CELESTIAL NAVIGATION

Celestial navigation in general depends upon being able to establish the orientation of the earth globe with respect to the celestial glboe (i.e., the time of day). If sidereal time is known it is possible to determine one's position on the earth by using the Uniglobe and spanner. It should be noted that stellar navigation is essentially based on sidereal time. If GMT is known, it must be converted to sidereal time before calculations can be made.

Refer to Figure 31. Assume that an observer aboard a ship knows that the time is 0430 GST, and looks into the heavens and observes the stars Rigel and Capella. Using the spanner, he measures the altitude (i.e., the angle from the horizon) of Capella as 60°. (Thus, zenith distance is 30°) and the altitude of Rigel as 35° (zenith distance 55°. By orienting the spanner rosetta over Capella, a circle of 30° radius can be drawn around the star. This circle represents all of the possible positions at which the observer may be. The circle represents all points having a zenith distance from Capella of 30°. If a circle of radius 55° is also plotted around Rigel on the Uniglobe, the circle will intersect the Capella circle at two points. Thus, there are only two places on the globe where these two conditions exist.

By orienting the two globes for 0430 GST, (24 on vernal equinox and 0430 on Greenwich) we find that at this time, one of the two points appears in the Atlantic, and the other in Iran. The ambiguity is resolved since the obeserver is known to be in the Atlantic. A third measurement and plot, perhaps of Deneb (altitude 36°) would also resolve the ambiguity, or the observer could note that Capella is in the eastern sky, not the western sky. This is an example of celestial navigation which does not require the use of an almanac or an ephemeris, just a catalogue of star positions. This type of calculation can be performed using any of the fixed celestial objects (the navigational stars).

The navigation process can also be performed using moving celestial objects such as the sun, moon or planets, but the positions of these objects for the particular date and time of day must be known. This requires an almanac or ephemeris, together with a knowledge of GST or GMT.

The normal SUN SHOT requires going to the ephemeris or almanac to determine the sun's position for the current time. The sun's declination is approximately known using only the date and without knowing the time of day. The determination of one's latitude alone may be performed by measuring the sun's altitude at local noon or transit.

For example, assuming it is the 15th of May, it is easy to approximate the sun's position from an interpolation of the dates on the ecliptic of the Uniglobe, which estabishes the declination of the sun for that date. The latitude is determined by first computing the zenith distance from the measured altitude at local noon. (90° - altitude= zenith distance). By drawing a circle about the sun of radius equal to the zenith distance, two possible latitudes are determined, one directly north of the sun and one south. This ambiguity is easily resolved by observation. (See Figure 32).

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