9. _Altitude and Zenith Distance._--The _altitude_ of a heavenly body is its distance above the horizon, and its _zenith distance_ is its distance from the zenith. Both the altitude and the zenith distance of a body are measured on the vertical circle which passes through the body. The altitude and zenith distance of a heavenly body are complements of each other. 10. _Azimuth and Amplitude.--Azimuth_ is distance measured east or west from the meridian. When a heavenly body lies north of the prime vertical, its azimuth is measured from the meridian on the north; and, when it lies south of the prime vertical, its azimuth is measured from the meridian on the south. The azimuth of a body can, therefore, never exceed 90°. The azimuth of a body is the angle which the plane of the vertical circle passing through it makes with that of the meridian. The _amplitude_ of a body is its distance measured north or south from the prime vertical. The amplitude and azimuth of a body are complements of each other. 11. _Alt-azimuth Instrument._--An instrument for measuring the altitude and azimuth of a heavenly body is called an _alt-azimuth_ instrument. One form of this instrument is shown in Fig. 16. It consists essentially of a telescope mounted on a vertical circle, and capable of turning on a horizontal axis, which, in turn, is mounted on the vertical axis of a horizontal circle. Both the horizontal and the vertical circles are graduated, and the horizontal circle is placed exactly parallel with the plane of the horizon. When the instrument is properly adjusted, the axis of the telescope will describe a vertical circle when the telescope is turned on the horizontal axis, no matter to what part of the heavens it has been pointed. The horizontal and vertical axes carry each a pointer. These pointers move over the graduated circles, and mark how far each axis turns. To find the _azimuth_ of a star, the instrument is turned on its vertical axis till its vertical circle is brought into the plane of the meridian, and the reading of the horizontal circle noted. The telescope is then directed to the star by turning it on both its vertical and horizontal axes. The reading of the horizontal circle is again noted. The difference between these two readings of the horizontal circle will be the azimuth of the star. [Illustration: Fig. 16.] To find the _altitude_ of a star, the reading of the vertical circle is first ascertained when the telescope is pointed horizontally, and again when the telescope is pointed at the star. The difference between these two readings of the vertical circle will be the altitude of the star.