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ROTUNIDITY OF THE EARTH. 15
light and shade, ascends the heavens, reaches the highest point and declines in the west. Star after star sinks beneath the western hills, and new ones rise in the east. Twelve hours pass away, when again the sun, rising with undiminished lustre, calls the busy world once more into bustle and activity.
The phenomena thus presented, convince us that there is no such thing as rest, for the whole heavens seem revolving around us, and the first step towards an accurate knowledge of our earth is, that either we, or the heavenly bodies, are in ceaseless and regular motion.
Suppose that before us the waters of some vast lake or ocean are spread out; far as the eye can reach there seems to be a place where the sky is resting upon the water, called the horizon from a Greek word meaning “to see.” As we stand, perhaps wondering how far from us this horizon is, a vessel sails out the harbor and moves steadily from us. Now our first idea is that we are looking out upon a vast plain, ald consequently we expect to see the vessel as it moves away, become fainter and fainter, until at last the straining eye will fail to catch the minute image. This appearance is shown in the engraving below.
Instead of this, however, a new and unexpected phenomenon greets the eye. The vessel sails away, and soon arrives at the horizon, and then slowly sinks from view. First the hull disappears, then the sails, and at last the flag, presenting the appearance shown in this engraving.
This then is the second step towards obtaining an accurate knowledge of our earth, and we learn that the surface of our lakes, and seas, is not an extended plain, but curved. If we were on a vessel at sea, we would perceive the horizon encompassing us like a vast circle, of which, we would be the centre. And in whatever direction we made an observation, we would find the surface of the water curving or bending from us in that direction. The same phenomenon is observed on land. If we ascend some high elevation, such as a mountain, or lofty monument, the horizon appears in every direction equally distant, or, in other words, a large circle, of which we are the centre. From this we rightly infer that the surface of the earth is convex, like the surface of an apple, or an orange. It becomes an interesting question, after the convexity of the earth is thus established, to determine its actual shape, whether it is a true sphere, or a spheroid, i.e., having the diameter through one direction longer than another, or, whether the curvature is of such a nature as to return into itself, for it is well known that there are curves, such are the parabola, and hyperbola, which, however far continued, never return into themselves like the curve of a circle. It was therefore a bold undertaking to circumnavigate the globe and thus demonstrate its spherical form, by actually sailing around it. This was accomplished however by Ferdinand Magellan, or rather by the expedition which he fitted
ROTUNDITY OF THE EARTH. . 17
out, for he himself did not live to witness the complete triumph of his bold attempt. Magellan was a Portuguese who had entered into the service of Spain. In the year 1519 he sailed for South America, and discovered the straits called by his name, and which separate the island of Terra del Fuego from the continent. He likewise discovered the Marian and Phillipine islands, which he took possession of in the name of the King of Spain, and was killed on one of the latter group. His fleet was mostly dispersed, but one ship with eighteen men, returned to Spain in 1522, having sailed westward completely around the world. The rotundity of the earth, by these means, was established beyond a doubt, though indeed this proof was not necessary, a great variety of phenomena giving the same result. For example, the shadow of the earth, which is cast upon the moon at the time of a lunar eclipse, is always bounded by a curved line or circle, and it can be shown mathematically, that a spherical form is absolutely necessary for the stability of the earth. The moon, and all the planetary bodies, are also observed to present discs, the same as a ball suspended in the sky. Having learned these two things, viz: that there is a great and unceasing motion somewhere, and that the earth is round, it becomes interesting to determine its actual size, its diameter and circumference. Previous to determining this and on the supposition that our earth is the grand centre of the universe, let us study the phenomena presented by the sun, planets, and stars in their apparent diurnal or daily revolution around the earth, premising however, that to certain directions upon its surface the arbitrary names, North, South, East, and West, have been assigned. For example, we call the part towards the north star north, the opposite south, and facing towards the north star, we call the right hand east, and the left hand west. These names are entirely arbitrary, i.e., they do not actually represent fixed directions in space, but are simply relative expressions, thus, what is east to one observer, may be west to another, for example, take the next diagram, representing the earth as round, the north pole being at the position N, and suppose two observers one at A, and the other at B, both facing towards the north. If questioned about some object C, B would declare it to be
west, being at his left hand, whilst A would assert it to be east,
being at his right hand. The terms therefore, north, south, east and west, are only relative expressions, and not absolute directions. It will be necessary to remember this, and we may also remark, the same is true of the expressions up, and down. What would be up to an observer at A, would be in the direction N A, but this would be down to an observer at B. Hence we must learn to consider up, as away from the earth, and down as the direction to its centre, and therefore not absolute directions in space but only relative terms. Now as the sun and the stars are observed after certain regular intervals to appear in the east, apparently move over the heavens, and set in the west, the natural inference is, that they are revolving in vast circles around the earth, which itself is the immovable centre. Below we have given
MoTION of THE SUN.” - 19
sun revolving around it in a circular orbit, and the stars still further beyond. Now on the supposition that this is the true system of the world, suppose the sun revolving in the direction A B, and an observer at a, facing towards the north N. He would perceive the sun appear to rise at his right hand, or in the east, and when the sun had travelled far enough round, say to B, to become visible to an observer at b, he would see it at his right hand, or in the east. The sun in his daily revolution, would thus track out in the heavens a certain line, which astronomers call a diurnal circle. Now suppose that some morning, just at sunrise, we observe a particular star, A, close to the sun, rising just before it. If the stars revolved around the earth in the same time as the Sun, as they seem to do from a casual observation, it is evident that after any definite interval, say one month, the sun and this star would still be found together, but this is not the case, for after one month, it will be found, that this star A, which rose just before the sun, will now rise two hours before him, and the sun will be near the star C, having apparently moved backward the distance A. C. -- If we should continue to observe this backward motion of the sun, we would find that after one year had elapsed, the sun would have moved completely around backward, contrary to the direction in which, each day he seems to move across the heavens, arriving again at A. Hence it would appear, that, the earth being the centre, the stars are revolving around it a little faster than the sun, but in the same direction, gaining upon the sun about 4 minutes. a day, so that in one month the star A would gain 120 minutes or two hours, and rise just so much sooner than the sun; and thus, in the course of a year, the stars would make one more revolution than the sun. Now suppose we were to observe carefully the stars near and over which the sun passed in this backward motion, for it is evident that this path would mark out a circle in the heavens. Astronomers have done this, and they call this path or line, which has a fixed position among the stars, the Ecliptic, or sun's path. On the next page we represent the ecliptic, and a certain space on each side of it. This space includes the orbits of all the planets,which also partake of the same backward motion as the sun.