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degree of Aries ; least, when her descending node is in that degree.
The full Moon, in summer, always runs low, but much lower in some years than in others. The running low seldom attracts particular notice, except when she is in that part of her orbit which is south of the ecliptic. The summer full Moon runs the lowest, when her latitude south is greatest ; or she is 90° from her nodes. When the Moon is north of the ecliptic, she runs high at the winter fulls; highest, when she is farthest north, or 90° from her nodes. The full Moons of summer not only run low, but continue a short time above the horizon. But this is at the time when the length of the days makes moonlight of little utility. On the contrary, the full Moons of winter not only run high, but continue long above the horizon. This is at the season when the light of the Moon is peculiarly useful, guiding and cheering the lonely traveller in the dreary and protracted nights.
At the poles, the full Moon, being below the horizon, is not seen for nearly half the year. This is in summer, when, were the Moon to shine, her light, immerged in the continued splendor of the Sun, would be of
In winter, when, at the benighted polar regions, the light of the Moon is most beneficial, she shines from about the first to the third quarter.
What claim can he have to rational being, who does not admire and adore the wisdom and benevolence of Him, who not only "gave the Sun for a light by day,” but " the Moon and Stars for a light by night!"
The years, when the harvest Moon is least, and when most beneficial, may be seen in the following table. L. stands over the columns least beneficial; M. over those most beneficial. In both the columns marked N., the harvest Moon is farthest north in the orbit ; in those marked S., it is farthest south.
HARVEST MOON OF THE NINETEENTH CENTURY.
Years least beneficial.
1806 1807 1808 1809 1810 1811 1812 1813 1814 1815
Years most beneficial.
1801 1802 1803 1804 1805
How far does the Moon move in a solar day? How much does the motion of the Moon exceed the apparent motion of the Sun? At the equator, what is the difference in the time when the Moon rises on each succeeding night? In high latitudes, what have farmers observed respecting the rising of the autumnal full Moon? What is there remarkable in the Moon's rising at the polar circles ? From what arise the phenomena of the harvest Moon? When is the harvest Moon? How would you illustrate the phenomena of the harvest Moon? What would give a very natural representation of this peculiarity in the Moon's rising? When there is the least difference in the time of the Moon's rising, how is her setting? As the Moon passes through the same signs in every revolution, why are not the phenomena of the harvest Moon observed at other seasons besides autumn? Why is not the statement of Mr. Ferguson respecting the Moon's rising at the polar circles, strictly true ? When the Moon is not full exactly at the equinox, what fulls are
taken for the harvest Moon? When have the inhabitants of south latitude the phenomena of the harvest Moon ? Does the inclination of the Moon's orbit to the ecliptic affect the phenomena of the harvest Moon? Is the motion of the Moon's nodes direct or retrograde? How long are they in performing a revolution ? During what part of the time of their revolution is the harvest Moon most beneficial, and what part least beneficial ? When does the full Moon run low? In what part of her orbit does it run lowest? How are the full Moons of winter? When do they run highest? Is the regulation of the high and low fulls calculated for our benefit? How long does , the Moon shine in winter in the polar regions ? Do the wisdom and benevolence of the Deity appear in the regulations of the Moon ?
The tides are the alternate ebbing and flowing of the sea. They
They are imperceptible in the midst of the ocean, and can only be known by the rising of the water on the adjacent land, or where tħe depth of water will admit of sounding. 1
Kepler was the first who discovered the true cause of the tides, and that the attraction of the Sun and Moon produced the constant flux and reflux of the water. But, a “ hint being given, the immortal Sir Isaac Newton improved it, and wrote so amply on the subject, as to make the theory of the tides in a manner quite his own, by discovering the cause of their rising on the side of the Earth opposite to the Moon. For Kepler believed that the presence of the Moon occasioned an impulse, which caused another in her absence."
The attraction of the Moon is the principal cause of the tides; but the attraction of the Sun operates to in
crease or diminish the height or depression of the water occasioned by the lunar attraction. But, were every part of the Earth equally attracted by the heavenly bodies, (no tide could be produced. The unequal attraction, or the attraction of one part of the terraqueous globe more forcibly than the other, may be considered as the true cause of the tides. The force of attraction in any body decreases, as the squares of the distances from that body increase. Hence the farther distant any body is from the centre of attraction, the less the operation on that body. The water, therefore, on the side of the Earth next to the Moon, is more forcibly attracted than the body of the Earth, and the body of the Earth than the water on the opposite side. Suppose_three particles of matter, one on the surface of the Earth next to the Moon, one at the centre of the Earth, and one on the surface opposite to the Moon. By the laws of gravitation, the particle nearest to the Moon would be more attracted by her, than that at the centre, and that at the centre more attracted than the particle on the opposite side. By the unequal attractions, the distances between these particles would be increased. One would be elevated from the centre, and the centre particle would be drawn from that on the side opposite to the Moon, amounting to the same thing as if the opposite particle were elevated. For, when the distance between the centre of the Earth and a particle at the surface is increased, the particle will appear raised from the surface.
raised from the surface. We take notice of a tide, because the water rises on the adjacent land. This will be the case when the distance between the surface of the water and the centre of the Earth is increased, whether the water be elevated from the Earth, or the Earth be withdrawn from the water. difficulty, therefore, arises in accounting for the tide on the side of the Earth opposite the Moon, than for that on the surface nearest to her, both being the effect of unequal attraction.
The points directly under and opposite to the Moon,
may be considered as the centres of highest elevation; and 90° from these, or half the distance between them, as the circle of low water. This extends wholly round the Earth, and moves as the Moon moves.
Let NEO E be the Earth, (Plate vi. Fig. 3,) C the centre; M the Moon; N the point on the Earth's surface next to the Moon; O a point on the opposite surface; E E the circle of low water. The attraction of the Moon, M, being unequal at the different parts, from the circle E E, the water on the side next to the Moon is more attracted than the centre, C, where the solid body of the Earth may be considered as concentrated; and the centre, C, more attracted than the water on the opposite side. The water, therefore, will rise at N, increasing the distance from the centre, C. On the same principle, C, the centre, and with it the mass of the Earth, will be attracted from the surface at O, enlarging the distance, and leaving the water, which, being farther from the centre, is higher to the observer ; high and low, in respect to the Earth, always relating to the distance from the centre.
Some have accounted for the tide on the side of the Earth opposite to the Moon by the motion of the Earth and Moon round a common centre. In the revolution of these bodies, the side of the Earth farthest distant from the Moon must have a swifter motion than the side nearest to her. The water, endeavoring to escape, must rise towards the highest part, the point opposite to the Moon. Some effect may be attributed to this ; but, without doubt, the unequal attraction is the principal cause.
The tides, travelling as the Moon travels, have her declination and the declination opposite. As the Moon revolves round the Earth, they revolve, following her in her perpetual motion. Below the polar circles, therefore, every place, in its diurnal rotation, must have two tides in about 24 h. 50 m. 28 s.
When the Moon is in the equator, the circle of low