Popular Astronomy: A Series of Lectures Delivered at Ipswich. George Biddell Airy

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      Fig. 9.

      acquainted with the celestial globe, knows that the brass meridian ought to be perpendicular to the horizon; for securing that condition in the curve described by the transit instrument, the two points AB must be exactly level. In the next place, the brass meridian of a common globe is not what is called a small circle, but it divides the globe into two equal parts. For that purpose it is necessary that ​the telescope CD should be square to its axis AB. The astronomer ascertains whether it be truly square or not, by looking at a distant mark, first with the pivots A,B, of the instrument resting on the piers a,b, and then with the axis turned over, so that the pivots A,B, rest on the piers b,a. If the telescope points equally well to the mark in both positions of the axis, the telescope is truly square to its axis. The third condition is, that when the axis is level, and the telescope is square to its axis—on turning the instrument round its axis, the line CDE shall pass through the pole of motion of the celestial sphere. Now, the way of obtaining this condition is as follows:—We take advantage of that admirable Polar Star, which is a blessing to astronomers of the northern hemisphere. The Polar Star, as I have said, turns round like the rest, although in a small circle. Let FGHIKL represent the circle in the sky in which the Polar Star turns round in the order FGHIKLF. Suppose that in turning the transit instrument round its axis AB, the line CDE prolonged will trace on the sky the line GI or FK, as the case may be. The Polar Star in its revolution passes that line twice. Now, what we want is, that that line CDE, carried on to the sky, should be so directed, that in the motion of the telescope it should pass exactly through the centre of the circle which the Polar Star describes, and therefore that it should divide into two equal halves the circle which the Polar Star describes. We ascertain it in this manner. We can measure the description of the parts of the circle of the Polar Star by time. One of the most important parts of the apparatus by which that astronomical observation is made, is a clock. The clock should go well, and should beat loudly and distinctly. The astronomer ​observes the Polar Star when it passes the transit instrument at its upper passage, as at K, and also when it passes its lower passage, as at F. If these are twelve hours apart, we know that the transit instrument is in its proper position. For as the star describes the whole circle, in twenty-four hours, if the times of passing at F and K are twelve hours apart, there must be exactly half the circle between them, and therefore the line FK must pass through the centre of the circle, or through the Pole of the heavens. But supposing the transit instrument were a little out of position, so that the line described by CDE prolonged would be GI, then the star would require more than twelve hours to pass from its visible upper passage at I, through KLF, to its visible lower passage at G, and fewer than twelve hours to pass from its visible lower passage at G, through H, to its visible upper passage at I. In this manner we are enabled to adjust this transit instrument to its position with the utmost accuracy.

      Having explained the manner in which the transit instrument is placed accurately in its proper position, I will now explain its use, I will assume you are looking to the south. The observer stations himself at his transit instrument, not looking at all parts of the sky, but waiting to observe the stars as they pass the meridian. The clock is going all the time. A star is seen to be approaching the meridian: the observer directs the telescope so as to observe the star when it actually crosses the meridian, and then looks into the telescope. In the telescope he sees the wires, and sees the image of the star travelling along, and he observes the passage of the star over every wire. Just before the star begins to pass, he looks to the clock face for the hours and minutes, ​and he then listens to the clock, which beats seconds—in that manner he gets the hour, the minute, the second, and the fraction of a second, at which that bright star passes every wire, and by taking the mean or average of these, he finds the time at which the star passes the meridian. He looks again, and he sees a planet coming into the field of view. He directs his telescope to that planet, and in like manner he gets the time by the clock at which that planet passes the meridian—the hour, the minute, the second, and the fraction of a second. He sees another star. The telescope is moved to the proper position he notes the time in the same manner, and he finds the hour, the minute, the second, and the fraction of a second, as before. Another star comes in the same way. Such is the duty which a transit-observer has to perform—the watching of objects which are passing the meridian in endless succession. He has this instrument, which is confined in its motions to the meridian, and which admits of no other motion; and the clock, by which he notes the hour, the minute, the second, and the fraction of a second; by the use of the various wires, he observes the time at which the object passes each wire; and by taking the mean of all, he finds very accurately the time at which the object passes the meridian: such are the duties of the transit-observer.

      The next thing is to ascertain the elevation of the object when it passes the meridian. Now before we enter upon the use of the Mural Circle, I must offer a word or two upon Geometry. I dare say everybody here, like myself, has in his time, studied books containing measures—so many barleycorns make an inch, so many inches make a foot, so many feet make a yard, etc., as well as so many yards make a mile, ​and so many miles make a degree. But the publication in a book of measures of such an expression as "69 miles make a degree" is in the highest degree reprehensible, as giving false ideas on one of the most important expressions in science. No schoolmaster ought to introduce books into his school, teaching that 69 miles make a degree. What do we mean by a degree? The use of the word degree is to define inclination, and it ought to be looked upon as defining a measure of inclination only, and not as defining a measure of length. If I had to describe the position of two arms of a pair of compasses, I should say they were inclined; but the notion of their inclination is entirely different from the notion of a measure of length. But we want some means for describing how much these two arms are inclined. Now the method of describing how much these two arms are inclined, is got at in this way: we use the word degree for a certain small inclination, such that if we first give one arm an inclination of one degree to the other, then incline it one degree further, then one degree in addition, and so on to 360 degrees, the arm will have gone through the whole circle of inclination, and will have returned back again to its first position. But these degrees, as you will perceive, have nothing to do with lineal measures; they СКАЧАТЬ