CHAPTER V.

SECONDARY PLANETS -THE MOON.

210. THE Secondary Planets are those that revolve statedly around the primaries, and accompany them in their periodical journeys around the sun. Of these, the earth has one; Jupiter, four; Saturn, eight; Uranus, six; and Neptune, one-in all twenty. Besides these, there is a strong suspicion among astronomers that Venus is attended by a satellite, and that Neptune has at least two, instead of one.

Sir John Herschel says Uranus is attended "certainly by four, and perhaps by six; and Neptune by two or more." Outlines, Art. 533. In regard to Venus, Prof. Hind, of London, says: "Astronomers are by no means satisfied whether Venus should be attended by a satellite or not. * * *It is a question of great interest, and must remain open for future discussion."

211. Though the secondary planets have a compound motion, and revolve both around the sun and around their respective primaries, they are subject to the same general laws of gravitation-of centripetal and centrif ugal force-by which their primaries are governed. Like them, they receive their light and heat from the sun, and revolve periodically in their orbits, and on their respective axes. In the economy of nature, they seem to serve as so many mirrors to reflect the sun's light upon superior worlds, when their sides are turned away from a more direct illumination.

The design of all the secondaries may be inferred from what is said of the purposes for which our own satellite was created. "And God said, Let there be lights in the firmament of heaven, to divide the day from the night; and let them be for signs, and for seasons, and for days and years; and let them be for lights in the firmament of heaven, to give light upon the earth: and it was so. And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also."-Gen. i, 14—16.

210. What are the Secondary planets? How many? How distributed? What supposition respecting Venus? Neptune? (Herschel's remark? Prof. Hind's?)

211. What said of the laws by which the primaries are governed? Light and heat? Uses? (From what may we infer their design?)

212. To the inhabitants of our globe, the earth's satellite or moon is one of the most interesting objects in all the heavens. Her nearness to the earth, and consequent apparent magnitude, her rapid angular motion eastward, her perpetual phases or changes, and the mottled appearance of her surface, even to the naked eye, all conspire to arrest the attention, and to awaken inquiry. Add to this her connection with Eclipses, and her influence in the production of Tides (of both of which we shall speak hereafter in distinct chapters), and she opens before us one of the most interesting fields of astronomical research.

213. The Romans called the moon Luna, and the Greeks Selene. From the former, we have our English terms lunar and lunacy. In mythology, Selene was the daughter of Helios, the sun. Our English word selenography-a description of the moon's surface-is from Selene, her ancient name, and grapho, to describe.

214. The point in the moon's orbit nearest the earth is called Perigee, from the Greek peri, about, and ge, the earth. The point most distant is called Apogee, from apo, from, and ge, the earth. These two points are also called the ap sides of her orbit; and a line joining them, the line of the apsides.

See the moon in apogee and perigee in the cut. The singular of apsides is apsis.

APOGEE.

PERIGEE.

215. The mean distance of the moon from the earth's center is, in round numbers, 240,000 miles; or, more accurately, 238,650. The eccentricity of her orbit amounting to 13,333 miles, of course her distance must vary, and also her apparent magnitude (56). Her average angular

212. What said of our moon? Why specially interesting?

213. Latin name of the moon? Greek? Derivation of words from Luna? Who was Selene in mythology? Selenography?

214. Perigee and Apogee? Derivation? What other name for these two points? What is the line of the apsides? (Apsis?)

215. Moon's distance? Does it vary?

Why? Eccentricity of orbit?

diameter is 31' 7", and her real diameter 2,160 miles. She is consequently only 4th part as large as the earth, and 70000000th part as large as the sun.

Then

The masses of globes are proportional to the cubes of their diameters. 2,160 × 2,160 × 2,160 10,077,696,000, the cube of the moon's diameter; and 7,912 7,912 × 7,912=495,289,174,428, the cube of the earth's diameter. Divide the latter by the former, and we have 49 and a fraction over, as the number of times the bulk of the moon is contained in the earth. Its mass, as compared with the sun, is ascertained in the same manner.

216. The plane of the moon's orbit is very near that of the ecliptic. It departs from the latter only about 51° (5°8′ 48′′).

A

INCLINATION OF THE MOON'S ORBIT TO THE PLANE OF THE ECLIPTIC.

Let the line A B represent the plane of the earth's orbit, and the line joining the moon at C and D would represent the inclination of the moon's orbit to that of the earth. At C the moon would be within the earth's orbit, and at D exterior to it; and it would be Full Moon at D, and New Moon at C.

217. The line of the apsides of the moon's orbit is not fixed in the ecliptic, but revolves slowly around the ecliptic, from west to east, in the period of about nine years.

In the adjoining cut, an attempt is made to represent this motion. At A, the line of the apsides points directly to the right and left; but at B, C, and D it is seen changing its direction, till at E the change is very perceptible when compared with A. But the same ratio of change continues; and at the end of a year, when the earth reaches A again, the line of the apsides is found to have revolved eastward to the dotted line IK, or about 400. In nine years, the aphelion point near A will have made a complete revolution, and returned to its original position.

MOTION OF THE APSIDES.

H

218. The line of the moon's nodes is also in revolution; but it retrogrades or falls back westward, making the circuit of the ecliptic once in about 19 years.

Angular diameter? In miles? How compare with earth? With sun? (How demonstrated?)

216. How is the plane of the moon's orbit situated with respect to the ecliptic? (Illustrate by diagram.)

217. Is the line of the moon's apsides stationary or not? What motion? Period? (Illustrate.)

218. What of the line of the moon's nodes? In what time does it make the circuit of the ecliptic? Amount of motion?

The amount of this motion is 100 35' per annum, which would require 18 years and 219 days for a complete revolution.

1 400

219. The diameter of the moon is only th part as great as that of the sun; and yet the apparent diameter of the moon is nearly equal to that of the sun. The former is 31' 7", and the latter 32′ 2′′, or only 55" difference. The reason why the moon appears to vie with the sun in magnitude, when she is only 7.000.000 as large, is, that she is 400 times nearer to us than he is. See Art. 56.

1. The cut in the margin will show how it is that a small object near us will fill as large an angle, or, in other words, appear as large, as a much larger object which is more remote. The moon at A fills the same angle that is filled by the sun at B.

2. This fact may serve to illustrate the comparative influence of things present and future upon most minds. The little moon may eclipse the sun; or even a dime, if held near enough to the eye, will completely hide all his. glories from our view. So in morals and religion. The things which are seen and temporal" are too apt to hide from our view the more distant but superior glories of the life to come.

220. The density of the moon is only about two-thirds that of the earth, and her surface th as great. The light reflected to the earth by her, at her full, is only 300,000th part as much as we receive on an average from the sun.

66

B

new

221. The daily apparent revolution of the moon is from east to west, with the sun and stars; but her real motion around the earth is from west to east. Hence, when first seen as a moon," she is very near, but just east of the sun; but departs further and further from him eastward, till at length she is seen in the east as a full moon, as the sun goes down in the west.

222. The moon performs a sidereal revolution around the earth in 27d. 7h. 43m.; and a synodic in 29d. 12h. 44m. The sidereal is a complete revolution, as measured by a fixed star; but the motion of the earth eastward in

219. Moon's diameter, as compared with that of the sun? With sun's apparent diameter? Why appear so near of a size? (Illustrate by diagram. Reflection of the author?)

220. Density of the moon? Her light?

221. Her daily apparent motion? Real motion? How traced?

222. What is her sidereal revolution? Her synodic? What difference? Why? (Illustrate by diagram.)

her orbit gives the sun an apparent motion eastward among the stars (119), and renders it necessary for the moon to perform a little more than a complete revolution each month, in order to come in conjunction with the sun, and make a synodic revolution.

SIDEREAL AND SYNODIO REVOLUTIONS OF THE MOON.

1. On the right, the earth is shown in her orbit, revolving around the sun, and the moon in her orbit, revolving around the earth. At A, the sun and moon are in conjunction, or it is New Moon. As the earth passes from D to E, the moon passes around from A to B, or the exact point in her orbit where she was 27 days before. But she is still west of the sun, and must pass on from B to C, or 1 day and 20 hours longer, before she can again come in conjunction with him. This 1 day and 20 hours constitutes the difference between a sidereal and a synodic revolution.

2. The student will perceive that the difference between a sidereal and synodic revolution of the moon, like that between solar and sidereal time, is due to the same causenamely, the revolution of the earth around the sun. See 135.

The estimate of 130 10' 35" is made for a sidereal day of twenty-four hours. In the above cut, the daily progress of the moon may be traced from her conjunction or "change" at A on the right, around to the same point again. This being a sidereal revolution, requires only 27 days.

223, Daily angular motion eastward? How detected? (For what day is this estimate made?)