The GP you see on the small sphere in figure 15-5 corresponds to the stars 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 earths surface. The latitude of a point on the terrestrial sphere is measured from the equator northward or southward along the points meridian to a maximum of 90. Declination of a body on the celestial sphere is measured in exactly the same wayfrom the celestial equator (equinoctial) northward or southward along the bodys 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 (d). Polar distance of a body whose declination has a different name from that of the elevated pole is always 90 plus d.

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.

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.

Figure 15-7.-Locating G on the time diagram location at 90 longitude.

In the time diagram (fig. 15-6), the observer is theoretically located outside the celestial sphere, over its south pole. The diagram consists of a circle representing the celestial equator. The center of the circle is the south celestial pole. Counterclockwise direction is westerly. The local meridian is drawn in as a vertical line, thus placing the upper branch(M), which is the arc of a celestial meridian, between the poles at the top of the diagram and the lower branch (m) at the bottom. To avoid confusion, we show the lower meridian as a dashed line.

You locate the Greenwich meridian (G) by means of your longitude (symbol l). If you were at longitude 90W, G would appear on your diagram 90 clockwise from M because you are counterclockwise or west of G. A glance at figure 15-7 will confirm this location. What you really do, then, is measure from M toward Greenwich, the direction depending upon whether you are in east or west longitude.

Figure 15-9.-Locating the vernal equinox and a star on a time diagram.

Figure 15-8 shows another time diagram on which GHA of the sun is indicated. The upper branch of the suns hour circle is shown as a solid line. The angle, or arc, of the celestial equator between the Greenwich meridian and the suns hour circle is 90. Therefore, GHA of the sun at this instant is 90. Remember, GHA is always measured westward from G.

The GHA of a star is measured in the same direction from Greenwich to the star; however, because the SHA enters the picture here, your method of locating a star on the time diagram is somewhat different. First, you must locate the vernal equinox by its tabulated GHA. Lets say the GHA of the vernal equinox for the time of your observation is 45. You locate the vernal equinox 45W from Greenwich, as shown in figure 15-9. The symbol that resembles a pair of rams horns represents the vernal equinox.

Itis easy to see herethat the GHA of Vega must be equal to the GHA of the vernal equinox plus the SHA of Vega (or GHA_Vega, = GHA_r + SHA_Vega. In this example, the GHA of Vega is 81 plus 45, or 126. Now lets use the time diagram to explain some more facts about nautical astronomy.