Magnetic amplitude
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the angular distance of a heavenly body, when on the horizon, from the magnetic east or west point as indicated by the compass. The difference between the magnetic and the true or astronomical amplitude (see 3 above) is the "variation of the compass." | |
etc. See under Amplitude, Attraction, etc. |
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.