Now there are three things to be done in order to adjust this instrument for observation. In the first place we must see that the line of sight is exactly at right angles to the axis on which the telescope turns, and when we have satisfied ourselves of that, we must, in the second place, take care, not only that the pivots on which the telescope rests are perfectly equal in size, but that the entire axis resting on these pivots is perfectly horizontal. Having made these two adjustments, we shall at all events be able, by swinging the telescope, to sweep through the zenith. Then, thirdly, if we take care that one end of this axis points to the east, and the other to the west, we shall know, not only that our transit instrument sweeps through the zenith, but sweeps through the pole which happens to be above the horizon—in England the north pole, in Australia the south pole. That is to say, by the first adjustment we shall be able to describe a great circle; by the second, this circle will pass through the zenith; and by the third, from the south of the horizon to the north, through the pole. Of course, if the pole star were at the pole, all we should have to do would be to adjust the instrument (having determined the instrument to be otherwise correct) so as simply to point to the pole star, and then we should assure ourselves of the east and west positions of the axis. Some details may here be of interest.
The first adjustment to be made is that the line of sight or collimation shall be at right angles to the axis on which the instrument moves: to find the error and correct it, bring the telescope into a horizontal position and place a small object at a distance away, in such a position that its image is bisected by the central wire of the transit, then lift the instrument from its bearings or Ys, as they are called, and reverse the pivots east for west, and again observe the object. If it is still bisected, the adjustment is correct, but if not, then half the angle between the new direction in which the telescope points and the first one as marked by the object is the collimation error, which may be ascertained by measuring the distance from the object to the central wire, by a micrometer in the field of view, and converting the distance into arc. To correct it, bring the central wire half way up to the object by motion of the wire, and complete the other half by moving the object itself, or by moving the Ys of the instrument. This of course must be again repeated until the adjustment is sensibly correct.
The second adjustment is to make the pivots horizontal. Place a striding level on the pivots and bring the bubble to zero by the set screws of the level, or note the position of it; then reverse the level east for west, and then if the bubble remains at the same place the axis of motion is horizontal, but, if not, raise or lower the movable Y sufficiently to bring the bubble half way to its original position, and complete the motion of the bubble, if necessary, by the level screw until there is no alteration in the position of the bubble on reversing the level.
Fig. 115.—Diagram explaining third adjustment, H, R, plane of the horizon; H, Z, A, P, B, R, meridian; A and B places of circumpolar star at transit above and below pole P.
The third adjustment is to place the pivots east and west. Note by the clock the time of transit of a circumpolar star, when above the pole, over the central wire, and then half a day later when below it, and again when above it; if the times from upper to lower transit, and from lower to upper are equal, then the line of collimation swings so as to bisect the circle of the star round the pole, and therefore it passes through the pole, and further it describes a meridian which passes through the zenith by reason of the second adjustment. This is therefore the meridian of the place, and therefore the pivots are east and west. If the periods between the transits are not equal, the movable pivot must be shifted horizontally, until on repeating the process the periods are equal.
In practice these adjustments can never be made quite perfect, and there are always small errors outstanding, which when known are allowed for, and they are estimated by a long series of observations made in different manners and positions. The error of the first adjustment is called the collimation error, that of the second the level error, and that of the third the deviation error. When the errors of an instrument are known the observations can be easily corrected to what they would have been had the instrument been in perfect adjustment.
Now what does the modern astronomer do with this instrument when he has got it? It is absolutely without circles, but the faithful companion of the Transit Instrument is the Astronomical Clock—and the two together serve the purpose of a circle of the most perfect accuracy, so that by means of these two instruments we shall be able to determine the right ascensions of all the stars merely by noting the time at which the earth’s rotation brings them into the field of view. The clock having been regulated to sidereal time, a term fully explained in the sequel, it will show 0h. 0m. 0s. when the first point of Aries passes the meridian, and instead of dividing the day into two periods of twelve hours each, the clock goes up to twenty-four hours. If now a star is observed to pass the centre of the field of view (that is the meridian) at 1h. by the clock, or one hour after the first point of Aries, it will be known to be in 1h. of right ascension; or if it passes at 12h. it will be 12h. right ascension, or opposite to the first point of Aries, and so on up to the twenty-four hours, the clock keeping exact time with the earth. The transit instrument thus gives us the right ascension of a star, or one co-ordinate: and now we want the other—the declination.