Instruments for measuring angles and distances were very early employed in the field of astronomy as well as in the field of geography. Of these instruments the Egyptian _gnomon_ appears to have been the oldest.[38] In its best form it consisted of a bowl having a perpendicular rod or staff erected at the central point of the inner curved surface. This rod cast a shadow upon the inner surface of the bowl, which had been graduated, giving a reading in degrees which furnished to the observer the information desired. Time brought improvements and variations in the construction of simple instruments of this character. As early as the third century before the Christian era, adjustable rings, or armillae, for example, were employed by astronomers to aid them in the solution of their problems, which instruments later developed, as noted below, into the more elaborate and complex armillary spheres. The simplest form of such an instrument appears to have been but a single graduated circle. To this, at a very early date, a second was added, thus providing an instrument in which one of the circles was regarded as fixed in the plane of the equator, the other, intersecting this at right angles, served as a meridian circle, being movable around an axis which could be called the world axis, the axis of the celestial sphere, or the axis of the universe. The position of a celestial body in declination could be determined on the meridian circle, and its right ascension on the fixed or horizon circle.[39] It seems altogether probable that Eratosthenes made use of such an instrument in his efforts to measure the obliquity of the ecliptic. He tells us that in his time one of large dimensions hung in the portico of the academy of Alexandria.[40] With the addition of other circles, and of an adjustable view-tube, that more accurate and detailed measurements might be made, this device, in Hipparchus’ day, came to be known as an astrolabe, and, after the addition of other rings in later years, to be known as an armillary sphere. Even in this last development it was not a true sphere on which could be represented the starry constellations, but an arrangement of circles forming a sort of imaginary sphere, the circles being intended to represent the relative position of the principal celestial circles. This instrument seems, at first, to have been suspended, when in use, but later was made to rest upon a base, the whole adjusted to revolve around an axis and within a graduated horizon circle. In the earliest examples, the earth at the center of the circles, it represented the Ptolemaic system (Fig. 9); in the later examples, having the sun at the center, it represented the Copernican system. [Illustration: Fig. 9. Armillary Sphere according to Ptolemy.]