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(AE. MaARINE. REVIEW cle, or 23%4° from the pole, have had overhead at noon for an observer on the continuous day--the sun being seen all day in every point of the horizon. On June 21 it is continuous day as far south from the pole as the arctic circle. Now, what is the cause of this? It is due to the fact that the earth's axis is inclined to the plane of the ecliptic at an angle of 2314°, and that in its revolution around the sun its path is elliptical in- stead of a true circle. The sun always throws its light from the same relative position, that is to say, it is not the shift- ing of the sun that is In anyway respon- sible for the variable manner in which the sun's light illuminates the earth. From the March equinox, when the plane of the ecliptic, or a straight line from the center of the sun to the center of parallel of 10°. As the sun's direct ray is north of the equator, the season is our summer, and as such, the sun's light must shine beyond the north pole--10° beyond it. In other words, it is continu- ous day from the pole to the 80th paral- lel, or vice versa. For an observer situ- ated anywhere on the 80th parallel would observe the sun on its horizon for 24 hours. If the stun's declination were in- creasing it would appear higher to him thereafter, and if diminishing he would only see its shadow during the 24 hours following, or its twilight. Twilight is the light perceived before 'he rising, and after the setting, of the sun, or when the sun is less than 18° below the horizon, occasioned by the il- ~~ ~ ~ ~s ~ ~ SUN'S ~ PERPENDICULAR RAY 'Y 23 yo x yes, RAYS 6 230 : About June 2r. About December 2r. FIG. 3. earth coincides with the equator, the lumination of the earth's atmosphere by north pole axis, in retaining its constant angle of inclination with the plane of the ecliptic, must tip forward, or toward the_sun, in moving along its elliptical path. The farther it goes from the March equinox the more the axis tilts, because the arc of the ellipse over which it moves, becomes smaller. From the March equinox to the summer solstice the axis tips 23!4°, which causes the sun's light to shine beyond the pole and causes continuous day as far as the light rays reach. It must not be understood that because the earth's axis is inclined to the plane of the ecliptic by an angle of 2314°, that it is from this cause alone that the poles tip toward and away from the sun in its annual revolution. the elliptical path in which it travels as well as the inclination of its axis. About June 21. About Dec. 21. The angular distance from the equator to where the vertical ray of the sun strikes the earth's surface is the sun's declination. It is also the angular dis- tance that the sun's light shines beyond or falls short of either pole. Suppos- ing the vertical ray from the sun shone on the earth at 10° north of the equator. The sun's declination is 10° north be- cause this is its angular distance from the equator. The sun would be directly ese the direct rays of the sun and their re- flection on the earth. EXPLANATION OF FIG. 3. About June 21 the north pole is 2314° within the lighted hemisphere; the per- pendicular rays of the sun fall on places 2314° north of the equator. As the earth rotates in this position, the sun rises and sets at 6 o'clock at the equator, and the days and nights there are equal, but the sun rises earlier and earlier at places nearer and nearer to the north pole, and later and later at places nearer and nearer to the south pole; hence in the northern hemisphere the days are longer than the nights. About Dec. 21 the north pole is within the dark, and the south pole within the lighted half of the earth, while the perpendicular rays of the run fall on places 23%4° south of the equator. Fig. 3 shows the positions of the earth in its orbit at the summer and winter solstices. These positions show the ex- treme limits of the sun's declination-- one north and one south. Note the man- ner in which the sun's perpendicular ray strikes the earth, and the positions of same. This perpendicular ray may be taken as the plane of the ecliptic, for a straight line from the sun's center to the earth's center is nothing more. Wher- ever the sun's vertical or perpendicular 31 ray occurs, the sun at noon is in the ze- nith of that parallel of latitude, or in other words, the sun's altitude is 90°. In either of the above positions the earth is at the turning point of its orbit. From June to December the earth in presenting and withdrawing its north pole to and from the sun, changes the vertical ray of the sun over the earth's surface, twice 23%°, or 47°. From the June turning to the September equinox the north pole in withdrawing from the sun changes its position by 234°, due to the obliquity of the ecliptic, or from the fact that the earth's path is irregular In its shape. At the September equinox the plane of the ecliptic and the plane of the equator co- incide, or the earth is in that position in its orbit that its obliquity is a minus quantity. From September to December the obliquity of the ecliptic is such that the earth's axis is again thrown out of line in addition to its constant angle of inclination. It turns its north pole away from the sun and presents its south pole to the sun. In the winter solstice the earth in its orbit merely retraces the movements it performed during the summer solstice. It must be borne in mind that the ecliptic is the real path of the earth as seen from the sun, or the apparent path of the sun in the sky. It is the sun in- stead of the earth that appears to move. It is the rotation of the earth on its axis from west to east that is responsible for the sun's apparent movement across the sky from east to west. The ecliptic, the earth's orbit or path, or the ellipse in which the earth revolves around the sun, are all one and the same thing; hence, do not become puzzled at the various expressions. A careful study of Fig. 3 will make this all very clear. The line marked per. pendicular ray with those marked equator measures the inclination of the earth's At the March equinox. FIG. 4, axis with the plane of its orbit, and also, why the inclination of the earth's equator to the plane of the ecliptic is responsible for the declination of the sun. This dia- gram also illustrates why the sun rises and sets on the equator always at 6 o'clock. AT THE MARCH EQUINOX. Showing the perpendicular ray of the sun coinciding with the equator. The in- tersection of the noon line with the per-