Page:In the high heavens.djvu/73

 been made, and the results to which astronomers have been thereby conducted.

Greenwich Observatory lies more than 2,000 miles from the North Pole, and yet if the Pole were to shift by as much as the width of Regent Street, the fact that it had done so would be quite perceptible at Greenwich. Let me endeavour to explain how such a measurement could be achieved. In finding the latitude at any locality we desire, of course, to know the distance between the locality and the Equator, expressed in angular magnitude. But though this is distinctly the definition of latitude, it does not at once convey the idea as to how this element can -be ascertained. How, for instance, would an astronomer at Greenwich be able to learn the angular distance of the observatory from the Equator? The Equator is not marked on the sky, and it is obvious that the observer must employ a somewhat indirect process to ascertain what he wants. Here, again, we have to invoke the aid of that celestial Pole to which I have so often referred.

Think of that point on the sky which is the common centre of the circles exhibited on Professor Barnard's photograph. That point is not indeed marked by any special star, but it is completely defined by the circumstance that it is the centre of the track performed by the circumpolar stars. We thus obtain a clear idea of this definite point in the sky, and the horizon is a perfectly definite circle, at all events from any station where the sea is visible. It is not difficult to imagine that by suitable measurements we can ascertain the altitude of this point in the heavens above the horizon. That altitude is the latitude of the place; it is, in fact, the very angle which lies between the locality on the earth and the Equator.