in the cut.

URANUS.

274. Uranus is supposed to be attended by six secondaries. Sir Wm. Herschel recorded that he saw this number, and computed their periods and distances; and on his authority the opinion is generally received, though

273. Size? Eclipses of? When? Why not oftener? (Illustrate.) 274. Satellites of Uranus? Upon what authority? Distances? Periods? Situation of orbits? Form? Direction in revolution Remark of Dr. Herschel?