102 CHANGES OF THE SEASONS. shine beyond a pole farther than twenty-three degrees and twenty-eight minutes; for that is the extent of his declination; and when he has declination from the celestial equator either north or south, he must shine beyond one pole and not to the other; the days, therefore, will be longest in one hemisphere when they are shortest in the other. The subject of this lesson may be illustrated, by hanging any round body above or below the level of a candle so as to correspond with the sun's declination. It will be seen, that the light shines over one pole and does not reach the other. If the ball be then turned round, it will be observed, that the circles performed by any parts of the surface are unequally divided by the light; that it will be constant day or night near the north pole, as the ball is depressed or elevated, and that all the phenomena will be reversed in the other, or lower hemisphere. QUESTIONS.-1. What phenomena appear from the diurnal motion of the earth? 2. Under what circumstances would the days and nights be every where of the same length? Why? 3. Why not the day and night always equal to an inhabitant at the equator, and to one on the same meridian towards the poles? 4. At what points does the equator cut the ecliptic? 5. When is the earth at those points of its orbit?-and what happens at these periods? 6. At other seasons? 7. What is said of day and night at the poles? 8. How may the subject of this lesson be illustrated? 9. Look at fig. 40, and illustrate the variations in the lengths of the days and nights. LESSON 46. Changes of the Seasons. Obliq'uity of the Ecliptic, the angle which the ecliptic makes with the equator. Look nature through, 'tis revolution all; All change, no death. Day follows night, and night Then melts into the Spring. Soft Spring, with breath CHANGES OF THE SEASONS. Recals the first. All, to reflourish, fades ; As in a wheel, all sinks, to reascend: 103 Emblem of man, who passes, not expires.-THOMSON. The orbit in which the earth revolves in his annual course round the sun is not a circle but an ellipse or oval; and we are more than three millions of miles nearer to the sun in December about the time of the winter solstice, than we are in June about the time of the summer solstice. Now as heat and light from the sun are greater as the distance is less, it is manifest that this circumstance would occasion à variation in the temperature of the air, like that of our seasons, if the equator always coincided with the ecliptic. But the seasons with us, in north latitude, are not in the least degree occasioned by this circumstance, but by the direction in which the sun's rays fall upon us. When they fall perpendicularly, or most nearly so, the season is warmest; and when they fall most obliquely, or in a slanting manner, the season is coldest. The cause of the difference in the obliquity of the sun's rays is the obliquity of the ecliptic. The effect of obliquity, in regard to rays will be evident, if a board be held perpendicularly before a fire. It will then receive a body of rays equal to its breadth. But if it be placed obliquely, at an angle of forty-five degrees, then only half the rays will fall on its surface, and the other half will pass over it; so it is with the surface of the earth in summer and winter. The circumstance also, that the days are longest, whether in north or south latitude, when the sun's rays fall in the greatest quantity and most directly at any place, contributes much to the warmth of summer and the cold of winter. In northern countries, where the days are eighteen or twenty hours long, or where the sun is above the horizon for any number of days together, the heat of summer is equal to that of any part of the world. Since the degree of heat from the sun increases as the earth's distance diminishes, and this distance is least when it is summer in south latitude, and greatest when it is summer in north latitude, a greater degree of heat, therefore, must be received in summer in south latitude, than in summer in north latitude. But to compensate for a less degree of heat, the inhabitants in north latitude have longer summers than those in south latitude. For as the sun is not in the centre of an ellipse but in the focus, the earth must move farther in its orbit in one part of its revolution than in the other. It moves slower also as it is farther from the sun; and our summers are found to be eight days longer than the summers in south latitude; that is, between the vernal and autumnal equinoxes there are eight days more, than between the autumnal and vernal. It is well known that the degree of heat is not greatest, when the days are longest. We have the warmest weather in the latter part of July, and in the first of August; and our coldest month is January. To account for this it has been stated, that a body once heated does not grow cold again instantaneously, but gradually; now as long as more heat comes from the sun in the day, than is lost in the night, the heat of the earth and air will be daily increasing, and this must evidently be the case for some weeks after the longest day, both on account of the number of rays which fall on a given space, and also from the perpendicular direction of those rays. It is for the same reason, that the warmest part of the day is not, when the sun is at the meridian, but about two or three o'clock in the afternoon. QUESTIONS.-1. When are those who live in north latitude nearest the sun? 2. What would be the consequence if the equator coincided with the ecliptic? 3. What occasions the seasons with us? 4. How may the effect of obliquity in regard to the sun's rays be made evident? 5. What contributes much to the warmth of summer? 6. What is said of north and south latitudes as respects the degree of heat?-Explain. 7. Why is not the degree of heat greatest when the days are longest? 8. Look at fig. 40, and illustrate the diversity of the seasons. LESSON 47. The Moon. Quad'rature, the first and last quarter of the moon. Lu'nar, relating to the moon. Luna'tion, the revolution of the moon. THE moon is a secondary planet, revolving round the earth in about twenty-nine days and a half, and is carried with the earth round the sun once a year. Its distance from the earth is about two hundred and forty thousand miles; and it turns on its axis in the same time that it per forms its revolution round the earth. The light of the sun illuminates one half of its surface, and leaves the other in darkness. Of this illumination we perceive different degrees, according to the various positions of the moon, with respect to the sun and the earth. We see one half of its body enlightened, or a full moon, when it is placed in opposition to the sun, or when the sun is in one part of the heavens, as west, and the moon in the opposite part, as east. When the moon is in conjunction with the sun, or in that part of its orbit which is between the earth and the sun, its enlightened surface is turned from us, which renders it invisible; this is the time of the new moon. When the moon appears in the intermediate part of its orbit, between the conjunction and opposition, it is in its quadratures, and about half of its illuminated surface is turned towards us. As the moon illuminates the earth by light reflected from the sun, so she is reciprocally illuminated by the earth which reflects the sun's rays to the surface of the moon. As the surface of the earth is more than thirteen times greater than that of the moon, the earth must appear to the inhabitants of the moon thirteen times larger than the moon does to us, and it will exhibit the same phases, but in an opposite order. As the rotation of the moon on her axis is performed in the same time that she goes once round the earth,—which is evident from her always presenting the same face to us during the whole of her monthly revolution,-it is plain, that the inhabitants of one half of the lunar world are totally deprived of a sight of the earth, unless they travel to the opposite hemisphere. The face of the moon appears to have shades of different colours. If viewed through an ordinary telescope, her surface will appear diversified with long tracts of mountains and cavities. It has been ascertained that these are mountains from the shadows which they cast, and some of them are supposed to be volcanic. The difference between the rising of the moon on one day and the preceding is generally about fifty minutes. But in places of considerable latitude, there is a remarkable difference about the time of harvest, when at the season of full moon she rises for several nights together only about twenty minutes later on the one day than on that immediately preceding. By thus succeeding the sun before the twilight is 106 THE HARVEST MOON. ended, the moon prolongs the light, to the great benefit of those who are engaged in gathering in the fruits of the earth; and hence the full moon at this season is called the harvest moon. It is believed that this was observed by persons engaged in agriculture, at a much earlier period than it was noticed by astronomers. The phenomenon may be easily explained by the assistance of a globe; and it is occasioned by the moon's orbit lying sometimes more oblique to the horizon than at others. The Harvest Moon. All hail! thou lovely queen of night, How lovely is the scene !-how bright The wood-the lawn-the mountain's breast, When thou fair moon of Harvest! hast Thy radiant glory all unfurled, And sweetly smilest in the west, Far down upon the silent world. Shine on, fair orb of light! and smile And labour hath forgot the toil He bore in summer's sultry ray; And when the reapers end the day, Tired with the burning heat of noon, They'll come with spirits light and gay, And bless thee-lovely Harvest Moon! W. MILLAR. QUESTIONS.-1. In what time does the moon revolve round the earth? 2. At what distance is it from the earth? 3. In what time does it turn on its axis? 4. What is said of the illumination of the moon? 5. How does the earth appear as seen from the moon? 6. How does the face of the moon appear when viewed through a telescope? 7. What is the Harvest Moon? 8. By what is it occasioned?-9. Look at fig. 41, and illustrate the phases of the moon. |