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Amplitude of the Sun, Continued
Labeling the
Amplitude
The amplitude of a body is given the prefix E (east) if the body is rising
and the prefix W (west) if the body is setting. Additionally, the
amplitude of a body is given the suffix N (north) if the body has
northerly declination and the suffix S (south) if it has southerly
declination.
Finding
Amplitude of
the Sun Using
the Celestial
Horizion
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.
Gathering Information: To observe the Sun when it is on the celestial
horizon, its lower limb must be about two-thirds of the diameter above
the visible horizon. You must know the Greenwich mean time (GMT)
of your observation to determine the Sun’s declination from the right-
hand daily pages of the Nautical Almanac, your DR Lat. at the time of
observation, and the true bearing of the Sun as observed using a
telescopic alidade.
Example
Problem
The DR latitude of your ship is 51°04.6'N. The declination of the
setting Sun was 19°00.4'N. Your true bearing (as observed by a
telescopic alidade) to the Sun was 300°.
From this known information, we can use table 27 of Bowditch to
determine the amplitude.
Figure 9-7 shows an excerpt from table 27. By inspection of figure 9-7,
you can see that you must enter the left-hand column with your ship’s
DR latitude. You can also see that the Sun’s declination is listed across
the top of the table. Since latitude 51° and declination 19° are closest to
our entering values, we determine that the amplitude of the Sun when it
is on the celestial horizon is 31.2°. Now that we have the amplitude,
what do we do with it? First of all, there are some basic rules that must
be applied that relate to our previous discussion of the assigned prefix
and suffix of an amplitude. Our amplitude was taken when the Sun was
setting, and its declination name is north. Using the rules for labeling
the amplitude, we label the amplitude as follows: W 31.2° N. We use
W because the Sun is setting and N because the Sun’s declination is N.
9-13
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