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