Latitude upon the earth is distance either North or South of the Equator, and is reckoned each way toward the Poles in Degrees, Minutes, and Seconds.

As the distance from the Equator to the Pole cannot be more than a quarter of a circle, or 90°, it is obvious that no place can have more than 900 of latitude; or, in other words, all places upon the earth's surface must be between the Equator and 90C of latitude, either north or south.

Parallels of Latitude are circles either North or South of the Equator, and running parallel to it.

We may imagine any conceivable number of parallels between the Equator and the Poles, though upon most Inaps and globes they are drawn only once for every ten degrees.

The Tropics are two parallels of latitude, each 23° 28' from the Equator.

The Northern is called the Tropic of Cancer, and the Southern the A Tropic of Capricorn.

1. The Tropical Circles are shown at E E in the an

nexed figure.

2. The sun never shines perpendicularly upon any points on the earth further from the Equator than the Tropics. Between these he seems to travel regularly, leaving the Southern Tropic on the 23d of December, crossing the Equator northward on the 20th of March, reaching the Northern Tropic on the 21st of June, crossing the Equator southward on the 23d of September, and reaching the Southern Tropic again on the 23d of December. In this manner he seems to cross and recross the Equator, and vibrate between the Tropics from year to year. The cause of this apparent motion of the sun will be explained hereafter.

The Polar Circles are two parallels of latitude, 23° 28′ from the Poles. (See F F in the last cut.)

The Northern is called the Arctic, and the Southern the Antarctic, Circle.

The Tropics and Polar Circles divide the globe into five principal parts, called Zones, namely, one Torrid, two Temperate, and two Frigid.

A zone properly signifies a girdle; but the term is here used in an accommodated sense, as only three of these five divisions at all resemble a girdle. The parts cut off by the polar circles are mere convex segments of the earth's surface.

The tropics Names? Polar circles? Names? Zones? Names? (Are there in reality any frigid zones?) Situation of the several zones? Merilinns Longitude on the earth? First meridian? (European and Ameri can charts and glóbes ?) How longitude reckoned? Its greatost extent i

The Torrid Zone is situated between the Tropics; the Temperate, between the Tropics and the Polar Circles; and the Frigid, between the Polar Circles and the Poles.

Meridians are imaginary lines drawn from pole to pole over the earth's surface.

Meridians cross the Equator at right angles; and the plane of any two Meridians directly opposite each other would divide the earth into Eastern and Western Hemispheres, as the Equator divides it into Northern and Southern. We may imagine Meridians to pass through every conceivable point upon the earth's surface. They meet at the Poles, and are furthest apart at the Equator.

Longitude upon the earth is distance either East or West of any given meridian.

THE FIVE ZONES.

FRIGID
TEMPERATE

FOR RID

TEMPERATE

FRIGID

MERIDIANS.

A degree of longitude at the Equator comprises about 693 miles, but is less and less as the meridians approach the Poles, at which points it is nothing. A degree of Latitude is about 69 miles on all parts of the globe.

The First Meridian is that from which the reckoning of Longitude is commenced.

On European charts and globes, longitude is usually reckoned from the Royal Observatory at Greenwich, near London; but in this country it is often reckoned from the Meridian of Washington. It would be better for science, however, if all nations reckoned longitude from the same Meridian, and all charts and globes were constructed accordingly.

As Longitude is reckoned both East and West, the greatest longitude that any place can have is 180°.

20. THE CELESTIAL SPHERE.

The Celestial Sphere is the appa

rent concave surface of the heavens, surrounding the earth in all directions.

TERRESTRIAL AND CELESTIAL SPHERES.

The relation of the Terrestrial to the Celestial Sphere may be understood by the above diagram, in which the stars surround the earth in all directions, as they seem to All che whole celestial vault.

The Axis of the Heavens is the axis of the earth produced or extended both ways to the concave surface of the heavens.

The Equator of the heavens, or Equinoctial, is the plane of the Earth's equator extended to the starry heavens.

Declination is distance either north or south of the Equinoctial.

Declination is to the heavens precisely what latitude is upon the earth. It is reckoned from the celestial equator, both North and South, to 900, or to the poles of the heavens. Celestial Lati tude can be explained better hereafter, and so with the terms Eiptic, Zodiac, &c.

Right Ascension is distance east of a given point, and is reckoned on the Equinoctial quite around the heavens.

In one respect, Right Ascension in the heavens is like longitude on the earth: they are both reckoned upon the equators of their respective spheres. But while longitude is reckoned both east and west of the first meridian, and can only amount to 1800, Right Ascension is reckoned only eastward, and consequently may amount to 8600, or the whole circle of the heavens. The principal difference between Right As cension and Celestial Longitude is, that the former is reckoned on the Equinoctial, and the latter on the Ecliptic.

The Sensible Horizon is that circle which terminates our view, or where the earth and sky seem

to meet.

The Rational Horizon is an imaginary plane, below the visible horizon, and parallel to it, which, passing through the earth's cen

SENSIBLE

RATIONAL

4000 MS

HORIZON

ter, divides it into upper and lower hemispheres.

HORIZON

1. These hemispheres are distinguished as upper and lower with reference to the ob server only.

20. Celestial sphere? (Relation to terrestrial?) Axis of the heavens? Equator of the heavens? Declination? (How illustrated by terrestrial latitude? How reckoned? Its limits?) Right ascension? (How resemble longitude? What difference?) Sensible horizou? Rational? Explain by

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 wer removed, and we stood on the rational horizon of that place.

The Poles of the Horizon are two opposite pointsone 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.

NMOGE

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.

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

EAST

DOWN

DOWNE

DOWN

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 direc tion.

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 advanceinent. 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 boun daries 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 grand divisionsviz., 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 sturs. 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 smaй 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.

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