Lesson 6b  Diagram on the Plane of the Celestial Meridian and the Navigational Triangle  
LESSON 1 
Recall
that there are two systems of coordinates depicted on the diagram on the plane of the celestial
meridian: the celestial horizon system of coordinates, and
the celestial equator system of coordinates. Once we have constructed our complete diagram, we have depicted the position of the body relative to two different sets of references:
Remember "The Answer" given to you early on? It said, "If we can know the exact position in the sky that a body would occupy at a precise moment in time, we can then compute the altitude (height above the horizon) that the body would have from a precise location on the surface of the Earth." Well, here we are. The fundamental problem solved by the celestial navigator is the determination of the position of the vessel on the surface of the Earth with as much precision as possible. The first step in that process is determining the computed altitude and azimuth of some visible bodies from a place near the position the vessel is thought to occupy. Using the diagram on the plane of the celestial meridian and the Nautical Almanac, you can accomplish that task. Use these next problems to practice the skill of determining altitude and azimuth of a known body at a known date and time from a known position. Draw them carefully; we'll use them later to compare methods.
