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632. Although there can be but two full Moons in the year that rise with so little variation of time, yet the phenomenon of the Moon's rising for a week together so nearly at the same time, occurs every month, in some part of her course or the other.
In Winter, the signs Pisces and Aries rise about noon; hence the rising of the Moon is not then regarded nor perceived.
In Spring, these signs rise with the Sun, because he is then in them; and as the Moon changes while passing through the same sign with the Sun, it must then be the change, and hence invisible.
In Summer, they rise about midnight, when the Moon, is in her third quarter. On account of her rising so late, and giving but little light, her rising passes unobserved.
633. To the inhabitants at the equator, the north and south poles appear in the horizon, and therefore the ecliptic makes the same angle southward with the horizon when Aries rises, as it does northward when Libra rises; consequently the Moon rises and sets not only with angles nearly equal, but at equal intervals of time, all the year round; hence, there is no harvest Moon at the equator. The farther any place is from the equator, if it be not beyond the polar circles, the angle which the ecliptic makes with the horizon gradually diminishes when Pisces and Aries rise.
634. Although, in northern latitudes, the autumnal full Moons are in Pisces and Aries; yet in southern latitudes it is just the reverse, because the seasons are so :-for Virgo and Libra rise at as small angles with the horizon in southern latitudes as Pisces and Aries do in the northern; and therefore the harvest Moons are just as regular on one side of the equator as on the other.
At the polar circles, the full Moon neither rises in summer, nor sets in winter. For the winter full Moon being as high in the ecliptic as the summer Sun, she must continue while passing through the northern signs, above the horizon; and the summer full Moon, being as low in the ecliptic as the winter Sun, can no more rise, when passing through the southern signs, than he does.
635. The great apparent magnitude of the Moon, and indeed of the Sun, at rising and setting, is a phenomenon which has greatly embarrassed almost all who have endeavored to account for it. According to the ordinary laws of vision, they should appear to be least when nearest the horizon, being then farthest from the eye; and yet the reverse of this is found to be true. The apparent diameter of the Moon, when viewed in the horizon by the naked eye, is two or three times larger than when at the altitude of thirty or forty degrees; and yet when measured by an instrument her diameter is not sensibly increased.
632. Does not the Moon rise with little variation for several nights in succession, every month? Why not always perceived? 633. Why is there no Harvest Moon at the equator? 634. What said of these lunar phenomena in the Southern hemisphere? 635. What said of the apparent diameter of the Moon in the horizon? How when
Both the Sun and the Moon really subtend a greater angle when on the meridian, than they do in the horizon; because they are then actually nearer the place of the spectator by the whole semi-diameter of the Earth; and one reason why they appear largest in the horizon is, that they are then compared with terrestrial objects, with whose magnitude we are acquainted.
This apparent increase of magnitude in the horizontal Moon, is chiefly an optical illugion, produced by the concavity of the heavens appearing to the eye to be a less portion of a spherical surface than a hemisphere. The eye is accustomed to estimate the distance between any two objects in the heavens by the quantity of sky that appears to lie between them; as upon the Earth we estimate it by the quantity of ground that lies between them. Now when the Sun or Moon is just emerging above the eastern horizon, or sinking beneath the western, the distance of the intervening landscape over which they are seen, contributes, together with the refraction of the atmosphere, to exaggerate our estimate of their real magnitudes.
636. Both the Sun and Moon are sometimes seen to be elongated horizontally, when near the horizon. This is often the case when the atmosphere is very dense. The cause of this phenomenon is this: All celestial bodies in the horizon are more or less elevated by atmospherical refraction (See page 300); and the amount of this apparent elevation depends somewhat upon the density of the atmosphere as well as upon the altitude of the object. When, therefore, the Sun or Moon are near the horizon, and viewed through a dense atmosphere, the refraction is greatest; and as their lower limb is seen through a denser stratum of atmosphere than their upper limb, its apparent elevation is greater, and the object seems to be flattened, while its horizontal diameter is not sensibly diminished.
This phenomenon and its cause may be easily illustrated by a diagram.
REFRACTION AND TWILIGHT.
637. THE rays of light, in passing out of one medium into another of a greater density, deviate from a straight course, and are bent towards a perpendicular to that course; and if the density of the latter medium continually increase, the rays of
measured? When do they subtend the greater angle? Why appear largest when in the horizon? What other explanation given? 636. What is meant by a Horizontal Moon? The cause of this phenomenon? 637. What is meant by the refraction of light? What principles govern it?
light in passing through it, will deviate more and more from a right line as they pass downwards, or towards the eye of the observer.
LIGHT REFRACTED BY WATER.
638. As air and water are both transparent, but of different densities, it follows that, when light passes obliquely from one to the other, it will be refracted. If it pass from the air into the water, it will be refracted towards a perpendicu lar.
Here the ray A C strikes the water perpendicularly, and passes directly through to B without being refracted. But the ray D C strikes the water at C obliquely; and instead of passing straight through to E, is refracted at C,
and reaches the bottom of the water at F. If, therefore, a person were to receive the ray into the eye at F, and to judge of the place of the object from which the light emanates from the direction of the ray C F, he would conclude that he saw the object at G, unless he made allowance for the refraction of the light at C.
639. When light passes obliquely from a denser to a rarer medium, as from water into air, it is refracted from a perpendicular towards a horizontal.
Here the lamp A shines up through water into air. The ray that strikes the surface perpendicularly passes on to B without being refracted; but the other rays
LIGHT PROCEEDING FROM WATER.
that leave the water obliquely are refracted toward a horizontal direction, in proportion to their distance from the perpendicular; or, in other words, in proportion to the obliquity of their contact with the surface of the water.
640. In consequence of the refraction of light towards a horizontal direction, in passing from water into air, a pole, half of which is in the water, seems bent at the surface, and the lower end seems nearer the surface than it really is. For the same reason, the bottom of a river seems higher, if seen obliquely, than it really is; and the water is always deeper than we judge it to be.
683. How refracted by air and water? 639. How when light passes from denser to rarer media? 640. Effect of refraction upon objects seen under water?
In this cut, the oar, the blade of which is in the water, seems bent at the surface of the water. The rays of light passing from the part under water to the surface at D, are refracted toward a horizontal direction at that point, and received into the eye of the observer at B, who, judging of the position of the immersed portion of the oar from the direction of the rays D B, locates the blade of the oar at C; thus reversing the effect illustrated at 638.
641. The refracting power of different transparent substances depends mainly upon their density. Water refracts more than air, glass more than water, and diamond most of all. But the angle of incidence, or the obliquity of the contact of the rays with the denser substance, has also much to do in determining the amount of refraction.
641. Upon what does the refracting power of different transparent media depend? 642. What other effect of refraction?
643. What discovery by refraction?
the light are more refrangible than others, so that the light is analyzed, or separated into its component parts or elements.
Let a ray of light from the Sun be admitted through a hole in the window shutter, A, into a room from which all other light is excluded; it will form on a screen placed a little distance in front, a circular image, B, of white light. Now interpose near the shutter a glass prism, C, and the light, in passing through it, will not only be refracted in the same direction, both when it enters the prism and when it leaves it, but the several rays of which white light is composed will be separated, and will arrange in regular order on the screen, immediately above the image B, which will disappear. The violet ray, it will be seen, is most refracted, and the red least; the whole forming on the scale an elongated image of the Sun, called the solar spectrum.-Johnston.
644. It is the refraction of the clouds that gives the sky its beautiful colors morning and evening; and the refracting power of the rain-drops produces the beautiful phenomenon of the rainbow.
645. The refracting power of the atmosphere produces many curious phenomena. Sometimes ships are seen bottom upwards in the air, single or double. At other times, objects really below the horizon, as ships or islands, seem to rise up, and to come distinctly in view.
646. A very important effect of refraction, as it relates to astronomy, is, that it more or less affects the apparent places of all the heavenly bodies. As the light coming from them strikes the atmosphere obliquely, and passes downward through it, it is refracted or bent towward the Earth, or toward a perpendicular. And as we judge of the position of the object by the direction of the ray when it enters the eye, we place objects higher in the heavens than they really are.
Let A, in the cut, represent the Earth; B, the atmosphere; C C, the visible horizon; and the exterior circle the apparent concave of the heavens. Now, as the light passes from the stars, and strikes the atmosphere, it is seen to curve downward, because it strikes the atmosphere C obliquely; and the air increases in density as we
approach the Earth. But
as the amount of refraction depends not only upon the density, but also upon the obliquity of the contact, it is seen that the refraction is greatest at the horizon, and gradually diminishes till the object reaches the zenith, when there is no obliquity, and the refrac
644. What other effects of refraction? 645. Atmospherical refraction? Effects an terrestrial objects? C46. Upon apparent places of stars, &c.?