Principles of Celestial Navigation

The Intercept Method » Basics: Recap

image of inner page of Nautical Almanac, with GHA and Dec data for planets and stars

At this point we have learned the basic ideas behind celestial navigation. We know a star's geographic position at any time by consulting an almanac.

Quartermaster 2nd Class Nicholas Gibson takes a sight aboard the amphibious transport dock ship USS Green Bay (LPD 20).

We can determine how far away we are from a star's geographic position by measuring the star's zenith distance. (In reality, we measure its altitude above the horizon using a sextant.)

This gives us a circle of position, and our position is somewhere along that circle. Doing this for more than one star will result in intersecting circles. Our position—or fix—is at the intersection of these circles.

The concept is straightforward, especially when looking at a globe. But what technique is utilized by sailors? That is, how is it really done in practice? As we progress through the next sections, Navy Chief Quartermaster Tim Sheedy will demonstrate these techniques and then take us through a typical day in the life of a navigator.