The following text may have been generated by Optical Character Recognition, with varying degrees of accuracy. Reader beware!

28 TAE Marine. REVIEW THE THEORY AND PRACTICE OF LAKE NAVIGATION BY CLARENCE E. LONG 'MOVEMENTS OF THE EARTH AND SUN. INCLINATION OF THE EARTH'S AXIS. As the earth travels around its orbit, the axis always points in nearly the same direction. The axis is not perpendicular to the orbit; it leans, or inclines, about 2314° from the perpendicular, as shown ie _ Diagram or Fig. 1, represents the earth at four points in its orbit around the sun. In its position of March 21 the sun at noon is on the equator. Each day as the earth advances in its orbit, the inclination of the axis causes places far- Fic. 1. ther and farther from the equator to be presented to the vertical rays of the noon sun, until June 21, the sun at noon is in the zenith or vertically over z, which is as far from the equator as the north pole, N., is distant from x; that is 2314°, or the angle of inclination of the earth's axis, or the number of degrees it is out of plumb with the plane of the ecliptic, which is represented by a straight line drawn from the center of the sun to the center of the earth. Note--If you do not thoroughly un- derstand what this means try the follow- ing simple experiment: Stick a knitting needle through the core center of an apple or an orange (get one as round as possible), and let the ends protrude above and below. The knitting needle repre- sents the poles and axis of the earth. Next take a piece of pasteboard and cut a circular hole in its center large enough so that the apple or orange will go through it. Now rest the ends of the pasteboard on a couple of books laid on a table in such a manner that the apple will have room enough below with- out touching the table. The piece of pasteboard must be level (horizontal). Place the apple in the hole of the paste- beard so that only one-half is to be seen on top and adjust it so that the end pro- truding above is at right angles, or 90°, from the sheet of pasteboard. The sheet of pasteboard represents the plane of the ecliptic, or a plane extending from the earth's center to the sun's center. If the curth's axis were exactly perpendicular to the plane of the ecliptic, as shown by your rude device, the days and nights would everywhere be of the same dura- tion, from one end of the year to the other; there would be no seasons, and in fact, nothing. Now incline the needle ends to an angle of 66%4° (as near as you can tell by the eye) with the sheet of pasteboard, or the ecliptic, which it represents; when you have done this shove another knitting needle through tne apple's equatorial center from a po- sition on top the pasteboard so that it will come out on the other side diametri- cally opposite and again rest on the pasteboard. The needle will now hold the apple in its place without further aid, and you can illustrate for yourself the precise manner in which the earth trav- cls around the sun, and the manner in which the sun's rays strike the earth's surface, in which portions and the cause of the seasons, the sun's declination, etc. As the earth passes around in its orbit, the axis assumes daily a position in re- lation to the sun more nearly similar to that shown of March in the diagram, and the earth presents to the vertical rays of the noon stn points nearer and near- eG to} the equator, until; in September, the sun is exactly over the equator again. Through the other half of the orbit the same phenomena takes place, but on the other side of the equator. Thus, the only part of the earth which ever receives the vertical rays of the sun lies between the parallels of 2314° latitude on either side of the equator; hence, these parallels are taken as the limits of the hottest OW tOnmcde The parallel on the north is called the tropic of Cancer, and the one on the south the tropic of Capri- corn. They are called tropics (turnings), because over them the sun appears to turn and retrace its course toward the equator. It will be observed in the dia- gram that when the earth is at December or June the region within 2314° of one of the poles is in darkness, and hence receives no light or heat during an en- tire rotation. of the earth. These re- gions, which, during at least one- day Of the year receive no heat rays, and during the rest of the year receive them ZONne. only very obliquely, must be the coldest parts of the earth; hence, the parallels of 23%4° from the pole, or in latitude 66%2°, are taken as the limits of the frigid zones. The polar circle near the north pole is the Arctic Circle; that near- est the south pole is the Antarctic Circle. It is because of this inclination of the earth's axis that the hours of sunrise and sunset change from day to day, and that one-half of the year is warmer than the other half. This inclination never changes. It is the annual revolution of the earth and the inclination of the axis that causes the change of the seasons. EXPLANATION OF FIGURE 1. C is the position of earth in its orbit, or path, at about March 21 (spring, or vernal equinox). With the earth in this position the declination of the sun is zero; in other words, the sun is directly over the equator, and its light shines from the north pole to the south pole, hence the day and night for this position is equal all over the world. On this date for an observer situated on the equator the sun would appear to describe in the sky a great circle like the equator itself. The sun .rises in the true east at 6 o'clock true time. It ascends vertically and continues to bear true east until it reaches the zenith--directly overhead-- of your meridian. From this position it descends describing the same great circle but bears true west until it sets at 6 o'clock. Only to the equator does the sun describe this great circle. Although, at all other places on the earth the day and night is of equal length, 12 hours of each, the sun rises and sets in the true east and west at 6 o'clock true time, but it travels at a less altitude across the sky. The higher the latitude, that is, the greater the distance from the equator, the smaller arc it will describe in the sky, or the closer it will hang to the horizon. 'For an observer at either pole on this date he would see the sun fol- lowing along his horizon at the same height from its time of rising till time of setting. Theoretically, he would only see one-half the sun's disk, while an ob- server at the opposite- pole would see the other half. The observer at the north pole would observe the upper half and the observer at the south pole, its lower half. On the equator at high noon (meridian height) the sun's_ altitude would be 90°. In our latitude, say 45°, the meridian height of the sun on that day would be 45°. We know how far north of the equator we are and the posi- tion of the sun, therefore, our latitude must equal the number of degrees the sun is above the horizon at noon for that day. Again, if we know how far we are from the stn and how far the sun is from the equator, we can easily determine our latitude. We have the altitude of the sun for three positions on the earth for that day, viz.: Equator Or Oe lat, -- 902 altitudesslat 452 alt. AS enontn pole, lat O0e ultras Ome