One of the very first facts noticed in the very early days of astronomy was that, as the stars seemed to move across the sky night by night, they seemed to move in one solid piece, as if they were lamps rigidly fixed in one and the same solid vault. Of course it has long been perfectly understood that this apparent movement was not in the least due to any motion of the stars, but simply to the rotation of the earth on its axis. This rotation is the smoothest, most constant, and regular movement of which we know. It follows, therefore, that the interval of time between the passage of one star across the meridian of Greenwich and that of any other given star is always the same. This interval of time is simply the difference of their right ascension. If we are able, then, to turn our transit instrument to the sun, and to a number of stars, each in its proper turn, and by pressing the tapping-piece on the instrument as the sun or star comes up to each of the ten wires in succession, to record the times of its transit on the chronograph, we shall have practically determined their right ascensions--one of the two elements of their places. The other element, that of declination, is found in a different manner. The celestial equator, like the terrestrial, is 90° from the pole. The bright star Polaris is not exactly at the north pole, but describes a small circle round it. Twice in the twenty-four hours it transits across the meridian--once when going from east to west it passes above the pole, once when going from west to east below the pole. The mean between these two altitudes of Polaris above the horizon gives the position of the true pole. [Illustration: THE TRANSIT CIRCLE.]