Sutcher 8.28-44 50626 LECTURES ON EXPERIMENTAL PHILOSOPHY, &c. LECTURE XXIII. ASTRONOMY. THE VICISSITUDES OF THE SEASONS, AND THE constant inclination of the axis of the earth to the plane of the ecliptic, and its parallelism, produce the vicissitudes of the seasons. That part of the earth which has the sun at midday as near to its zenith as possible, consistently with its latitude, enjoys the pleasant season of summer; and it is winter when the sun is as far distant as possible from its zenith. As, therefore, the track of the ecliptic lies between the two tropics, it is evident that our midsummer must be on the day of the summer solstice, or that on which the sun is vertical, or in the zenith, to all that part of the globe which is under the tropic of Cancer, and our midwinter or winter solstice must be on that day on which that part which lies under the tropic of Capricorn has the sun vertical, or in the zenith, and that period is midsummer to all that part of the southern hemisphere which lies beyond that tropic. Those places also which are situated under the equator will evidently have the sun in their zenith twice a year, at the time of the equinoxes. In order that this vicissitude of seasons should take place, that is, that the solar rays should be vertical at the tropic of Cancer at the time of our summer solstice, at that of Capricorn at our winter solstice, and also at the equator at the time of the equinoxes, it is necessary that the axis of the earth should have an inclination of 234 degrees to the plane of the ecliptic, and that this axis should preserve its parallelism during the whole of the earth's annual revolution. To explain this more clearly by a diagram, let S (Pl. I. fig. 1.) be the sun; T and t two points diametrically opposite in the annual orbit of the earth; T the point where it is found towards the 21st of June; t the point where it is found towards the 21st of December; EF, or ef, the equator; It the ecliptic or the orbit of the earth; GH, or gh, the tropic of Cancer at the distance of 234 degrees from the equator; IK, or ik, the tropic of Capricorn at the same distance from the equator on the opposite side; PA, or pa, the axis of the earth; P, or p, the north pole; A, or a, the south pole. Now the equator of the earth being inclined at an angle of 234 degrees to the ecliptic, the sun is bright so many degrees nearer to the north pole P about the 21st of June, and consequently he will be immediately over the point H in the tropic of Cancer, and all the places of the earth situated under the parallel GH will pass in their diurnal revolution directly under the sun. Six months after, the earth will be found on the other side of the sun in the point t of her orbit, diametrically opposite to the point T. Suppose then (which really takes place) that the axis pa is parallel to the axis PA, so as to point exactly to the same part of the heavens to which it pointed six months before. Now it is evident that the terminator MN, ormn, (drawn at right angles to It) will divide the parallels to the equator or the circles of daily motion into unequal parts-those on the same side of the equator with the sun having a greater portion within the illumined than within the dark hemisphere. All the countries of the earth, therefore, which lie on this side of the equator will have their days longer than their nights, and of course enjoy summer. On the contrary, all the countries on the other side of the equator, having a greater portion of their parallels included within the dark than the illumined hemisphere, must have their nights longer than their days, and consequently it will be their winter. This is not only the case when the sun is at his greatest distances from the equator, or when he is in either of the tropics, but it takes place in a less degree at all his intermediate distances from it, the terminator continually dividing the parallels unequally, so as to comprehend a greater or a less portion of them within the illuminated part of the earth, according as the sun is on the same or a different side of the equator from that on which the parallels lie. When he is in the equator, the terminator will be in the direction PA, or pa, and will manifestly divide the parallels equally, so as to make day and night of the same length throughout the globe. This is the time of spring to the inhabitants of that part of the earth who are placed on the same side of the equator on which the sun is passing, and autumn to those who are placed on the other side of the equator from which he is retiring. It is easy, therefore, to see that it is the inclination of the axis of the earth to the plane of the ecliptic, and its constant parallelism, which occasion the change of the seasons. The sun being of all the heavenly bodies the most familiar to us, and the most easy of observation, has from the earliest ages furnished us with the measure of time. Time is divided into ages, years, mouths, weeks, days, hours, minutes, and seconds. That space of time is called a day in which the earth performs one rotation on its axis, and in which consequently the sun appears to make an entire revolution round the earth from east to west. The natural or astronomical day is that in which the sun appears to revolve from the eridional line which passes through any given. place to the same meridian again. The astronomical days are, however, not always of the same length, while the civil day is always of equal duration. This difference has given rise to a distinction which is necessary to be known. The former, which is usually measured by the sun-dial, is called solar, and the latter, which is measured by the clock, is called mean or equal time. The difference between mean and solar time depends upon three causes. * 1st, The earth (according to the third law of Kepler) does not pass equal portions of its orbit in equal times. It proceeds sometimes quicker, and sometimes slower, and consequently the sun appears to advance more or less rapidly in the ecliptic. In the first case the day will be some-. what longer, because the earth to rejoin the sun, or to present to him the same meridian, must make somewhat of a larger turn upon its axis. In the second case (for the opposite reason) the day will be in some degree shorter. 2dly, The earth's motion on its own axis is always equal and regular; and if the plane of the ecliptic was parallel to the equator, there would be no difference in the time marked by either of these circles, for fifteen degrees of each of these circles passing over any meridian would be equivalent to an hour in time. But, from the inclination of the earth's axis, as already described, the ecliptic is oblique to the equator, and consequently the earth's rotation on its axis. carries. |
