2. The sensible horizon is half the diameter of the earth, or about 4,000 miles from the rational; and yet so distant are the stars, that both these planes seem to cut the celestial arch at the same point; and we see the same hemisphere of stars above the sensible horizon of any place that we should if the upper half of the earth were removed, and we stood on the rational horizon of that place.

The Poles of the Horizon are two opposite points-one directly above, and the other directly beneath, us. The first is called the Zenith, and the latter the Nadir.

The points Up and Down, East and West, are not positive and permanent directions, but merely relative.

EAST

WEST.

NMOO

EAST

1. As the earth is a sphere, inhabited on all sides, UP AND down, and EAST AND the Zenith point is merely opposite its center, and the Nadir toward its center. So with the directions Up and Down one is from the center, and the other toward it; and the same direction which is up to one, is down to another. This fact should not merely be acknowledged, but should be dwelt upon until the mind has become familiarized to the conception of it, and divested, as far as possible, of the notion of an absolute up and down in space. We should remember that we are bound to the earth's surface by attraction, as so many needles would be bound to the surface of a spherical loadstone.

EAST

DOWN

DOWNE

DOWN

2. East and West also are not absolute, but merely relative, directions. East is that direction in which the sun appears to rise, and West is the opposite direction; and yet, so far as absolute direction is concerned, what is East to one, as to the observer at A, is West to B, and so with C and D. And as the earth revolves upon its axis every twenty-four hours, it is obvious that East and West upon its surface must, in that time, change to every point in the whole circle of the heavens. The same is true of the Zenith and Nadir, or of up and down.

Space, in Astronomy, is that boundless interval or void in which the earth and the heavenly bodies are situated, and extending infinitely beyond them all, in every direction.

Space has no limits-or, in other words, is boundless, or infinite. Suppose six persons were to start from as many different points upon the earth's surface-as, for instance, one from each pole, and one from each of the positions occupied by observers in the next figure. Let them ascend or diverge from the earth in straight lines, perpendicularly, to its surface, and though they were to proceed onward, separating from each other, with the speed of lightning, for millions of ages, none of these celestial voyagers would find an end to space, or any effectual barrier to hinder their advance.ment. Should they chance to meet another world in the line of their flight, it would soon be passed, like a ship met by a mariner upon the ocean, and beyond it space would still invite them onward to explore its immeasurable depths. And thus they might go on forever, without changing their position in respect to the center or boundaries of immensity; for as eternity has no beginning, middle, or end, so space is without center or circumference-an ethereal ocean, without bottom or shore.

diagram. Poles of the horizon? Names? Up and down-positive or rela tive points? (Illustrate by diagram; also east and west.) Term space in astronomy? (Has it any limits? Illustration.)

21. FIRST GRAND

DIVISIONS OF THE
UNIVERSE.

The visible uni

verse may be considered under two divisions—

grand

viz., the SOLAR SYSTEM and the SIDEREAL HEAVENS.

The Solar System consists of the sun and all the planets and comets that revolve around him.

The Sidereal Heavens include all those bodies that lie around and beyond the Solar System, in the region of the Fixed Stars.

1. The word Sidereal is from the Latin sideralis, and signifies pertaining to the stars. The Sidereal Heavens are, therefore, the heavens of the fixed stars.

2. The relation of the Solar System to the Sidereal Heavens is shown in the annexed cut, where the sun appears only as a star, at a distance from all others, and surrounded by his own retinue of worlds. The Solar System is drawn upon a small scale, and the Sidereal Heavens are seen around and at a distance from it in every direction.

In considering the general subject of Astronomy, we shall proceed according to the foregoing classification, treating first of the SOLAR SYSTEM, and, secondly, of the SIDEREAL HEAVENS.

(Derivation of term

21. How visible universe divided? Define each? sidercal? Relation of solar system to the sidereal heavens? Illustrate by drawing.) Of which division does the author first treat?

22. The Solar System derives its name from the Latin term sol, the sun. It signifies, therefore, the System of the Sun. It includes that great luminary, and all the planets and comets that revolve around him.

23. The Sun is the fixed center of the system, around which all the solar bodies revolve, and from which they receive their light and heat.

24. The Planets are those spherical bodies or worlds that revolve statedly around the sun, and receive their light and heat from him.

The term planet signifies a wanderer, and was applied to the solar bodies because they seemed to move or wander about among the stars.

The Orbit of a planet is the path it pursues in its revolution around the sun.

25. The planets are divided into Primary and Secon dary planets.

The Primary Planets are those larger bodies of the system that revolve around the sun only, as their center of motion.

The Secondary Planets are a smaller class of bodies,

22. Of what does Part II. treat? What meant by the Solar System? In cludes what?

23. What is the sun?

24. Describe the planets.

(The term?) The orbit of a planet? 25. How planets divided? Describe each. (What other names for secon daries?)

that revolve not only around the sun, but also around the primary planets, as their attendants, or moons.

The secondary planets are also called Moons or Satellites. A satellite is a follower of attendant upon another.

In this cut, the sun may be seen in the center. The white circles are the Orbite of the primary planets. The planets may be seen in those orbits at various distances from the sun. The numerous orbits so close together are those of the Asteroids. The secondary planets may be seen near their respective primaries, revolving around them, while they all go on together around the sun. On the right is seen a Comet plunging into the system, with his long and fiery train. His orbit is seen to be very elliptical. All these bodies are opake, the sun excepted. Even the blazing comet shines only by reflection.

26. The planets are again divided into Interior and Exterior planets.

The Interior Planets are those whose orbits lie within the orbit of the earth, or between it and the sun.

26. What meant by interior and exterior planets? (Why not inferior and superior?)

The Exterior Planets are those whose orbits lie without the orbit of the earth.

Some Astronomers speak of these two classes respectively as Inferior and Superior. The reason seems to be, that as those nearer the sun than the earth are lower than she is that is, nearer the great center of the system-they are, in this respect, inferior to her; while, on the other hand, those that are above, or beyond her, are her superiors. But as the distinction is founded upon, and is intended to denote, the position of the planets with respect to the earth's orbit, it is obvious that interior and exterior are the more appropriate terms. It seems hardly ai.wable to call the Asteroids superior planets, and Mercury and Venus, which are mu larger, inferior.

27. Eighteen of the smaller primary planets are called Asteroids.

Asteroid signifies star-like, and is applied to these small planets because of their comparative minuteness. They are never seen except through telescopes, and through ordinary instruments are not always readily distinguished from the fixed stars.

28. Comets are a singular class of objects, belonging to the solar system, distinguished for their long trains of light, their various shapes, and the great eccentricity of their orbits.

NUMBER AND NAMES OF THE PRIMARY PLANETS.

29. The Primary Planets are thirty-five in number. They are denoted, in astronomical works, by certain characters, or symbols; and as the names of the planets are mostly derived from Mythology, their symbols generally relate to the imaginary divinity after whom the planet is named. The names of the planets and their symbols are as follows:

Several, recently discovered, are not in this table.

27. What are the asteroids? How many? (Term? Are they visible to the naked eye?)

28. What are comets? (Describe the preceding cut. Where sun? Prim