Lesson 7  Sight Reduction  

A simple description of sight reduction could be the determination of the exact altitude and azimuth of a known body from a precise location on the surface of the Earth at a given time. You've constructed diagrams on the plane of the celestial meridian which have allowed you to approximate altitude and azimuth of a known body from an approximate position on the surface of the Earth on a given date and time. In these diagrams you learned the importance of the navigational triangle with Z (zenith), Pn or Ps (elevated pole) and the M (the body) as its three vertices, and colatitude, codeclination, and coaltitude as its three sides. You discovered that you need to find Z (azimuth angle), which is formed between the hour circle and the meridian at the elevated pole, and h (altitude), which actually falls outside the triangle between M (the body) and the horizon. In this lesson, we'll begin to use one of the actual tools of the navigation trade, the sight reduction table, to determine h_{c}, the computed altitude of the body and Z, the azimuth angle of the body, from a known position on Earth at a given moment in time. Sight reduction tables actually present resolutions for many, many celestial triangles. They save the navigator from having to "do the arithmetic" necessary to solve the particular triangle he has found. Of course, with the advent of seagoing calculators and computers, "doing the arithmetic" is often easier than using the tables. Although there are several sight reduction tables available, we'll use the one in the back of the Nautical Almanac because it is widely available. If you want to try other sight reduction tables, I recommend H.O. Publication 229 as an easy one. H.O. Publication 249, the Air Almanac, is used by many for star reductions. It is laid out to be used very quickly as it was developed for celestial navigation from aircraft. Have fun, and if you don't understand something, use the discussion board or mail navigator@cyberacademy.org.
