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Question:

In the scheme of USCG license questions, they have Compass Error by Moon Amplitude questions. OR as the CFR states “Amplitude by “ANY BODY"

However, as you probably are aware, Moon amplitudes due to parallax and declination reasons, is not practical from a professional mariner stand point, with the moon moving so fast, we just don’t use Amplitudes by the Moon.

However we have to deal with the reality of USCG questions, that my business partner and I have been experincing, that NMC is posting these questions to exam recipients.

So when you read all through Bowditch and a few other resources, what is NOT clear and seams confusing is the wording in Bowditch on WHEN you take the bearing of the moon for an amplitude.

Bowditch Vol II, Page 514 of 1975 version says: “When the center of the Moon is on the celestial horizon, its UPPER LIMB is on the visible horizon.”  In the same paragraph it explains other positions of the bodies, so you can figure out when to use Table 27 and or both Table 27 and 28.

My questions is I just don’t seam to get the above statement and it what perspective this statement is in. Are they saying that the UPPER LIMB of the Moon is at the visible horizon, even though you actuallly are viewing the Moon at the celestrial horizon with your Azmimuth while you are standing on the bridge wing taking an amplitude?  If the center of the moon is on the celestial horizon, who can the UPPER LIMB be on the visible horizon? I understand maybe parallax is in play here? 

I’m a bit stumped on this one, can you help?