BEGINNING AND LENGTH OF THE SEASONS. h. m. s. Sun enters V (Winter begins) 1833, Dec. 21st, 7 25 46 M. T. Wash. (Spring 1834, March 20,8 56 38 d. h. m. S. 92 21 6 31 93 13 55 22 89 17 23 26 186 11 1 53 178 18 54 18 7 16 7 35 365 5 56 11 365 5 48 48 The north pole of the Earth is denominated the elevated pole, because it is always about 231° above a perpendicular to the plane of the equator, and the south pole is denominated the depressed pole, because it is about the same distance below such perpendicular. As the Sun cannot shine on more than one half the Earth's surface at a time, it is plain, that when the Earth is moving through that portion of its orbit which lies above the Sun, the elevated pole is in the dark. This requires six months, that is, until the Earth arrives at the equinox, when the elevated pole emerges into the light, and the depressed pole is turned away from the Sun for the same period. Consequently, there are six months day and six months night, alternately, at the poles. When the Sun appears to us to be in one part of the ecliptie, the Earth, as seen from the Sun, appears in the point diametrically opposite. Thus, when the Sun appears in the vernal equinox at the first point of Aries, the Earth is actually in the opposite equinox at Libra. The days and nights are then equal all over the world. As the Sun appears to move up from the vernal equinox o the summer solstice, the Earth actually moves from the iutumnal equinox down to the winter solstice. The days tow lengthen in the northern hemisphere, and shorten in the outhern. The Sun is now over the north pole, where it is nid-day, and opposite the south pole, where it is mid-night. Why is the north pole denominated the elevated pole? Why is the south pole denomiated the depressed pole? Why are there six months day and six month night, alternately, the poles? What is always the relative position of the Sun and Earth in the ecliptic? live an example. When do the days lengthen in the northern hemisphere, and shorten in ne southern? When is it mid-day at the north pole, and mid-night at the south? As the Sun descends from the summer solstice towards the autumnal equinox, the Earth ascends from the winter solstice towards the vernal equinox. The summer days in the northern hemisphere having waxed shorter and shorter, now become again of equal length in both hemispheres. While the Sun appears to move from the autumnal equinox down to the winter solstice, the Earth passes up from the vernal equinox to the summer solstice; the south pole comes into the light, the winter days continually shorten in the northern hemisphere, and the summer days as regularly increase in length in the southern hemisphere. While the Sun appears again to ascend from its winter solstice to the vernal equinox, the Earth descends from the summer solstice to the autumnal equinox. The summer days now shorten in the southern hemisphere, and the winter days lengthen in the northern hemisphere. When the Sun passes the vernal equinox, it rises to the arctic or elevated pole, and sets to the antarctic pole. When the Sun arrives at the summer solstice, it is noon at the north pole, and midnight at the south pole. When the Sun passes the autumnal equinox, it sets to the north pole, and rises to the south pole. When the Sun arrives at the winter solstice, it is midnight at the north pole, and noon at the south pole; and when the Sun comes again to the vernal equinox, it closes the day at the south pole, and lights up the morning at the north pole. There would, therefore, be 186 days during which the Son would not set at the north pole, and an equal time during which he would not rise at the south pole; and 1781 days in which he would not set at the south pole, nor rise at the north pole. At the arctic circle, 23° 27' from the pole, the longest day is 24 hours, and goes on increasing as you approach the pole. In latitude 67° 18′ it is 30 days; in lat. 69° 30′ it is 60 days, &c. (See Table XII.) The same takes place between the antarctic circle and the south pole, with the exception, that the day in the same latitude south is a little shorter, since the Sun is not so long south of the equator, as at the north of it. In this estimate no account is taken of the refraction of the atmosphere, which, as we shall When do the summer days in the northern hemisphere grow shorter and shorter? When do they become of equal length in both hemispheres? When do the winter days shorten in the northern hemisphere, and the summer days lengthen in the southern? When do the summer days shorten in the southern hemisphere, and the winter days lengthen in the northern? When does the sun rise to the north pole, and set to the south? When is it noon at the north pole, and mid-night at the south pole? When does the Sun set at the north pole, and rise to the south? When is it mid-night at the north pole, and noon at the south? What is the length of the day at the north pole? What at the south pole? At the arctic circle? Between the antarctic circle and the pole? see hereafter, increases the length of the day, by making the Sun appear more elevated above the horizon than it really is. THE SEASONS-UNEQUAL LENGTHS OF DAYS AND NIGHTS. The above cut represents the inclination of the Earth's axis to its orbit in every one of the twelve signs of the ecliptic, and consequently for each month in the year. The Sun enters the sign Aries, or the vernal equinox, on the 20th of March, when the Earth's axis inclines neither towards the Sun, nor from it, but sideways to it; so that the Sun then shines equally upon the Earth from pole to pole, and the days and nights are every where equal. This is the beginning of the astronomical year; it is also the beginning of day at the north pole, which is just coming into light, and the end of day at the south pole, which is just going into darkness. By the Earth's orbitual progress, the Sun appears to enter the second sign, Taurus, on the 20th of April, when the north pole, N, has sensibly advanced into the light, while the south pole, S, has been declining from it; whereby the days become longer than the nights in the Northern Hemisphere, and shorter in the Southern. On the 21st of May, the Sun appears to enter the sign Gemini, when the north pole, N, has advanced considerably further into the light, while the south pole, S, has proportionally declined from it; the summer days are now waxing longer in the Northern Hemisphere, and the nights shorter. The 21st of June, when the Sun enters the sign Cancer, is the first day of summer, in the astronomical year, and the longest day in the Northern Hemisphere. The north pole now has its greatest inclination to the Sun, the light of which, as is shown by the boundary of light and darkness, in the figure, extends to the utmost verge of the Arctic Circle; the whole of which is included in the enlightened hemisphere of the Earth, and enjoys, at this season. constant day during the complete revolution of the Earth on its axis. The whole of the Northern Frigid Zone is now in the circle of perpetual illu mination. On the 23d of July, the Sun enters the sign Leo, and as the line of the Earth's axis always continues parallel to itself, the boundary of light and darkness begins to approach nearer to the poles, and the length of the day, in the Northern Hemisphere, which had arrived at its maximum, begins gradually to decrease. On the 23d of August, the Sun enters the sign Virgo, increasing the appearances mentioned in Leo. On the 23d of September, the Sun enters Libra, the first of the autumnal signs, when the Earth's axis, having the same inclination as it had in the opposite sign, Aries, is turned neither from the Sun, nor towards it, but oblique ly to it, so that the Sun again now shines equally upon the whole of the Earth's surface from pole to pole. The days and nights are once more of equal length throughout the world. On the 23 of October, the Sun enters the sign Scorpio; the days visibly decrease in length in the Northern Hemisphere, and increase in the South ern. On the 22d of November, the Sun enters the sign Sagittarius, the last of the autumnal signs, at which time the boundary of light and darkness is at a considerable distance from the north pole, while the south pole has proportionally advanced to the light; the length of the day continues to increase in the Southern Hemisphere, and to decrease in the Northern. On the 21st of December, which is the period of the winter solstice, the Sun enters the sign Capricorn. At this time, the north pole of the Earth's axis is turned from the Sun, into perpetual darkness; while the south pole, in its turn, is brought into the light of the Sun, whereby the whole Antarctic region comes into the circle of perpetual illumination. It is now that the Southern Hemisphere enjoys all those advantages with which the Northern Hemisphere was favoured on the 21st of June; while the Northern Hemis phere, in its turn, undergoes the dreariness of winter, with short days and long nights. HARVEST MOON-HORIZONTAL MOON. The daily progress of the Moon in her orbit, from west to east, causes her to rise, at a mean rate, 48 minutes and 44 seconds later every day than on the preceding. But in places of considerable latitude, a remarkable deviation from this rule takes place, especially about the time of harvest, when the full Moon rises to us for several nights together, only from 18 to 25 minutes later in one day, than on that immediately preceding. From the benefit which her light affords, in lengthening out the day, when the husbandmen are gathering in the fruits of the Earth, the full moon, under these circumstances, has acquired the name of Harvest Moon. It is believed that this fact was observed by persons engaged in agriculture, at a much earlier period than that in which it was noticed by astronomers The former ascribed it to the goodness of the Deity; not doubting but that he had so ordered it for their advantage. About the equator, the Moon rises throughout the year with nearly the equal intervals of 483 minutes; and there the harvest moon is unknown. What is the mean difference of time in the daily rising of the Moon? Under what circumstances is there a material deviation from this rule? Whence the name of Harvest Moon? By whom was this phenomenon first observed, and to what did they attribute it? Why is the Harvest Moon unknown at the equator? At the polar circles, the autumnal full Moon, from her first to her third quarter, rises as the Sun sets; and at the poles, where the Sun is absent during one half of the year, the winter full Moons, from the first to the third quarter, shine constantly without setting. By this, it is not meant that the Moon continues full from her first to her third quarter; but that she never sets to the North Polar regions, when, at this season of the year, she is within 90° of that point in her orbit where she is at her full. In other words: as the Sun illumines the south pole during one half of its yearly revolution, so the Moon, being opposite to the Sun at her full, must illumine the opposite pole, during half of her revolution about the Earth. The phenomenon of the harvest Moon may be thus exemplified by means of the globe: Rectify the globe to the latitude of the place, put a patch or piece of wafer in the ecliptic, on the point Aries, and mark every 120 preceding and following that point, to the number of ten or twelve marks on each side of it; bring the equinoctial point marked by the wafer to the eastern edge of the horizon, and set the index to 12; turn the globe westward till the other marks successively come to the horizon, and observe the hours passed over by the index; the intervals of time between the marks coming to the horizon, will show the diurnal difference of time between the Moon's rising. If these marks be brought to the western edge of the horizon in the same manner, it will show the diurnal difference between the Moon's setting. From this problem it will also appear, that, when there is the least difference between the times of the Moon's rising, there will be the greatest difference between the tines of her setting, and the contrary. The reason why you mark every 120 is, that the Moon gains 12° 11' on the apparent course of the Sun every day, and these marks serve to denote the place of the Moon from day to day. It is true, this process sup poses that the Moon revolves in the plane of the ecliptic, which is not the case; yet her orbit so nearly coincides with the ecliptic, (differing only 509 from it,) that they may, for the convenience of illustration, be considered as coinciding; that is, we may take the ecliptic for the representative of the Moon's orbit. The different lengths of the lunar night, at different latitudes, is owing to the different angles made by the horizon and different parts of the Moon's orbit; or in other words, by the Moon's orbit lying sometimes more oblique to the horizon than at others. In the latitude of London, for example, as much of the ecliptic rises about Pisces and Aries in two hours as the Moon goes through in six days; therefore while the Moon is in these signs, she differs but two hours in rising for six days together; that is, one day with another, she rises about 20 minutes later every day than on the preceding. The parts or signs of the ecliptic which rise with the smallest angles, set with the greatest; and those which rise with the greatest, set with the least. And whenever this angle is least, a greater portion of the ecliptic rises in equal times than when the angle is larger. Therefore, when the How is it at the polar circles, and the poles? What is meant by the full Moon's shining from the first to the third quarter? How may the phenomenon be exemplified by means of the artificial globe? Why do you mark every 12° of the ecliptic in this problem? What does this process of illustration suppose, which is not true, and why is it adopted? To what is the different lengths of the lunar night, in differen latitudes, owing? Give an example. How do those parts of the ecliptic set, which rise with the smallest angles, and the contrary? |