PARALLAX. 39 CHAPTER III. Parallax. "The broad circumference Hung on his shoulders like the moon, whose orb Or in Valdarno, to descry new lands, Rivers or mountains, in her spotty globe."— Milton. We have now shown that our earth is revolving around the sun, which is the grand central luminary, and that within its orbit are the orbits of Venus and Mercury, while exterior are the orbits of Mars, Jupiter, and Saturn. We have learned to look upon these bodies as orbs, or balls like our own earth, and suppose them to revolve like our earth upon an axis. We now desire to know something of their distance from us, and the actual velocity with which both we and they are moving. The diameter of our earth we have assumed at 8000 miles, or equal lengths, we can, from knowing this, ascertain the distance of the moon from the earth, and of the earth from the sun. Every one is familiar with the fact, that every change of position of a spectator, causes an apparent change of place in the object viewed. Thus, if while in a certain position, we observe a particular house to be in the range, or same line with a distant tree, then upon changing our position, the house will no longer be in a line with the tree, but will appear to have moved in the contrary direction. This apparent change of place of the object, due to a real change of place in the observer, is called parallax, and by its means, we can determine the distances of the heavenly bodies, Thus, supposing spectators on opposite portions of the earth's surface, as at A and B, to view the moon or a planet, at c, the observer at A, will see the object c, apparently at a, while the observer at B will perceive it at the same time at b. Here is an apparent change of place, viz from a to b, due to a real change in the position of ъ the spectator. This change, enables us to ascertain the distance of the object with much precision, for supposing A and B joined by a line, we have a triangle A B C, in which one side A B, is known, and all three angles - for the observers at A and B determine with some graduated instruments, the inclinations of the lines A c and B c to the line A B. We can illustrate the method by which the distance of an object is ascertained by means of graduated instruments thus: Suppose a spectator at B, to observe by means of á graduated MEASUREMENT OF DISTANCES. 41 circle, the number of degrees subtended by a distant object, as a church, at A C, and let this angle be two degrees; we have here a triangle A B C, and knowing its angles, and any one side, we can determine the other sides. Suppose we know the side B C, or the distance of the Church, to be 1 mile, we can ascertain the height A C thus: Twice B C, or 2 miles, will be the diameter of a circle whose centre is the eye of the spectator, and whose radius, the distance of the Church. Three times this (nearly), or 6 miles, will be the whole circumference, and six miles divided by 360 will give the length of one degree, and twice this, since the angle A B C is 2 degrees, will give Allowing 5000 feet to the mile, 6 miles would be 30,000 feet, and this divided by 360, gives 83 of one degree, consequently 2 degrees are 166 height required. Now in any triangle whatever, we can determine the length of all its sides, provided the length of one side is given and also the angles. We do not mean to be understood that this is the actual process employed by astronomers to determine the distance of the moon, and other heavenly bodies, but simply introduce it as an explanation of the principle. the height A C. feet for the length feet, which is the By means of parallax, the distance from the moon to the earth has been ascertained to be 60 semi-diameters of the latter, and the distance of the earth from the sun has been determined to be 95,000,000 of miles. When we reflect upon this vast distance, the absurdity of that system which denied to the earth a revolution on its axis, once in 24 hours, is strikingly apparent. We could not conceive of the amazing velocity with which the sun must move, at the immense distance which it is situated from the earth, if it was obliged to travel once around in 24 hours. It would require a rate of about 24,000,000 miles per hour, or 400,000 miles in one minute, and 6,666 miles each tick of the clock. Such velocity is absolutely incredible, and this would be to save our little globe from turning on its axis at the rate of 1000 miles an hour, or about 17 miles in one minute. When the distance of any of the heavenly. bodies becomes known, its actual diameter in miles can be easily ascertained. It is no more difficult to obtain the diameter of the moon, when her distance from the earth is known, than to determine the height of a church steeple when we know how far it is from the observer. We here represent the moon and a part of B its orbit, the earth being supposed to be at A. The distance A B or A C, is 240,000 miles, and the angle B AC, which is observed with a graduated circle, is about 30 minutes, or half a degree. Proceeding as in the case of the Church, twice A C is 480,000 miles, and three times this is 1,444,000 miles which is the circumference of a circle whose centre is the centre of the earth, and whose radius, or half diameter, is the distance of the moon. This circumference divided by 360, gives 4000 miles for the length of one degree, and half this is 2000 miles the length of half a degree, which is the diameter of the moon. The actual diameter of the moon is 2140 miles, for the angle B A C is nearly 31 minutes, or a little over half a degree. In precisely the same manner the diameter of the sun is ascertained to be 880,000 miles. Hence we learn, that if a spectator at the sun, should look towards the earth, it would appear only the one hundredth the diameter which the sun appears to us, or not larger than a very small star, How absurd then is the idea that the sun revolves around the earth.-We now have a just conception of the solar system, and have learned to look upon the sun as the central body, around which the planets revolve in order, our earth being one of the smallest. Far beyond it, other magnificent orbs are moving silently in the depths of space, peopled with myriads of intelligent beings. Very far beyond the boundary of our own system, we believe there are others more beautiful, and IMMENSITY OF CREATION. 43 that every star which adorns the heavens, and upon which we turn such unheeding eyes, is a sun, giving light, and warmth, and happiness to its own attendant planets. Nay, more than this, we believe that all those countless myriads of stars which the telescope reveals, twinkling from distances so far, that if blotted from existence, their light would continue a thousand years, so long it would take to travel thence to us, are all centres of systems, around which, worlds peopled with intelligences of the highest order, are revolving, and yet, we have obtained but a faint idea of the immensity of Creation. Where is the central throne from which all power emanates? The throne of the Eternal. Imagination fails. Reason shrinks back abashed, but Faith, with more than telescopic eye, pierces to that centre, and sometimes catches a gleam, a faint ray of the brightness of its glory. What wonder that astronomy should be called the noblest science, since it affords scope for the highest order of intellect, and presents truths unequalled for their grandeur and sublimity. Unconsciously we are moving on, life and death is every where around us, but the heavens seem unchangeable, the type of eternity. We are unwilling to believe that the principle within us, whatever it may be called, soul, spirit, or reason, which is thus capable of comprehending sublime truths, perishes, and becomes inanimate, like the dead flowers, and withered leaves. We feel an ardent aspiration after higher and purer knowledge, and cannot doubt that such longings will one day be gratified. These may be called "flights of the imagination," but we would do well to remember, that there are things, which are as far beyond the imagination to conceive, and which are more strange than this, yet of whose reality we cannot doubt. Such is the progression of light, and of electricity. The eye cannot follow them, nor the imagination, as they rush on, with a speed of 200,000 miles in one second! And, quicker than this is the transmission of that mysterious influence, called gravitation, which acts with allcontrolling force, through distances, utterly inconceivable to the human mind, causing the immense masses of the planetary orbs to rise and fall like bubbles on the ocean wave. Shall we then call all these flights of the imagination, or mere fancy, and with |
