The celestial horizon differs from the one you see (the visible horizon) because it runs through the center of the earth. There are a lot of computations that must be done to determine the celestial horizon of a body, but for now we will just say that it is the horizon that a navigator uses for all celestial computations. When the center of the sun is on the celestial horizon, its lower limb (lower edge) is about two-thirds of the diameter of the sun above the visible horizon. When planets and stars are on the celestial horizon, they are a little more than one sun diameter above the visible horizon. If the amplitude were taken on the visible horizon, the compass bearing would require a correction from Table 28 in Bowditch.
AMPLITUDE ON THE CELESTIAL HORIZON
As discussed above, the amplitude of a body can be taken directly from table 27 of Bowditch, volume II, if the body is observed when its center is on the celestial horizon. Since the sun is most commonly used for amplitudes, it will be the topic of our discussion. First of all, to observe the sun when it is on the celestial horizon, its lower limb should be about two-thirds of the diameter above the visible horizon, note the time and your compass bearing as observed by a bearing or azimuth circle to the sun. Next, with the Greenwich Mean Time (GMT) of your observation, you can use the right-hand daily pages of the Nautical Almanac to determine the sun's declination. From this known information, we can use table 27 of Bowditch to determine the amplitude. Using Problem # 1 as example, it will help explain the procedures:
