_{Copyright © 1999 by Scott Diener}

The SEXTANT and Celestial Navigation

This section of my website may eventually present a complete tutorial on celestial navigation.  However, my current intent is not to write yet another text on celestial, but instead to present the concepts in a fun and graphical way, so that students of all ages will find it both interesting and helpful for further study.
	Introduction to the Sextant A sextant does one thing only...it measures an angle.  In the case of navigation, the sextant measures the angle between the horizon, you and the sun (or star).  Pretty simple.When you look through the sextant you see an image of the sun, along with the line of the horizon.
As you adjust the sextant the image of the sun is lowered until it just touches the line of the horizon, and then the measurement is complete.  The image above shows just how simple this measurement is.  Of course, on the deck of a bouncing boat, with big seas on the horizon, you can easily make mistakes!
Once the measurement has been made, it is a simple process to look in the Nautical Almanac to find out just what point on the earth the Sun is situated directly above at the moment we made our observation (this is called the G.P.).  Looking at the graphic, if the angle of the red line is smaller, then we will be further aways from the G.P.  As it gets larger, we will be closer to the sun's G.P.
In fact, if the angle is exactly 90 degrees, we are directly under the sun, right on the G.P.!    So, armed with our angle measurement we can use published Sight Reduction tables to tell just how far away we are from that G.P. point and establish a circle of position. We could theoretically be at any point on that circle, but hopefully we have a pretty good idea of which hemisphere or ocean we are in, so we are able to narrow down our position with confidence and can determine, within just a few miles, just exactly where we are on the earth.  This ancient form of navigation is highly accurate and is still practiced by sailors all over the world today.  In spite of the fact that satellite navigation systems are so reliable, they can still fail.  The sun, moon and stars are not likely to do so!
References Campbell, Stafford.  The Yachting Book of Celestial Navigation.  Dodd, Mead & Company, N.Y.   1984 This little book may be out of print, but regardless,  I think it is probably the most readable, simple discussion of celestial navigation I've ever seen.  If you find a source for this excellent work, please send me the information! Turner, Merle B.  Celestial for the Cruising Navigator.  Cornell Maritime Press. 1986. At the opposite end of the spectrum to Stafford Campbell's book, this remarkable reference is much more mathematically oriented and will be of interest only to advanced students.  Turner has an excellent section on using a normal scientific calculator to replace sight reduction tables.  I used the formulae in this book to write my own computer program for sight reductions.  I'm not sure if this volume is still in print. The following excellent titles can be ordered online from amazon.com: Celestial Navigation Jeff E. Toghill, 1988 Celestial Navigation for Yachtsmen Mary Blewitt & Tomas Bergel, 1996
.....more to follow.  Please check back
updated 05/18/03