Imágenes de página
PDF
ePub

nomena, it will be necessary to define some of the principal circles of the globe. The reader will comprehend more fully these definitions, and attain more accurate ideas of these circles, by placing, while he is reading them, a terrestrial globe or armillary-sphere before him. It may, however, bé necessary to pre mise, that we are at liberty to suppose as many circles as we please to be described on the earth ; and the plane of any of these to be continued from the earth, until it marks a corresponding circle in the concave sphere of the heavens.

Among these circles, the horizon is the most frequently named. Properly speaking, there are two: circles by this name, but distinguished from 'each other by added epithets; the one being called the sensible, the other the rational horizon.

In general terms, the horizon may be defined to be an imaginary circle, that, separates the visible from the invisible part of the heavens.

If a spectator supposes the floor or plane on which he stands, to be extended every way, till it reach' the starry heavens, this plane is his sensible horizon.

The rational horizon is a circle, whose plane is parallel to the former, but passing through the centre of the earth.

The rational horizon divides the concave sphere of the heavens into two equal parts, or hemispheres ; the objects that are in the upper hemisphere will be visible; such as are in the lower hemisphere will be invisible to the spectator.

Though the globe of the earth appears so large

to those who inhabit it, yet it is so minute a speck when compared to the immense sphere of the heavens, that at that distance the planets of the rational and sensible horizons coincide; or, in other words, the distance between them in the sphere of the heavens is too small for admeasurement.

To illustrate this, let ABCD, plate 3, fig. 1, represent the earth; zhno the sphere of the starry heaven. If an inhabitant of the earth stand

upon the point A, his sensible horizon is se, his rational one ho; the distance between the planes of these two horizons is AF, the semi-diameter of the earth, which is measured in a great circle upon the sphere of the heaven, by the angle eFo, or the arc eo; this arc, in so small a circle, zhno, would amount to several degrees, and consequently the difference between the sensible and rational horizon would be great enough to be measured by observation. If we represent the sphere of the heaven by a larger circle, the semi-diameter of the earth AF, measured in this circle, will amount to fewer degrees; for the arc EO is less than the arc éo; and the larger the sphere of the heaven is, in proportion to the globe of the earth, the less sensible is the difference between the two horizons. Now, as the sphere of the earth is but as a point when compared to the starry heaven, the difference between the sensible and rational ho.. rizon will be insensible.

From what has been said, it appears that the only distinction between the sensible and rational horizon,

/

[ocr errors]

arises from the distance of the object we are looking at..

The sensible horizon is an imaginary circle, which terminates our view, when the objects we are looking at are upon the earth's surface.

The rational horizon is an imaginary circle, which terminates our view, when the objects we are looking at are as remote as the heavenly bodies.

As the rational horizon divides the apparent celestial sphere into two equal hemispheres, and serves as a boundary, from which to measure the elevation or depression of celestial objects ; those in the upper, or visible hemisphere, are said to be high, or elevated above the horizon; and those in the other hemisphere are called low, or below the horizon.

The earth being a spherical body, the horizon, or limits of our view, must change as we change our place; and, therefore, every place upon the earth has a different horizon. Thus, if a man lives at a, plate 3, fig. 2, his horizon is GC; if he lives at b, his horizon is HD; if at ç, it is AE. From hence we obtain another proof of the sphericity of the earth ; for if it were fiat, all the inhabitants thereof would have the same horizon.

The point in the heavens, which is directly over the head of a spectator, is called the zenith.

That point which is directly under his feet, is called the nadir.

If a man lives at a plate 3, fig 2, his zenith is A, his nadir E; it he lives at b, his zenith is B, his nadir F. Consequently, the zenith and horizon

of an observer remain fixed in the heavens, so long as he continues in the same place; but he no sooner changes his position, than the horizon touches the earth in another point, and his zenith answers to a different point in the heavens.

The aris of the earth is an imaginary line, conceived to be drawn through the centre of the earth, upon which line its revolutions are made.

The poles of the earth are the extremities of its axis, or those two points on its surface, where its axis terminates; one of these is called the north, and the other the south pole. The poles of the heavens, or of the world, are those twa points in the heavens, where the axis of the earth, if produced, would terminate; so that the north pole of the heavens is exactly over the north pole of the earth, and the south pole of the heavens is directly over the south pole of the earth.

The equator is an imaginary circle, which is supposed to be drawn round the earth's surface, in the middle between the two poles. It divides the earth into two equal parts; one of which is called the northern, the other the southern hemisphere.

If we suppose the plane of the earth's equator to be extended all ways as far as the heavens, it will mark there a circle, that will divide the heavens into two equal parts; this circle is called sometimes the equinoctial, sometimes the celestial equator.

The meridian of any place is a circle supposed to páss through that place and the poles of the earth: we may therefore imagine as many meridians as there

are places upon the earth, because any place that is ever so little to the east or west of another place, has a different meridian.

By the foregoing definition, we see that the meridian of any place is immoveably fixed to that place, and carried round along with it by the rotation of the earth. The meridian marks upon the plane of the horizon the north and south points.

The circle which the sun appears to describe every year, in the concave sphere of the heavens, is called the ecliptic. It is thus denominated, because, in all eclipses, the moon is either in or near the plane of it. But as the earth moves round the sun, in the plane of the ecliptic, it is likewise the plane of the earth's orbit.

If we conceive a zone, or belt, about sixteen de grees broad, in the concave sphere of thé heaven, with the ecliptic passing through the middle of it, this zone is called the zodiac. The stars in the zodiac were divided by the ancients into twelve equal parts or signs, to correspond with the months of the year; and because the number twelve with them was always expressive of fulness or completion, it is used in that sense in sacred writ. The signs are named, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces. We

may imagine as many circles as we please drawn on a globe, parallel to the equator, and these will decrease in their diameter as they approach nearer the poles. The tropics are two lesser cir.

« AnteriorContinuar »