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6. The ecliptic, colures, equinoctial and solstitial points, are situated on the celestial globe just as on the terrestrial; and therefore, it is unnecessary to take any farther notice of them here, as they have already been sufficiently defined in the first book.

It is also proper to observe that the tropics, polar circles, and parallels of declination, on the celestial sphere, correspond to the tropics, polar circles, and parallels of latitude op the terrestrial globe.

7. The poles of the ecliptic are situated on the celestial globe, at the distance of 23° 28' from the poles of the equinoctial.

For the pole of any great circle on the surface of the sphere, is 90 degrees distant from every part of its circumference, and the angle which the ecliptic makes with the equinoctial is equal to 23° 28'; consequently, the north pole of the ecliptic must be 230 23 distant from the north pole of the equinoctial, and the south poles must likewise be similarly situated.

8. Secondaries to the ecliptic are called circles of celestial latitude, or circles of latitude; because the arc of the secondary, intercepted between any celestial object, and the ecliptic is called its latitude, north or south; according as the object is on the north or south side of the ecliptic.

Every point on the surface of the celestial sphere is supposed to have a circle of celestial latitude passing through it, though, to prevent confusion, there are, in general, only twelve drawn on most of the celestial globes, the rest being supplied by the quadrant of altitude.

9. The longitude of a heavenly body is an arc of the ecliptic intercepted, in the order of the signs, between the equinoctial point Aries, and a circle of celestial latitude passing through the body.

Hence, the latitudes and longitudes of the heavenly bodies are ascertained by secondaries to the ecliptic, and the latitudes and longitudes of places upon the earth, are found by secondaries to the equator.

10. The right ascension of a heavenly body is an arc of the equinoctial intercepted, reckoning in the order of the signs, between the vernal equinoctial point and a circle of declination passing through the body. And the arc of the circle of declination intercepted between the celestial object and the equinoctial, is called the declination of the object.

The definitions contained in this article agree exactly with those which are given in Art. 5, page 21, and Art. 12, page 36.

In the practice of astronomy, the most general and convenient method of ascertaining the position of any celestial object on the concave surface, is to determine its position with respect to the equinoctial, or celestial equator, and the vernal equinoctial point, that is, to determine its declination and right ascension. The position of a celestial object, with respect to the equinoctial, being ascertained, it is very often necessary to determine its position with respect to the ecliptic, that is, to determine its latitude and longitude. See the foregoing two articles.

11. Diurnal arc is the arc described by the sun, moon, or stars, from their rising to their setting. The sun's semidiurnal arc is the arc described in half the length of the day.

12. Nocturnal arc is the arc described by the sun, moon, or stars, from their setting to their rising.

13. That parallel of declination, in an oblique sphere, which is as many degrees distant from the elevated pole of the heavens, as the place itself is distant in degrees from the equator, is called the circle of perpetual apparition; because all the stars included within this circle, are continually

above the horizon of the place, and consequently

never set.

14. The circle of perpetual occultation is another parallel of declination, opposite the former, and at a like distance from the depressed pole of the heavens. All the stars contained within this circle, never appear above our horizon, and consequently

never rise.

All the stars contained between these two circles, do alternately rise and set at certain moments of the diurnal rotation.

QUESTIONS.

How are the stars represented on the celestial globe?

What does the rotation of this globe on its axis from east to west represent; and what is the axis of the celestial sphere called ?

How is the wooden horizon of the celestial globe divided?

What is the equinoctial or celestial equator? What are the celestial meridians, and what are they usually called?

At what distance in degrees is the north pole of the ecliptic from the north pole of the equinoctial?

What are the circles of celestial latitude, and what is the latitude of a heavenly body ?

What is the longitude of a heavenly body ?

What is the right ascension and declination of a

heavenly body?

What are the diurnal and nocturnal arcs ?

What is the circle of perpetual apparition?

What is the circle of perpetual occulation?

CHAPTER II.

Of the fixed stars-division of the stars into constellations, &c.

1. Those celestial bodies, which have always been observed to keep the same relative distances with regard to each other, are called fixed stars, or simply stars.

From continued observations on the heavens, in clear nights, we shall soon see that the fixed stars constitute by far the greater number of the celestial bodies. It will likewise follow that they do not appear to have any proper motion of their own; but that the several apparent motions of the fixed stars are really caused by the diurnal motion of the earth, the precession of the equinoxes, properly called the recession of the equinoctial points, the aberration of light, &c. For, the apparent diurnal motion of all the heavenly bodies from east to west, is caused by the real motion of the earth on its axis, in a contrary direction; and the recession of the equinoctial points, will cause the fixed stars to have an apparent motion backwards from west to east, in circles parallel to the ecliptic, at the rate of 50" nearly in a year: in consequence of this motion, the longitude of the stars will be always increasing; their latitude remaining the same, because it is found by observation, that the equinoctial moves on the ecliptic, contrary to the order of the signs, while the ecliptic keeps nearly the same position in the heavens. The nutation of the earth's axis, the aberration of light, &c., cause some small change in the places of the stars. There are other changes in the apparent magnitude, lustre, &c. of the fixed stars, which shall be considered in a subsequent part of this work.

2. Those celestial bodies that are constantly changing their places, as well with regard to the fixed stars as to one another, are called planets, or wandering stars.

A planet may be known from a fixed star, by the steadiness of its light; for a fixed star appears to emit a twinkling light, but a planet gives a mild steady light. The planets, besides their apparent diurnal motions, have apparent motions that at first seem not easily brought under any general laws. Sometimes they appear to move in the same direction in the heavens as the sun and moon; at other times in a contrary direction; and sometimes they appear nearly stationary, or fixed in the same point of the heavens.

There are ten planets, whose names are, Mercury, Venus, Mars, Ceres, Pallas, Juno, Vesta, Jupiter, Saturn, and Uranus. Five of these planets have been observed from the remotest antiquity; the other five, lately discovered, are only visible by the assistance of telescopes. The motions, magnitudes, distances, &c. of the planets, shall be fully considered in the next book.

3. The fixed stars are divided into orders or classes, according to their apparent magnitudes. Those stars which appear largest, are called stars of the first magnitude; the next to them in lustre, stars of the second magnitude; and so on to the sixth, which are the smallest that are visible to the naked eye. All those stars which cannot be seen without the aid of a telescope, are distinguished by the name of telescopic stars.

The stars of each class are not all of the same apparent magnitude; there being considerable difference in this respect; and those of the first magnitude appear almost all different in lustre and size. There are also others of intermediate magnitudes, which astronomers cannot refer to one class in preference to another, and therefore they place them between the two. For instance, in M. Laland's catalogue of 600 principal stars visible at Paris, and which contains none less than of the fifth magnitude, there are no fewer than 126 stars of intermediate magnitudes. So that instead of six magnitudes, we may say that there are almost as many orders of stars as there are stars; such considerable varieties being observable in their magnitude, colour, brightness, &c. Whether these varieties of appearance are owing to a diversity in their real magnitude, or from their different distances, is impossible to determine; but it is highly probable that both of these causes contribute to produce those effects.

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