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d-Value

Numbers given in celestial navigation tables that are used for interpolation.

In the Nautical Almanac, the d-value for declination (Workform, Box 2) is the number of arc minutes the declination changes each hour. We must determine its sign () by inspection. This varies with the sun from 0 (near solstices) to 1' per hour (near equinoxes). For the stars this d is effectively 0, as they do not change. For the moon it can be rather large, up to 13' per hour or so, and it changes rather quickly with time. Declination of Venus can change up to 1.3' per hour; the other planets have very low d-values, less than 0.3' or so.

In sight reduction tables, the d-value for the calculated height (Hc, Workform, Box 5) is the number of minutes that Hc changes if declination changes by declination increases by 1; its sign is given. This use and definition of "d-value" is totally different from the one above.

See declination, calculated height


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