DIMINISHING OBLIQUITY OF THE ECLIPTIC. 101 NUTATION OF THE EARTH'S AXIS. The attraction of the Moon, on the elevated matter of the Equator, is slightly affecting the parallelism of the Earth's axis. That is, the north pole does not point, precisely, in the same direction in the heavens one year as another, but varies with the variation of the Moon's nodes, and points of apogee, and perigee. The greatest extent of their variation, is less than 19" of a degree, and is periodical in between 18 and 19 years. This may be called the NUTATION OF THE EARTH'S AXIS. DIMINISHING OBLIQUITY OF THE ECLIPTIC. Complicated attractions, are affecting both the equinoxes, and the angle which the Ecliptic makes with the Equator. Consequently, for some thousands of years, the obliquity of the Ecliptic has been diminishing. More than 2000 years ago, it was found by observation to be about 23° 51'. Later observations have proved it diminishing, till it is now a trifle less than 23° 28'. From these premises, the conclusion was once drawn, that the torrid zone would continue to have its dimensions contracted, till the Sun should be vertical the whole year, at the Equator. But La Place asserts, that it admits of mathematical demonstration, that this variation of the obliquity of the Ecliptic is confined within narrow limits, and can never exceed 20 and of a degree. Science thus unintentionally confirms the promise, that summer and winter, seed-time and harvest, shall not cease. What is the NUTATION OF THE EARTH'S AXIS? How much has the obliquity of the Ecliptic diminished in 2000 years? What effect has this on the breadth of the torrid zone? What is the full extent to which this variation can arrive, according to La Place? CHAPTER XXII. TIDES.-METEOROLOGY. If the Earth were at rest, and felt no influence from other material worlds, the laws of gravitation, as discovered by Newton, would keep the waters of the ocean perfectly spherical, as seen in Plate vii. Fig. 1. But daily experience proves that the ocean is agitated with continual ebbing and flowing, which we call TIDES. Or if the Earth had no motion on its axis, and the Moon no revolution round it, the waters of the ocean would become, and remain elevated directly under the Moon; as seen in Plate vii. Figure 2. Consider the Earth as having a diurnal rotation on its axis from west to east, a tide would pass to the west every 24 hours. The Moon, however, is falling back to the east in its orbit so much, that it comes to the meridian about 50 minutes later each day, therefore the tides will be later. The full effect of the Moon on the water, in accumulating it in tide, is not felt instantly; consequently, the tide under the Moon becomes highest some hours after that luminary has past the meridian. This accounts for one tide in 24 hours and 50 minutes. If the Earth felt no influence from other material worlds, and had no motion, what form would the waters of the ocean always exhibit? What other appearance is daily witnessed? What would be the state of the water, if the Earth had no rotation and the Moon no revolution? If one of these motions existed without the other, what would be the result. How much later is the Moon in the meridian each evening? Why does not a TIDE acquire its full elevation when the Moon is in the meridian? When treating of gravitation it was remarked, that its power diminishes as the squares of the distance increase. While the Moon attracts the water nearest to it, more than the solid part of the Earth more distant, it will attract the fluids less on the opposite side, than it does the intermediate solids; and this will cause the water to rise in the nadir, when the Moon is in the zenith. This may be illustrated by Plate vii. Fig. 4.. When the water is elevated two sides, it must be depressed on the sides at right angles. The rotation of the Earth will, consequently, give two elevations and two depressions of the water, in 24 hours and 50 minutes. It may also be remarked, that the motions of the Earth on its axis, round the centre of gravity between that and the Moon and even in its orbit round the Sun, may possibly all contribute to impart most centrifugal force to the water on that side of the primary, which is opposite to the secondary, and thus a tide would be raised. When the Moon is over the Equator, those two tides would be of about equal elevation in any given place, other things being equal. But if the Moon were 281° north of the Equator, northern Latitudes would have the highest tide with the Moon in the meridian, and that corresponding tide would pass over the southern hemisphere 12 hours after. When the Moon is south of the Equator, the reverse of this would be true. The higher the Latitude, the more unequal these two What causes two tides in 24 hours? When the Moon is highest in the northern hemisphere, which tide with us will be the most elevated? Which, when the Moon has greatest southern declination? How often will the tide then rise at the poles? What effects in tides, result from the Moon over the Equator between the polar circles?-at the poles? tides. At the poles no tide would rise when the Moon was over the Equator. At other times one tide would occur in 24 hours and 50 minutes; and that would be highest, when the Moon was most remote from the Equator. Thus far, the influence of the Moon ALONE, has been considered in producing tides. But the Sun raises tides likewise. At the change of the Moon, the influence of both luminaries will be combined, and the tides will rise very high. See Plate vii. Fig. 4. A similar effect results from their influence when in opposition, as seen in Plate vii. Fig. 6. These are called spring tides, and the Moon's place syzygies. When the Moon is in quadrature, as seen in Plate vii. Fig. 5, the influence of the Sun will diminish the tides. Those are called neap tides. The swell of the ocean, is not greatest with the Moon in the meridian, but a few hours afterwards, and the spring tides do not occur precisely at the change and full, but about the third tide after, and the neap tides, as much after the quadratures. The Moon's distance from the Earth, and the distance of both from the Sun, are very variable, and these variations affect the tides. When the Moon is in conjunction, the attraction of the Sun is stronger on that, than on the Earth; when in opposition, this attraction is strongest on the primary. In both cases, it tends to separate them. At the quadratures, the effect of the Sun's influence, is to bring the Earth and Moon nearer together. What other cause but the Moon's influence as affecting the tides? When will the joint influence of the Sun and Moon produce the highest tides? What are these called? Do they occur exactly at the Moon's syzygies? What are NEAP TIDES? When will they occur? In what manner are the varying distances of the Earth from the Sun, and Moon from the Earth, affecting the tides ? |