Captain's Log

6/16/2007

Celestial navigation is a combination of art and science.  A long time (2000 years) ago philosophers determined that the earth moved within the stars and around the sun.  In time, the British and other astronomers were able to map out the stars in the sky and determine their exact location above the earth at any time.  They were also able to do this for the sun, moon and planets, but these bodies were more difficult as they move in space.  The stars do not move, but appear to as we revolve around the sun. 

The nautical almanac will tell you the location of any navigational body, star, planet, sun or moon at any time ( universal coordinated time).  The almanac gives the location in Hour angle and Declination, but these can easily be converted to Latitude and Longitude.  If the body is directly over our location on the earth where we use our sextant, its measured angle from the horizon will be 90 degrees.  As we move away from that location, the angle above the horizon gets smaller.  This deviation is a measure of distance from the geographic position of the body at the time of our sighting.   

With some fairly complex spherical trigonometry, we can make this work for any location.  Simply put, we pick a position close to where we think we are and call it an assumed position.  We then use the spherical trigonometry to calculate how high the body would be if we were at the assumed position at the time of our sextant observation.  We then compare that result to our sextant observation: the difference represents the distance from the assumed position and the direction will point towards or away from the body.  We then have a line of position that we can plot on our chart.  Once we have two lines, we get a cross indicating our position. Often we take a third observation which confirms our location. 

Captain John Keever