Amplitude of the Sun
An amplitude of the Sun or other celestial body can be used to
determine gyro error. An amplitude (A) is the arc of the horizon
between the prime vertical circle (the vertical circle through the east end
west points of the horizon) and the observed body. The prime vertical
circle may be true or magnetic depending upon which east or west
points are involved. If the body is observed when its center is on the
celestial horizon, the amplitude can be taken directly from table 27 of
Bowditch, Volume II.
The celestial horizon differs from the one you see (the visible horizon)
because it runs through the center of 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 the center of the Moon is on the celestial
horizon, its upper limb (upper edge) is on the visible horizon.
Figure 9-6 shows the relationship of the visible horizon to the celestial
horizon. When planets and stars are on the celestial horizon, they are a
little more than one Sun diameter above the visible horizon.
Figure 9-6. The visible and celestial horizons.