Now it will at once be obvious that there must be a strict law connecting the position of the sun (or a star) with its place of rising or setting. Stars at the same distance from either of the celestial poles will rise or set at the same point of the horizon, and if a star does not change its place in the heavens it will always rise or set in the same place. Here it will be convenient to introduce one or two technical terms. Every celestial body, whether we deal with the sun, moon, planet, or star, occupies at any moment a certain place in the sky, partly, though not wholly, defined by what we term its declination, _i.e._, its distance from the celestial equator. This declination is one of the two co-ordinates which are essential for enabling us to state accurately the position of any body on the celestial vault; and we must quite understand that if all these bodies rise and set, and rise and set visibly, the place of their rising or setting must be very closely connected with their declination. Bodies with the same declination will rise at the same points of the horizon. When the declination changes, of course the body will rise and set in different points of the horizon. Next we define points on the horizon by dividing the whole circumference into four quadrants of 90° each = 360°, so that we can have _azimuths_ of 90° from the north or south points to the east and west points. Azimuths are not always reckoned in this way, navigators preferring one method, while astronomers prefer another. Thus azimuth may also be taken as the distance measured in degrees from the south point in a direction passing through the west, north, and east points. On this system, a point can have an azimuth varying from 0° to 360°. [Illustration: SHOWING AMPLITUDES RECKONED FROM THE EAST OR WEST POINTS TO N.P., NORTH POINT OF HORIZON, AND S.P., SOUTH POINT OF HORIZON.] It is next important to define the term _amplitude_. The amplitude of a body on the horizon is its distance north and south of the east and west points; it is always measured to the nearest of these two latter points, so that its greatest value can never exceed 90°. For instance, the south point itself would have an amplitude of 90° south of west (generally written W. 90° S.), or 90° south of east (E. 90° S.), while a point 2° to the westward of south would have an amplitude of W. 88° S., and not E. 92° S. We can say then that a star of a certain declination will rise or set at such an _azimuth_, if we reckon from the N. point of the horizon, or at such an amplitude if we reckon from the _equator_. This will apply to both north and south declinations. The following table gives for Thebes the amplitudes of rising or setting (north or south) of celestial bodies having declinations from 0° to 64°; bodies with higher declinations than 64° never set at Thebes if they are north, or never rise if they are south, as the latitude (and therefore the elevation of the pole) there is nearly 26°.