Fig. 121.—Diagram illustrating how the pole is found.
Let the circle, H, Z, R, Fig. [121], represent a great circle of the heavens, the meridian in fact, and let the centre of this circle represent the centre of the transit instrument. Now what we want is, not only to be able to measure degrees of arc along this circle, but to determine some starting-point for those degrees. One arrangement is to observe the reflection of the wires in the eyepiece of the transit circle, from the surface of mercury in a vessel which is placed below the telescope, turned with its object-glass downwards; the vessel containing the mercury is out of sight, between the two piers, but in Fig. [118] are seen the two parallel bars, with weights at the ends, carrying it, by which it may be brought into any position for the purpose referred to, so that the light from the wires in the eyepiece may pass through the tube and be reflected back by the mercury (the surface of which is of course perfectly horizontal), up through the tube again to the eyepiece. When the telescope is absolutely in the vertical position the images of the cross wires will be superposed over the cross wires themselves; and then an observation will give the actual reading of the circle when the instrument is pointing at 180° from the zenith; deduct 180° from this reading, and we get the reading when the instrument is pointing at the zenith—the zero required. This should be 0°, and the quantity by which it differs from 0° must be applied to the observed position of stars, so that the distance of a star from the zenith can be at once determined.
But this is not all. If we assume for the moment that the observer is at the north pole, the pole star will be exactly over head, and therefore, supposing the pole star to absolutely represent the pole of the heavens, all the observer has to do is simply to take a reading of the pole star on the arc of his circle—call it 0° O´ 0˝—and then use it as another zero to reckon polar distance from, seeing that every particular star or body we observe has so many degrees, minutes, seconds, or tenths of seconds, from the pole star.
But we are not at the north pole. Still we are in a position where the pole is well above the horizon, and from that fact we can determine the polar distance, although the absolute place of the pole is not pointed out by the pole star. Thus, if we suppose any star, A, Fig. [121], to be a certain distance from the pole, and the earth carrying the instrument to be in the centre of the circle H, Z, R, we can observe the zenith distance of that star, Z, A, when it transits our meridian above the pole, P; and we can then observe its distance, Z, B, when it transits below the pole; and it is clear that the difference between those two measures will give the distance A, B, or double the polar distance of that star, and the mean of the readings will give the distance, Z, P, the zenith distance of the pole, so that it is perfectly easy to determine the distance between the pole and the zenith, which, subtracted from ninety degrees, gives us the latitude of the place. It is therefore perfectly easy by means of this instrument to determine either the zenith or polar distance, and, knowing the polar distance, we get the declination, or distance from the equator, by subtracting it from ninety degrees.
In our case it is the north polar distance or declination of any object in the heavens that we record; and if we take the precaution to do so with this instrument at the time given by the clock, when the object passes the meridian, we have the actual apparent place of that body in the sky; and in this way all the positions of the stars and other bodies, and their various changes, and the courses of the planets, have been determined.
The transit circle is the most important instrument of astronomy, and such is the perfection of the Greenwich instrument that nothing could be more unfortunate for astronomy than that that instrument should be in any way damaged. And though many of us are admirers of physical astronomy, we have yet to find the instrument that is as important to physical astronomy as the transit circle at Greenwich is to astronomy of position.
The room in which these transit circles are worked—the transit room—is required to be of special construction. A clear space from the southern horizon through the zenith to the north must at any time be available; this entails the cutting of a narrow slit in the roof and both walls, without the intervention of any beams across the room. This slit is closed by shutters or windows which are made to open in sections, so that any part of the meridian can be observed at pleasure.
CHAPTER XVII.
THE TRANSIT CLOCK AND CHRONOGRAPH.
We have now to consider the way in which the transit instrument is used and the functions which both it and the transit circle fulfil.