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in the southern hemisphere, and the winter days lengthen in the northern hemisphere.
622. 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 Sun would not set at the north pole, and an equal time during which he would not rise at the south pole; and 178 days in which he would not set at the south pole, nor rise at the north pole.
623. 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. 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 see hereafter, increases the length of the day, by making the Sun appear more elevated above the horizon than it really is. All these apparent motions of the Sun are due to the inclination of the Earth's axis (or the obliquity of the ecliptic), and her revolution around the Sun.
The following cut represents the inclination of the Earth's axis to its orbit in every one of the twelve signs of the zodiac, and consequently for each month in the year. It is such a view as a beholder would have, situated in the north pole of the ecliptic, at some distance from it, and consequently, is a perpendicular view, the north pole of the Earth being towards us. The Sun enters the sign Aries, or the vernal equinox, on the 20th of March, when the Earth enters Libra, and when her axis inclines neither towards the Sun, nor from it, but stands exactly sideways to it; so that the Sun then shines equally upon the Earth from pole to pole, and the days and nights are everywhere 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 has sensibly advanced into the light, while the south pole 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
622. How are the light and darkness of the poles affected by the Sun's apparent motion? 623. What said of the length of the days within the arctic circle? In latitude 67° 18'? In latitude 69° 80'? How at the other pole? To what are these various apparent motions of the Sun really due?
has advanced considerably further into the light, while the south pole 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 revoiution of the Earth on its
axis. The whole of the Northern Frigid Zone is now in the circle of perpetual illumi
PHILOSOPHY OF THE SEASONS.*
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 obliquely 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 23d of October, the Sun enters the sign Scorpio; the days visibly decrease in length in the northern hemisphere, and increase in the southern.
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 into 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 favored on the 21st of June; while the northern hemisphere, in its turn, undergoes the dreariness of winter, with short days and long nights.
*This diagram and the accompanying explanations should be carefully studied till they are thoroughly understood by the learner. The cause of the seasons and of the unequal lengths of the days and nights, is a matter of which no professedly educated person ought to be ignorant, or to entertain confused and indefinite notions. By all means let this point be studied till the student can tell the cause of every particular phenomenon of the seasons and the length of the days, without any particular interrogation.
624. By carefully observing the figure, it may be seen that the orbit of the Earth is slightly elliptical, that the Sun is to the left of the center, and that consequently, the Earth is nearer the Sun on the 21st of December, than on the opposite side of the ecliptic, on the 21st of June. This may seem strange to the learner, that we should have our winter when nearest the Sun, and our summer when most distant; but it must be remembered that the temperature of any particular part of the Earth is not so much affected by the distance of the Sun, as by the directness or obliquity of his rays. Hence, though we are farther from the Sun on the 21st of June than on the 21st of December, yet, as the north pole of the Earth is turned more directly into the light at that time, so that the Sun's rays strike her surface less obliquely than in December, we have a higher temperature at that period, though at a greater distance from the Sun.
625. The difference, however, between the aphelion and perihelion distances of the Earth is so slight, in comparison with the whole distance, as scarcely to cause a perceptible difference in the amount of light received at her respective positions. The eccentricity of the Earth's orbit, or the distance of the Sun from its center, is only about 1,618,000 miles, so that the variation is only 3,236,000 miles, or about one-thirtieth of the mean distance. The true orbit of the Earth could not be distinguished from a circle.
The only effect of the eccentricity of the Earth's orbit upon her temperature is, that she has probably a greater degree of heat, during summer in the southern hemisphere, when the Earth is at her perihelion, than we ever have at the north in the same latitude. But this difference must be very slight, if indeed it is at all perceptible.
THE HARVEST MOON AND HORIZONTAL MOON.
626. 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
624. What said of the form of the Earth's orbit? Why is it not then the warmest in the United States? Earth's variation in distance from the Sun ? What effect upon the light and heat of the Earth? 626. Subject of this chapter? Mean rate of the Moon's daily delay in rising?
When are we nearest the Sun? 625. What is the amount of the
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.
627. About the equator, the Moon rises throughout the year with nearly the equal intervals of 48 minutes; and there the harvest Moon is unknown. 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 12° 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 times of her setting, and the contrary.
The reason why you mark every 12° 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 supposes 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 5° 9' 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.
628. 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
What remarkable deviation? What is the Moon then called, and why? How anciently was this phenomenon observed? To what attributed? 627. Is the Harvest Moon known at the equator? How at the Polar circles? At the poles? Does she there exhibit her usual phases? Can you illustrate the phenomenon of the Harvest Moon by a globe? 628. To what is the different lengths of the lunar nights attributable?
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.
629. 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 Moon is in those signs which rise or set with the smallest angles, she rises or sets with the least difference of time; but when she is in those signs which rise or set with the greatest angles, she rises or sets with the greatest difference of time.
Let the globe, for example, be rectified to the latitude of New York, 40° 42′ 40′′, with Cancer on the meridian, and Libra rising in the east. In this position, the ecliptic has a high elevation, making an angle with the horizon of 72°.
But let the globe be turned half round on its axis, till Capricorn comes to the meridian, and Aries rises in the east, then the ecliptic will have a low elevation above the horizon, making an angle with it of only 25%. This angle is 47° less than the former angle, and is equal to the distance between the tropics.
630. In northern latitudes, the smallest angle made by the ecliptic and horizon is when Aries rises; at which time Libra sets; the greatest is, when Libra rises and Aries sets. The ecliptic rises fastest about Aries, and slowest about Libra. Though Pisces and Aries make an angle of only 25° with the horizon when they rise, to those who live in the latitude of New York, yet the same signs, when they set, make an angle of 72°. The daily difference of the Moon's rising, when in these signs, is, in New England, about 22 minutes; but when she is in the opposite signs, Virgo and Libra, the daily difference of her rising is almost four times as great, being about one hour and a quarter
631. As the Moon can never be full but when she is opposite to the Sun, and the Sun is never in Virgo or Libra except in our autumnal months, September and October, it is evident that the Moon is never full in the opposite signs, Pisces and Aries, except in those two months. We can, therefore, have only two full Moons in a year, which rise, for a week together, very near the time of sunset. The former of these is called the Harvest Moon, and the latter, the Hunter's Moon.
629. What said of the angles under which the signs rise and set? What result follows as to time of the Moon's rising and setting? How illustrate by globe? 630. When is the angle smallest in northern latitudes? When greatest? What difference of angle at the rising and setting of Pisces? Daily difference of the Moon's rising? When in Pisces and Aries? What when in Virgo and Libra? 631. Why have we not more than one Harvest, and one Hunter's Moon in a year?