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learned nations in Europe, the Sun's mean horizontal parallax was settled, as the result of their united observations, at 0° 0' 8.5776. Now the value of radius, expressed likewise in seconds, is 206264".8; and this divided by 8".5776, gives 24047 for the distance of the Sun from the the Earth, in semi-diameters of the latter. If we take the equatorial semi-diameter of the Earth, as sanctioned by the same tribunal, at (7924+2=) 3962 Liles, we shall have 24047 x 3962-95,278,869 miles for the Sun's true distance.
A TABLE OF THE SUN'S PARALLAX IN ALTITUDE.
Sun's Horizontal Parallax.
8.47 8.11 8.21 8.31
6.44 6.51 6.59 6.66
662. The change in the apparent position of the fixed stars, caused by the change of the Earth's place in her revolution around the Sun, is called their annual parallax. So immense is their distance, that the semi-annual variation of 190,000,000 of miles in the Earth's distance, from all those stars that lie in the plane of her orbit, makes no perceptible difference in their apparent magnitude or brightness.
The following cut will illustrate our meaning:
Let A represent a fixed star in the plane of the Earth's orbit, B. At C, the Earth is 190,000,000 of miles nearer the star than it will be at D six months afterward; and yet this semi-annual variation of 190,000,000 miles in the distance of the star is so small a fraction of the whole distance to it, as neither to increase or diminish its apparent brightness.
663. It is only those stars that are situated near the axis of the Earth's orbit whose parallax can be measured at all, on
662. What meant by Earth's annual parallax? Effect of variation of Earth's dis tance on the fixed stars? Diagram. 668. What stars have perceptible parallax?
account of its almost imperceptible quantity. So distant are they, that the variation of 190,000,000 miles in the Earth's place causes an apparent change of less than 1' in the nearest and most favorably situated fixed star.
Let A represent the Earth on the 1st of January, and B a star observed at that time. Of course, its apparent place in the more distant heavens will be at C. But in six months the Earth will be at D, and the star B will appear to be at E. The angle A B D or C B E will constitute the parallactic angle. In the cut, this angle amounts to about 48°, whereas the real parallax of the stars is less thanth of one degree, or 390th part of this amount. Lines approaching each other thus slowly would appear parallel; and the Earth's orbit, if filled with a globe of fire, and viewed from the fixed stars, would appear but a point of light 1' in diameter ! For a splendid diagram illustrative of the annual parallax of the stars, see Map I., of the Atlas.
ABERRATION OF LIGHT.
664. In the year 1725, Mr. Molyneux and Dr. Bradley fixed up a very accurate and costly instrument, in order to discover whether the fixed stars had any sensible parallax, while the Earth moved from one extremity of its orbit to the other; or which is the same, to determine whether the nearest fixed stars are situated at such an immense distance from the Earth, that any star which is seen this night, directly north of us, will, six months hence, when we shall have gone 190,000,000 of miles to the eastward of the place we are now in, be then seen exactly north of us still, without changing its position so much as the thickness of a spider's web.
665. These observations were subsequently repeated, with but little intermission, for twenty years, by the most acute observers in Europe, and with telescopes varying from 12 feet to 36 feet in length. In the mean time, Dr. Bradley had the honor of announcing to the world the very nice discovery made while endeavoring to ascertain the parallax of the fixed stars, that the motion of light, combined with the progressive motion of the Earth in its orbit, causes the heavenly bodies to be seen in a different position from what they would be, if the eye were at rest. Thus was established the principle of the Aberration of Light.
666. This principle, or law, now that it is ascertained, seems
Amount? Diagram, and explanation. 664. What experiment by Molyneux and Bradley? With what results? 665. What further observations for the same purpose? What discovery made while investigating the subject of parallax? What is the aberra tion of light? 666. What remarks upon the principle or law of observation? How is
not only very plain, but self-evident. For if light be progressive, the position of the telescope, in order to receive the ray, must be different from what it would have been if light had been instantaneous, or if the Earth stood still. Hence the place to which the telescope is directed will be different from the true place of the object.
The quantity of this aberration is determined by a simple proposition. The Earth describes 59' 8" of her orbit in a day =3548", and a ray of light comes from the Sun to us in 8′ 13′′ =493" : now 24 hours or 86400": 493: : 3548: 22′′; which is the change in the star's place, arising from the cause abovementioned.
PRACTICAL ASTRONOMY-REFLECTION AND REFRACTION OF LIGHT.
667. Practical Astronomy has respect to the means employed for the acquisition of astronomical knowledge. It includes the properties of light, the structure and use of instruments, and the processes of mathematical calculation.
In the present treatise, nothing further will be attempted than a mere introduction to practical astronomy. In a work designed for popular use, mathematical demonstrations would be out of place. Still, every student in astronomy should know how telescopes are made, upon what laws they depend for their power, and how they are used. It is for this purpose mainly that we add the following chapters on practical astronomy.
PROPERTIES OF LIGHT.
668. Light is that invisible ethereal substance by which we are apprised of the existence, forms, and colors of material objects, through the medium of the visual organs. To this subtile fluid we are especially indebted for our knowledge of those distant worlds that are the principal subjects of astronomical inquiry.
669. The term light is used in two different senses. It may signify either light itself, or the degree of light by which we are enabled to see objects distinctly. In this last sense, we put light
the quantity of aberration determined? 667. Subject of Chapter XVIII.? What is practical astronomy? How far discussed in this treatise? 668. Define light. For what indebted to it? 669. Different senses in which the term is used? What is
in opposition to darkness. But it should be borne in mind, that darkness is merely the absence of that degree of light which is necessary to human vision; and when it is dark to us, it may be light to many of the lower animals. Indeed, there is more or less light, even in the darkest night, and in the deepest dungeon.
"Those unfortunate individuals," says Dr. Dick, "who have been confined in the darkest dungeons, have declared, that though, on their first entrance, no object could be perceived, perhaps for a day or two, yet, in the course of time, as the pupils of their eyes expanded, they could readily perceive rats, mice, and other animals that infested their cells, and likewise the walls of their apartments; which shows that, even in such situations, light is present, and produces a certain degree of influence."
670. Of the nature of the substance we call light, two theories have been advanced. The first is, that the whole sphere of the universe is filled with a subtile fluid, which receives from luminous bodies an agitation; so that, by its continued vibra tory motion, we are enabled to perceive luminous bodies. This was the opinion of Descartes, Euler, Huygens, and Franklin.
The second theory is, that light consists of particles thrown off from luminous bodies, and actually proceeding through space. This is the doctrine of Newton, and of the British philosophers generally.
Without attempting to decide, in this place, upon the relative merits of these two hypotheses, we shall use those terms, for convenience sake, that indicate the actual passage of light from one body to another.
671. Light proceeds from luminous bodies in straight lines, and in all directions. It will not wind its way through a crooked passage, like sound; neither is it confined to a part of the circumference around it.
As the Sun may be seen from every point in the solar system, and far hence into space in every direction, even till he appears but a faint and glimmering star, it is evident that he fills every part of this vast space with his beams. And the same might be said of every star in the firmament.
672. As vision depends not upon the existence of light merely, but requires a certain degree of light to emanate from the object, and to enter the pupil of the eye, it is obvious that if we can, by any means, concentrate the light, so that more may enter the eye, it will improve our perception of visible objects, and even enable us to see objects otherwise wholly invisible.
Some animals have the power of adapting their eyes to the existing degree of light. The cat, horse, &c., can see day or night; while the owl, that sees well in the night, sees poorly in the day-time.
673. Light may be turned out of its course either by reflection
darkness? Can it be dark and light at the same time? Is there any place without light? Quotation from Dr. Dick? 670. What theories of the nature of light, and by whom supported respectively? Remark of author? 671. How light proceeds from luminous bodies? Radiations from Sun and stars? 672. How improve vision, and why? Animals? 673. How is light turned out of its course?
or refraction. It is reflected when it falls upon the highly polished surface of metals and other intransparent substances; and refracted when it passes through transparent substances of different densities, as already illustrated in Chapter XVI.
REFRACTION BY GLASS LENSES.
674. A lens is a piece of glass, or other transparent substance, of such a form as to collect or disperse the rays of light that are passed through it, by refracting them out of a direct course. They are of different forms, and have different powers.
674. What is a lens? Draw and describe different kinds. 675. Refracting power of double-convex lens? Focal distance? Diagram, and illustrate. 676. How focal distance governed? Diagram, and illustrate. 677. What is the focal distance of s