semi-diameter

The Oxford Companion to Ships and the Sea | 2006 | 514 words | Copyright

semi-diameter, one of the altitude corrections to the sextant reading of the sun or moon. It compensates for the fact that the navigator will observe the altitude of either the lower or upper limb of either body, whereas the altitude required is that of their centre. This is necessary because these two bodies are close to the earth whereas the stars, which require no such correction, are at an infinite distance. The values of semi-diameter are given in nautical almanacs and most nautical tables.

Mike Richey

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"semi-diameter." The Oxford Companion to Ships and the Sea. 2006. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>.

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