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the pole until that part of the ecliptic can be made to coincide with the given amplitude, as marked on the wooden circle of the globe; then the degrees of elevation of the pole, or the degree of declination in the zenith, will show the latitude.
The sun, on any day, sets so many degrees north or south of the west, as he rises north or south of the east.
1. In what latitude does the sun rise 25° north of the east, and set 25° north of west, on the 21st of June?
Here finding the first point of Cancer to be the sun's place, I elevate or depress the pole until that point of the ecliptic can be made to coincide with 25°, the given amplitude, and find that this can occur only in lat. 23° north or 23° south.
2. In how many different latitudes does the sun rise duly east and set duly west, on the 21st of March, and on the 23d of September?
3. What was the variation of the magnet, and in what direction from the true north, at a place where, on either of these two days, he appeared to be rising 24° to the south of the east point of my compass, and to set 241° to the north of its western point?
It will help the pupil to answer this question, if he consider whether the east point of the compass was too far north or too far south.
4. In what latitude north, and in what latitude south, does the sun rise north-east and set north-west on the 21st of June.
5. On what points of the compass does the sun of the 21st December rise and set in the latitudes last found?
6. In what latitude north on the 21st of June, did the sun rise with amplitude 80° north of east, having been a very little time below my horizon; and to the inhabitants of what latitude south, does the sun, on that day, rise with the same amplitude to give a very short day, having been very many of the twenty-four hours below their horizon?
7. In what latitudes (north and south) has the sun 220 of amplitude on the 5th of May or on the 7th of August, on both of which days he has 16° of north declination;
ANNUAL DISTRIBUTION OF LIGHT AND HEAT. 113
and what will be the difference of effect of his declination in these latitudes ?
8. On the 3d of February, in what latitude north does the sun set 20° south of the true west; and whether does the north point of the magnet vary eastward or westward in a certain longitude on that parallel of latitude, if the sun, on that day, is found to be setting 30° south of the west of the compass?
ANNUAL DISTRIBUTION OF LIGHT AND HEAT.
The distribution of the light and heat* of the sun over the surface of our earth, is unequal and variable from several
B First. Its globular form, as its surface, in the higher latitudes, curves off and retires, receiving the sunbeams obliquely.
A board twelve inches square will entirely intercept the rays of light and heat which, at a corresponding distance from a lamp, exactly spread themselves over the more extensive half-surface of a twelve inch globe. The pupil can readily conceive over which portions of the globe-surface these rays are cast most thinly.
Secondly. The nature of the surface whether water or land; and the situation, whether at a greater or less height above the level of the ocean. To such circumstances may be added, the particular configuration and geographical relations of places; as their aspect to the north or south; their being sheltered or exposed; the composition and nature of the soil, particularly its colour and state of aggregation, on which depends its power* of absorbing and of radiating heat and light, and of retaining or of parting with humidity, &c.; the proximity, or absence of seas; the predominancy of certain winds; the frequency of clouds, fogs, &c."-Prout's Bridgewater Treatise, p. 216.
Thirdly. The obliquity of the earth's axis of rotation to the plane in which it performs its annual revolution.
*See articles Light and Heat (Index).
The pupil is requested to observe, particularly, the latitude in which the vertical ray falls, as indicating the declination caused by the relative position of the axis.
We hope the explanation given on pages 106-8, have fully prepared the pupil for a just idea of the circumstances illustrated above. He will recollect that the axis (our button peg) maintains its parallelism during its 366 rotations, (otherwise our equinoctial plane would be a varying one); and he will now perceive that the consequence to our globular earth, is an ever-varying distribution of the sun's influence as it respects the latitudes of its surface: the sun (represented above by the taper) becoming, by this wondrous ordination of the Creator, a star of ever-changing declination; and, in the course of the year, or period of complete revolution, shedding its vertical rays in a range of latitude from 23° 28′ north, to 23° 28' south of the equator. Hence the seasonal changes of summer, autumn, winter, and spring.
Our purpose will be answered by referring to the five positions particularly noted above: we begin with the position on the right, representing the extreme circumstances
21st OF JUNE, (our SUMMER SOLSTICE.)
* Here it may be seen that the northern extremity of the axis, or arctic pole, is bowed towards the sun to the exact extent of its inclination to the plane of the orbit. The
ANNUAL DISTRIBUTION OF LIGHT AND HEAT. 115
taper flame, or sun, is therefore vertical in latitude 23° 28' north; and affording, notwithstanding the earth's daily rotation, continuous light to the whole segment within the arctic circle; i. e., to the whole of the north frigid zone, leaving a corresponding segment (the south frigid zone) continuously unenlightened.
Referring to Problem VI. of Section I., it will readily be understood that the equatorial inhabitants, as those of Quito or the north of Celebes, have an invariable period of twelve hours' light; but that of all the places which, at any instant, are rising into the sun's beams along the western edge of the terminator, those most southern will first arrive at its eastern edge to lose his light; whilst those most northern will enjoy it longest. And thus, since these are the extreme circumstances of our earth's position in sunlight, we have a representation of the longest day of the northern hemisphere, and the shortest day of the southern
AUGUST, (SECOND WEEK OF).
G We pass towards the left to view the condition of the earth in the early portion of August; and we remark that, in consequence of the curvature of its path, the unvaried direction of its axis is now, so to speak, considerably on one side of the sun; and the latitude now receiving his vertical beams about 15°. Hence his rays reach over the north pole to that extent only, and a considerable portion of the arctic or frigid zone is spending a part of the diurnal rotation of twenty-four hours out of their direct influence.
The equator still retaining its period of twelve hours, the summer days of the northern hemisphere are now considerably shortened, whilst the winter ones of the southern hemisphere are lengthening.
SEPTEMBER 23, (our AUTUMNAL EQUINOX).
H The earth has now attained a position with regard to the sun, at right angles* with that which it had at the solstices.
* As may be seen in the cut, at the solstices there is a common plane passing through the axis, the polar point in the heavens, and the sun. This plane is represented on the celestial globe by the solstitial colure.
Its poles now are neither inclined towards the sun nor away from it: hence, its equator is receiving his vertical rays, and the terminator or boundary of his light, passing through the poles, coincides with the whole meridian of any place rising or setting, and divides every parallel of latitude into two equal parts. The sun now appears in the heavens without declination, coinciding, like y of Virgo, or the star in the belt of Orion, with the equinoctial, and gives the name to that line, because when he is thus circumstanced, the nights and days of every latitude are of equal duration.
NOVEMBER, (FIRST WEEK OF).
K The circumstances of the southern hemisphere are now precisely those of the northern hemisphere in the position marked out for August. The north pole is inclined 15° from the sun; and that portion of the arctic zone which is not farther than 15° from the pole, or has above 75° of latitude, cannot rise into sun-) n-light; a similar portion of the antarctic zone having it continuously. Hence, too, the sun is vertical in latitude 15° south; and appears in corresponding south declination. The inhabitants of the northern hemisphere have a shortness of day depending on their distance from the equator, whilst those of corresponding latitudes in the southern hemisphere, have corresponding shortness of night.
21st DECEMBER, (our WINTER solstice).
The position of the earth on the day of the winter solstice, may be explained by referring to that of the 21st of June. It will be seen that the circumstances of the two hemispheres are now completely reversed. The arctic zone, the whole of which, on that occasion, had continuous light, is now entirely enveloped in darkness; whilst to the whole of the antarctic zone, the sun is constantly above the horizon. He is shedding his vertical rays in the latitude answering to the declination of the southern tropic, and the inhabitants of the southern hemisphere have now attained their longest day; the inhabitants of corresponding northern latitude having their night of corresponding length, and their day the shortest.