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E6 - - - The Nautical Almanac - - - 10/03/2004

Object.

The object of the Nautical Almanac is to provide, in a convenient form, the data required for the practice of astronomical navigation at sea.

Principle Features

The main contents of the Almanac consist of data from which the Greenwich Hour Angle ( G.H.A.) and the Declination (Dec.) of all the bodies used for navigation can be obtained for any instant of Greenwich Mean Time (G.M.T.). For the Sun, Moon, and navigation planets (Venus, Mars, Jupiter, and Saturn), the G.H.A. and Dec. are tabulated directly for each hour of G.M.T. throughout the year.

For the stars, the Sidereal Hour Angle , (S.H.A.) for each star is given, and the G.H.A. of the star is obtained from the Greenwich Hour Angle of Aries as follows:


    ( G.H.A. Star ) = ( G.H.A. Aries ) + ( S.H.A. Star )

The S.H.A. and Dec. of the stars change slowly and may be regarded as constant over periods of several days. The G.H.A. of Aries, or the Greenwich Hour Angle of the first point of Aries (the Vernal Equinox), is tabulated for each hour of the year.

The Local Hour Angle (L.H.A.) of the observer is identical to the difference in longitude ( DLO or t ) and can be obtained from the longitude of the observer by means of the formula:


    t = L.H.A. =( GHA of sidereal object) - (West longitude of observer)
or
    t = L.H.A. =( GHA of sidereal object) + (East longitude of observer) 

Permanent tables give the appropriate increments and corrections to the tabulated hourly values of G.H.A. and Dec. for the minutes and seconds of G.M.T.

dip

Dip is normally found by looking it up in the tables, but it can also be calculated as follows:
 Correction for dip =
   (-1'.76) times the square root of (height of eye in meters)
 or
 Correction for dip =
   (-0'.97) times the square root of (height of eye in feet)

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