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DEFINITIONS, ILLUSTRATIONS, ETC.

CLASS II.

1. PLANETS (λarns, a wanderer) are bodies belonging to our solar system; which, pursuing their courses to receive the light and heat of the sun, are seen reflecting his beams in constantly varying situations. The following are the chief, or primary planets, with their symbols and relative sizes.*

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45 Thous. 8 Thous. 7 Thous. 44 Thous. 3 Thous.

Of these, Mercury, although brilliant, is seldom favourably seen on account of his proximity to the sun. Venus has the most splendid appearance, on account of her nearness to the sun and our earth; and Mars also is conspicuous, from his little remoteness from us. Jupiter and Saturn, although at great distances, are even more splendid than Mars, from the extent of their reflecting surfaces;

* See also" Planetarium," in the Appendix.

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whilst Uranus or Herschel, (called also the " Georgium Sidus," or "Georgian,") although of magnitude so considerable, at a distance double that of Saturn, is barely discernible by the naked eye.

2. MINOR PRIMARY PLANETS are those that have been detected between the orbits or courses of Mars and Jupiter ; viz., Ceres, Pallas, Vesta, Juno . They are termed Asteroids; and being very minute, are never visible without the telescope.

3. SECONDARY PLANETS, MOONS, OR SATELLITES, (Satelles, Lat., an attendant,) are those which are seen to revolve about some of the primary planets, as auxiliaries to reflect the light of the sun upon the darkened portions of their surfaces.

Of these, besides that attending our earth, seventeen others have been discovered by the telescope; viz., four belonging to Jupiter, seven to Saturn, and six to Uranus.

4. FIXED STARS are distinguished from the planetary or moving bodies of our system, by the constant twinkling of their light. About 2000 only are visible to the naked eye; but it is calculated that, in the whole expanse of the heavens, there are a hundred millions within the reach of telescopic vision,—the whole number registered down to the seventh magnitude, amounting to between 15,000 and 20,000 (L on page 6).

5. THE MAGNITUDE OF A STAR is a term used to denote its apparent brilliancy. We see, without a glass, five or six gradations of star-light. Telescopic research has detected stars, which, from their small portion of brilliancy, might be classed as of the sixteenth magnitude; but the disks, even of those of the first magnitude, dwindle more nearly into points, in proportion as the construction of the telescope approaches perfection.

A scale of these gradations is given near the North Pole of every celestial globe. Thus, a star of the first magnitude is, in some editions, marked with eight petals or rays; a star of the second magnitude with seven petals, &c.

6. A CONSTELLATION is an imaginary arrangement of a number of fixed stars into the outline of a figure; as a bear, a dragon, a swan, &c. Constellations are referred to in the works of Homer and Hesiod, (900 years before the

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Christian æra). They occur in the prophecy of Amos, near the same period; and Arcturus, Orion, and the Pleiades are mentioned twice, even in the book of Job).*

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"The similarity of the constellations recognised in different countries, is very remarkable. The Chaldean, the Egyptian, and the Grecian skies, have a resemblance which cannot be overlooked. Some have conceived that this resemblance may be traced also in the Indian and Arabic constellations, at least in those of the Zodiac. It is clear that fancy, and probably superstition, had a share in forming the collection of constellations. The stars were supposed to influence the character and destiny of man ; and to be, in some way, connected with superior natures and powers.† We may, I conceive, look upon the formation of the constellations, and the notions thus connected with them, as a very early attempt to find a meaning in the relations of the stars, and as an utter failure."‡

7. MORNING OR EVENING STAR. Venus and Jupiter, as the most conspicuous of the planets, receive the names of Morning or Evening Stars, according as they are situated westward of the sun, and are therefore seen to rise before him; or are eastward of that luminary, and therefore appear in the evening after he has set in the west.

8. THE TERRESTRIAL GLOBE is a model of our earth, the third planet in order of distance from the sun; and is designed to exhibit the relative situations of its several countries and seas, and their varying positions in the light of any heavenly body, as our planet revolves on its axis and in its orbit.

Not more than one half of the surface of any globe can be enlightened, at any instant, by the direct beams of a distant luminary.

9. AXIS OF THE EARTH is the diameter about which it revolves with a uniform motion from west to east, (C on page 8).

10. POLES OF THE EARTH are the points where its axis meets its surface, (F on page 8). The northern extremity of the earth's axis is called the Arctic Pole, because it is situated nearly beneath that part of the heavens occupied by the constellation of the Bear, (αρктоç, arctos). Hence also the southern or opposite Pole is called Antarctic. For great Circles and small Circles, see D on page 8. 11. THE CELESTIAL GLOBE is a model of the apparent

* See" Rhymes on the Constellations," Appendix.

+ See "Astrology."

Whewell's Hist. of the Inductive Sciences, pp. 134, 136.

sphere of stars, formed on the supposition of the observer's being situated at its centre, and viewing the stars on its concave surface.

12. POLES OF THE CELESTIAL SPHERE are those points which are exactly vertical to the two poles, or stationary points, of the earth's surface; and which, therefore, are not affected with that apparent westward motion which is common to all other points in the Celestial Sphere, as our earth's surface rotates eastward within it.

13. EQUATOR of the earth, or terrestrial globe, is a great circle passing, or supposed to pass, through all those parts of the surface which are equally distant from both poles; and which, being most distant from the axis, are carried the most rapidly eastward by the rotation of the globe.

14. EQUINOCTIAL of the celestial globe or heavens, is a line passing, or supposed to pass, over all those stars or points in the sphere which are equally distant from both Polar points; and to which, successively, the head of any Equatorial inhabitant is pointed as he is carried eastward by the earth's diurnal rotation.

When the earth is so situated in space, as that its inhabitants behold the sun coinciding with this line of stars, our nights are equal to our days; hence the name Equinoctial.

15. LATITUDE OF A PLACE is its distance northward or southward of the Equator, and is reckoned in degrees and parts of a degree.

For "Latitude of a Heavenly body," (see Index).

16. PARALLELS OF LATITUDE are small circles parallel to the Equator, which, on globes and in maps of the world, are generally drawn through every ten degrees.

17. DECLINATION OF A HEAVENLY BODY is its distance northward or southward of the Equinoctial, reckoned in degrees and parts of a degree.

Since the Equinoctial stars appear to pass vertically over the Equator, and the Polar points are vertical to the Poles, it is evident that the declination of any star, shows the latitude in which that star appears to pass vertically.

18. PARALLELS OF DECLINATION are small circles parallel to the Equinoctial, and corresponding to the parallels of latitude of the terrestrial globe.

19. PLANE OF THE MERIDIAN of any place, is an ima

DEFINITIONS, ETC.

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ginary plane, which, passing through the place and the whole axis of the earth, passes likewise through all situations northward and southward of that place, and through their several Antipodes, (Note to G on p. 9).

20. BRAZEN MERIDIAN is the circle of brass within which the artificial globe is suspended by its poles.

On the terrestrial globe, the flattened and graduated surface of the Brazen Meridian represents a portion of the plane of a meridian extending beyond the earth's surface, and serves to find what places agree in meridian and time.

On the Celestial globe, this graduated surface of the brass meridian represents a portion of the plane of a meridian extended beyond the Heavens; and serves to show what heavenly bodies are situated in the same plane with our whole axis, and are therefore brought by the earth's rotation into the plane of a meridian at the same instant of time.

21. Divisions of the Brazen Meridian. The brazen meridian of either globe, is divided into semicircles, which have different uses. The upper semicircle has its quadrants graduated from the Equator or Equinoctial, northward and southward, and is used to point out latitude and declination. The under semicircle has its quadrants graduated from the Poles, to enable the student to elevate them above the wooden circle, as may be required.

22. MERIDIANS (or, "Semi-meridians," or "Meridional Lines,") of the terrestrial globe, are semi-circumferences, passing through the poles, and showing what portions of the surface have a common meridian plane. They are sometimes drawn through every 15°, and sometimes, but less conveniently, through every 10° of the globe's circumference.

23. LONGITUDE OF A PLACE is the inclination of the plane of its meridian towards the east or towards the west, of a given standard meridian plane; and is reckoned in degrees and parts of a degree. For this purpose, the Equator of the terrestrial globe is divided into semicircles, one of which is graduated eastward and the other westward. The greatest longitude any place can have, is 180°, or half the circumference of the globe.

24. THE FIRST MERIDIAN is that from which geogra

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