Figure 15-6.-Time diagram.Figure 15-7.-Locating G on the time diagram location at 90°longitude.In the time diagram (fig. 15-6), the observer istheoretically located outside the celestial sphere, overits south pole. The diagram consists of a circlerepresenting the celestial equator. The center of thecircle is the south celestial pole. Counterclockwisedirection is westerly. The local meridian is drawn in asa vertical line, thus placing the upper branch(M), whichis the arc of a celestial meridian, between the poles atthe top of the diagram and the lower branch (m) at thebottom. To avoid confusion, we show the lowermeridian as a dashed line.You locate the Greenwich meridian (G) by meansof your longitude (symbol i). If you were at longitude90°W, G would appear on your diagram 90° clockwisefrom M because you are counterclockwise or west of G.A glance at figure 15-7 will confirm this location. Whatyou really do, then, is measure from M towardGreenwich, the direction depending upon whether youare in east or west longitude.Figure 15-8.-GHA of the sun on a time diagram.Figure 15-9.-Locating the vernal equinox and a star on a timediagram.Figure 15-8 shows another time diagram on whichGHA of the sun is indicated. The upper branch of thesun’s hour circle is shown as a solid line. The angle, orarc, of the celestial equator between the Greenwichmeridian and the sun’s hour circle is 90°. Therefore,GHA of the sun at this instant is 90°. Remember, GHAis always measured westward from G.The GHA of a star is measured in the same directionfrom Greenwich to the star; however, because the SHAenters the picture here, your method of locating a staron the time diagram is somewhat different. First, youmust locate the vernal equinox by its tabulated GHA.Let’s say the GHA of the vernal equinox for the time ofyour observation is 45°. You locate the vernal equinox45°W from Greenwich, as shown in figure 15-9. Thesymbol that resembles a pair of ram’s horns representsthe vernal equinox.From the Nautical Almanac you find the SHA of thestar in question. You already know that the SHA ismeasured to the west from the vernal equinox (first pointof Aries). All you have to do here is find the SHA of thisstar and measure the SHA westward from the vernal15-12
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business