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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,

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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

an engraving which represents the earth as the centre, and the

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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,

not moving on uniformly with the stars. The middle black line

represents the ecliptic and the whole space or belt is called the Zodiac. The ancients divided the zodiac into twelve equal parts, and gave them names, indicative of the peculiar employment of that season of the year, when the sun happened to be in any one of them. For example, the sun, in the preceding diagram, is in the sign called Virgo, or the Virgin; this sign was represented by a virgin bearing sheaves of wheat, as the sun was near these stars in the fall of the year, when the harvest was gathered. We shall refer to this again when we explain the phenomena of the seasons. The ecliptic was divided into twelve parts, or signs, because the moon makes the complete circuit in one-twelfth of the time the sun does, hence the twelfth of the year is called a moon, or a month. The time of a lunation, or interval from new moon to new moon, being thirty days, and twelve of these lunations happening in a year, the number of days to the year, when reckoned by lunar months is 360. This number of days however is not strictly correct, for the sun makes 365+ revolutions apparently, around the earth, while moving from any particular star around to that star again. It would be inconvenient to subdivide the ecliptic into 365 parts as this number cannot be halved, or quartered. So the early astronomers, adopting the lunar year, divided the whole circle into 360 parts, which they called degrees. This division, it will be understood from what we have said, was ANGLES.

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perfectly arbitrary. The circle might have been divided into just 100, or 1000 parts, and these called degrees, but it was convenient to adopt for the length of a degree, a space which would represent the progress of the sun in one day as nearly as was possible. When we speak of a degree, it must be remembered that an absolute length is not meant, but only the 1-360 part of some circle. The length which belongs to a degree will vary with every different circle. Thus in this diagram, we have two circles with

a common centre, and two lines drawn from that centre, including 20 degrees of each circle.

All circles are supposed therefore, to be divided into 360 parts, and the 1-360 part of any circle is called a degree. Two kinds of circles are supposed to be traced on the earth, as also in the heavens, viz, great and small circles; this name does not arise from the fact that one circle is actually greater than another, the distinction is more marked, and is this

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Let A B C D, &c., represent the earth, and let G C be a circle

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