Figure 15-5.—The celestial sphere.DeclinationThe GP you see on the small sphere in figure 15-5corresponds to the star’s location on the celestial sphere.The letters GP stand for geographic position andrepresent a point where a line drawn from the center ofthe earth to the body would intersect the earth’s surface.The latitude of a point on the terrestrial sphere ismeasured from the equator northward or southwardalong the point’s meridian to a maximum of 90°.Declination of a body on the celestial sphere is measuredin exactly the same way—from the celestial equator(equinoctial) northward or southward along the body’shour circle.‘The polar distance is the number of degrees,minutes, and tenths of minutes of arc between theheavenly body and the elevated pole. The elevated poleis the one above the horizon; in other words, the one withthe same name as your latitude.From the foregoing description, it follows that thepolar distance of a body whose declination has the samename (north or south) as the elevated pole is always 90°minus its declination (6).Polar distance of a body whosedeclination has a different name from that of theelevated pole is always 90° plus 6.Declination of any navigational star is listed in theNautical Almanac for each date. Declination of eachbody of the solar system is listed for every hour GMT.Time DiagramSo far you have learned that a heavenly body islocated on the celestial sphere by its Greenwich hourangle (corresponding to longitude) and its declination(corresponding to latitude). You have seen how both ofthese coordinates are measured and how, from them, theGP of a heavenly body can be located on the terrestrialsphere.Before going further into nautical astronomy, youwill probably find it helpful to learn something aboutusing a diagram (called a time diagram) of the plane ofthe celestial equator. Not only will this make it easier foryou to understand the ensuing discussion, but it will alsosimplify the solution of celestial navigation problems.15-11

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