follows that the moon and sun must both vary in their respective apparent magnitudes. Hence some central eclipses of the sun are total, while others are partial and annular.

Annular.
B

TOTAL AND ANNULAR ECLIPSES OF THE SUN.

Total.

1. At A, the earth is at her aphelion, and the sun being at his most distant point, will have his least apparent magnitude. At the same time, the moon is in perigee, and appears larger than usual. If, therefore, she pass centrally over the sun's disk, the eclipse will be total.

2. At B, this order is reversed. The earth is at her perihelion, and the moon in apogee; so that the sun appears larger, and the moon smaller than usual. If, then, a central eclipse occur under these circumstances, the moon will not be large enough to eclipse the whole of the sun, but will leave a ring, apparently around herself, unobscured. Such eclipse will be annular.

258. As the solar ecliptic's limits are further from the moon's nodes than the lunar, it results that we have more eclipses of the sun than of the moon. There may be seven in all in one year, viz., five solar and two lunar; but the most usual number is four. There can never be less than two in a year; in which case, both must be of the sun. Eclipses both of the sun and moon_recur in nearly the same order, and at the same intervals, at the expiration of a cycle of 223 lunations, or 18 years of 365 days and 15 hours. This cycle is called the Period of the Eclipses. At the expiration of this time, the sun and the moon's nodes will sustain the same relation to each other as at the beginning, and a new cycle of eclipses begins.

259. In a total eclipse of the sun, the heavens are shrouded in darkness, the planets and stars become visible, the temperature declines, the animal tribes become agitated, and a general gloom overspreads the landscape. Such were the effects of the great eclipse of 1806. In a lunar eclipse, the moon begins to lose a portion of her

258. Which kind of eclipses is most frequent? Why? The greatest number in a year? How many of each? Least number, and which? Usual number? What said of the order of eclipses? Time of cycle?

259. Describe the effects of a total eclipse of the sun. The process of a lunar eclipse?

light and grows dim, as she enters the earth's penumbra, till at length she comes in contact with the umbra, and the real eclipse begins.

260. In order to measure and record the extent of eclipses, the apparent diameters of the sun and moon are divided into twelve equal parts, called digits; and in predicting eclipses, astronomers usually state which "limb" of the body is to be eclipsed-the southern or northern-the time of the first contact, of the nearest approach of centers, direction, and number of digits eclipsed.

261. The last annular eclipse visible in the United States occurred May 26, 1854. The next total eclipse of the sun will be August 7, 1869.

Some of the ancients and all barbarous nations formerly regarded eclipses with amazement and fear, as supernatural events, indicating the displeasure of the gods. Columbus is said to have made a very happy use of this superstition. When the inhabitants of St. Domingo refused to allow him to anchor, in 1502, or to furnish him supplies, he told them the Great Spirit was offended at their conduct, and was about to punish them. In proof, he said the moon would be darkened that very night; for he knew an eclipse was to occur. The artifice led to a speedy and ample supply of his wants.

262. Eclipses can be calculated with the greatest precision, not only for a few years to come, but for centuries

260. How are eclipses measured and recorded?

261. When the next annular eclipse visible in this country? The next total? How have the ignorant and superstitious regarded eclipses ? Anecdote of Columbus ?

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.

263. JUPITER is attended by four satellites or moons. They are easily seen with a common spy-glass, appear

ing 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 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.

264. The size of these satellites is about the same as our moon, except the second, which is a trifle less. The first is

TELESCOPIC VIEWS OF THE MOONS OF

JUPITER.

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.

8d.

2d.

1st.

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

268. How many moons has Jupiter? How seen? Why not all seen at once? 264. Their size? Distances? Periods? 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 satellites 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 Jupiter'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? How 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, whenever 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.

1. 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.

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 Copernican 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 navigation? (Illustrate method. Much used?)