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. 21 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— Let A B C D, &c., represent the earth, and let G C be a circle the plane of which passes directly through the centre of the earth; this is a great circle. So is A E for the same reason, for if the globe were to be divided through these circles it would be exactly halved, but a circle passing through H B, or FD, is called a small circle, since the plane of the circle does not pass through the centre of the sphere on which the circle is drawn. From this definition it will be perceived that the circle A I E K, (the part behind the sphere being shown by the dotted line) is a great circle, because the plane of this circle passes through the centre of the sphere. Every great circle, has what is called a pole, that is, a point ninety degrees, or one quarter of a circle, distant from it in every direction, thus—A is the pole of the circle G C, for from whatever point on the circle G C, the distance is measured up to A, it will be found 90°. For instance the arcs AG, AI, AO, A K, AC, are all # of their respective circles. Now suppose the circle G C, to represent the equator, then A will be the north pole of the earth, and Ethe south pole. Suppose now this great circle which we have called the equator to be actually traced around the earth and divided into 360 parts called degrees, marked (9), and suppose these degrees subdivided into minutes marked ('), and call these minutes miles, how many miles would the earth be in circumference? Evidently sixty times 360, or 21,600 miles. This is not so much as the circumference is usually stated to be; viz, 24,000 miles, and for this reason; the mile at the equator, is longer than the English statute mile. Referring to the preceding figure, it will be readily perceived that if the circle H B was divided into 360 parts and these again subdivided into 60 parts each, called miles, these miles would be much smaller than the equatorial miles, indeed it would require 69% English statute miles to constitute 19, or 60 equatorial, or geographical miles. Now if we take 694 miles for the length of a degree, it is evident the circumference of the earth will be 360 times this, or 25,020 miles, and as the diameter is a little less than the circumference, the diameter is called in round numbers 8000 miles. When therefore we assert that the earth is 8000 miles in diameter, we mean simply this, if the equator, or any great circle drawn upon the MEASUREMENT OF A DEGREE. 23 earth, is divided into 360 parts, and these subdivided into sixty parts each, and their length ascertained, that it would take 8000 of them to measure the diameter of the earth. The length of a mile therefore, instead of determining the diameter of the earth, or its circumference, is itself determined by that diameter or circumference. The circle might have been divided into 1000 parts, and these subdivided into 100 each, this would give 10,000 minutes or miles for the circumference, but the mile in this case would be shorter. Having assumed the earth’s circumference 24,000 miles, we next desire to know when we have passed over a mile on its surface. This would seem a difficultundertaking at first thought, for how can we determine when we have passed over a degree upon the earth 7 A diagram will explain the manner this is accomplished. Let A B C D represent the earth, A C being the equator. A spectator at the pole B, would see the pole star directly overhead, but a spectator at A, on the equator, would see the pole star in the horizon. Hence, in travelling from the north pole to the equator, the elevation of the pole star changes from directly overhead, or in the zenith as it is called, to the horizon, or 909, changing its altitude 19 for every degree traveled over the earth's surface, either north or south. The astronomer is furnished with the means of measuring the altitude of the pole star, or its distance above the horizon by means of the quadrant, or the astronomical circle which we shall describe, together with some other astronomical instruments in the next chapter. We have now learned three important facts in regard to our earth, and the celestial bodies, viz: The ceaseless and uniform motion, the rotundity of the earth, and the actual length of a degree upon its surface, and this is no small progress, supposing we commeneed entirely unacquainted with the subject. Fortunately, as we proceed to show the gradual improvement in astronomical knowledge, we can also give a history of the science, and briefly notice those eminent men, and their discoveries, whose labors have brought astronomical science to its present state of perfection. Supposing that we are ignorant of the nature of the motion perceived in the heavenly bodies, we will lay aside further observation for the present, and notice some of the instruments employed in astronomical discoveries. |