It is quite true that as the Pole is implied by these circles rather than directly marked by them, the measurement of the altitude cannot be effected quite directly. The actual process is to take the Polar Star, or some one of the other circumpolar stars, and to measure the greatest height to which it ascends above the horizon and the lowest altitude to which it declines about twelve hours later. The former of these is as much above the Pole as the latter is below it, so between them we are able to ascertain the altitude of the Pole with a high degree of accuracy.
It is true that in a fixed observatory such as Greenwich there is no visible sea horizon, and even if there were it would not provide so excellent a method as is offered by the equivalent process of first observing the star directly and then observing its reflection in a dish of mercury. In this way the altitude of the star above the horizon is determined with the utmost precision. The practical astronomer will, however, remember that, of course, he has to attend to the effects of atmospheric refraction, which invariably shows a star higher up than it ought to be. This can be allowed for, and in this way the latitude of the observatory is ascertained with all needful accuracy. When the highest degree of precision is sought for, and it is only observations with a very high degree of precision which are available for our present purpose, a considerable number of stars have to be employed, and very many observations have to be taken at different seasons of the year, so as to eliminate as far as possible all sources of casual error. When, however, due attention has been paid to those precautions which the experience of astronomers suggests, the result that is obtained is characterized by extraordinary precision.
