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not more than 150° from it, their difference of longitude will be found by subtracting the less from the greater. Should it, for instance, be required to know the difference of longitude between Constantinople, situated in longitude 29°, o', 52', and Pekin, lying in 116°, 30', 37'' both east from London, we have only to take the less quantity from the greater, for the difference of longitude required, which will be 87°, 29', 45". But when the two places are on opposite sides of the first meridian, and neither of them 90 degrees from it, the difference of longitude between them will be found by adding their respective longitudes together : thus the difference of longitude between Constantinople, situated in 29°, 0', 52" east from London, and Kingston in Jamaica, lying in longitude 77o, 0', 28" west from London, will be 106°, 1', 20".

Besides the revolution round its axis, in that portion of time which we call a day, the earth has another motion, by which it revolves round the sun, in that portion of time which we call a year. The path or figure described by the earth, in this annual motion round the sun, approaches nearly to a circle, but is in fact an ellipse, and is named the ecliptic. Whatever may have originally been the case, the plane of the ecliptic does not now coincide with the plane of the equator of the earth; that is, the axis of the earth is not perpendicular to the plane of the ecliptic, being inclined to it in an angle which, in 1769, was ascertained, by accu. rate observations, to be 66°, 31, 50"; and, consequently, the plane of the earth's equator must then have formed, with the plane of the ecliptic, an angle equal to the complement of the former, or 28°, 98, 10"; which angle is called the obliquity of the ecliptic. Had the plane of the equator coincided with that of the ecliptic, the earth in its progress round the sun would have had its axis perpendicular to its path, the globe would have been illuminated by the sun's rays constantly in the same manner, all the way from

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the north to the south pole, and the days and nights, produced by the diurnal revolution of the earth, by which all parts on its surface are successively exposed to his light, would have been constantly of the same length throughout the whole year. No sensible variation in the degrees of heat and cold, constituting that variety of seasons we now obscrve, especially in regions removed from the equator, would have been perceived: but from the obligue position of the earth’s axis to the plane of its path in the ecliptic, proceeds that constant and regular variation we now experience, in the length of the day and night, and in the degree of heat and cold by which the different seasons of the year are distinguished :-in treating of astronomy this subject will be rendered more intelligible to the student.

As the earth in its annual course round the sun, moves in a plane not coinciding with that of its equator, these two planes must intersect each other in two points distant asunder one half of the earth's orbit, and when the earth is in these points, the earth’s axis must of course be perpendicular to both planes, so that the sun's light is received equally on all parts of the globe, extending from pole to pole, and thereby causing the day and the night all over the world to be of equal length. From this circumstance it arises that these points of intersection are termed the equinoctial points, and simply the equinoxes.

In the middle point between these equinoctial points, when the earth's path is at its greatest distance from the plane of its equator, which is equal to the angle of the obliquity of the ecliptic, already stated to be 23', 28', 10' the earth seems for some days to be stationary: and hence those intermediate points at equal distance from the equinoxes are called the solstices, a term signifying that the sun stands still, agreeably to the common notion, that all the phenomena of the seasons were occasioned not by the motion of the earth, but by that of the sun. When the earth is in

one

one of these solstitial points, we have the longest day of the year, and when it is in the opposite solstice, we have the shortest day.

If from the equator on the earth's surface a space equal to the obliquity of the ecliptic, or 23, 28', 10', be set off towards cach pole, upon any meridian, and through that point circles be drawn from each pole, which must of course be parallel to the equator, these two circles will represent the earth's situation at the time of the solstices, where it begins to turn back towards the equator: and hence these circles are called tropics, from a Greek term signifying to return. That which lies on the north side of the equator is the tropic of Cancer, and that on the south side of the equator is the tropic of Capricorn, so named from certain clus. ters of stars, or constellations, in which they have a relation.

As the planes of the equator and the ecliptic intersect each other at an angle of 23o, 28', 10", it follows that the poles of a sphere, of which the plane of the ecliptic is a section perpendicular to its axis, must be situated at a corresponding distance from the poles of the equator, or of the earth : if then from the poles of the equator as centres, with a radius equal to 23°, 28', 10", circles be described, these circles will represent the various positions in which the poles of the ecliptic must be placed, and the path which they will describe, in the course of an annual revolution of the earth round the sun. The circle described about the north pole, is called the arctic ; and that described about the south pole, is called the antarctic circle; denominations borrowed from the name of a constellation in that part of the heavens to which the northern extremity of the earth's axis is always directed. This constellation was called by the Greeks arctos, or the bear, in which is a bright star, known by the name of the Pole star, from its situation very near to the point in the heavens, through which the earth's axis, if produced northerly, would pass : in the Latin language this constellation is called Ursa Min'r, and by us the Little Bar.

By means of these circles, which are in general called the polar circles, and the tropics, the surface of the globe is divided into five portions, extending from pole to pole, termed zones, or girdles : that which lies on both sides of the equator, and is bounded by the two tropics, is in breadth 23', 28', 10" X 2 = 46°, 56', 20", and is called the torrid zone, on account of the intense heat which prevails in that middle region of the earth.

The spaces comprehended between the poles and the polar circles, each being in breadth 23°, 28', 10”, are called the frigid zones, on account of the extreme cold experienced in those tracts of the globe.

The intermediate spaces lying between the tropics and the polar circles, each in breadth 43°, 03', 40", as extending from latitude 23o, 28', 10", to latitude 66°, 31', 50", are termed the temperate zones, on account of the moderate temperature of the atmosphere in those regions, equally removed from the heat of the torrid, and the cold of the frigid, zones.

Thus the surface of the earth is divided into one torrid zone, begirting its middle, two frigid zones surrounding the poles, and two temperate zones situated between the torrid and the frigid zones.

This division of the earth into zones being, however, too general for the purpose of ascertaining the positions of places with respect to each other, or to the equator, the ancients thought it necessary to subdivide its surface into other portions more minute.

The principle on which this subdivision was made, was the respective length of the day and the night, in different situations on the globe, at the summer solstice, or at the longest day of the year. Supposing the day and the night VOL, II.

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to consist each of twelve hours at the equator, it was found that at the distance of 8°, 34', on either side, the day consisted of 12 hours : again, at the distance of 8°, 10' more, or in latitude 16', 44' north and south, the day was observed to consist of 13 hours : and, in this manner, by observing the places where the day exceeded by 1 hour the length of that at a place before observed, a number of concentric circles was supposed to be described round the poles, through those places, and parallel to the equator, as high up as to the polar circles : and the narrow bands, or zones, included between these circles, were termed climates, from a Greek word signifying a gradual inclination ; so that whatever was the length of the day at the spot through which was drawn the circle nearest to the equator, it was half an hour longer at the spot through which was drawn the adjoining circle nearest to the pole : but this division of the earth's surface is now seldom employed, and at any rate can be of very little service, since the introduction of the use of the parallels of latitude.

It was already observed that the earth not being a perfect sphere, but flattened at the poles, the difference between its axis and the diameter of the equator has by some been calculated at 34 English miles ; and that a degree of latitude on either side of the equator, which may be considered as equal to a degree of longitude on the same line, contains about 69,9 English miles. Had the earth been a regular figure produced by the revolution on its shortest axis, of an ellipse whose transverse and conjugate axes differed by 34 miles, we might easily calculate the number of miles contained in a degree of latitude, at any distance from the equator; but from the measurements hitherto made, of degrees in different parts of the globe, so much uncertainty has resulted, that we must conclude either that the earth is not a solid produced by the revolution on its axis of any regular curve whatever, or that errors have occurred in the several

measure.

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