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Odyssey Deckhand, Jacob Kasper 'shooting' a star with a sextant.
Photo: Chris Johnson

January 2, 2001
Celestial Navigation
  Real Audio
  28k


Log Transcript

This is Jacob Kasper, deck hand aboard the Odyssey. I was awakened at 0300 this morning for dawn watch. Tired and groggy with just 5 hours of sleep inside of me I stumbled out of bed and put on my clothes. Arriving in the pilothouse, which was illuminated by the green glow of the navigation instruments, Brian and I relived the previous watch.

As the ship rolled, I stumbled to the starboard doorway to look outside and visit some friends. Looking up I was surprised by the brilliance of this morning's stars. When I had gone to sleep the moon was outshining most of the stars, but now that the moon had set I could clearly see thousands of stars all trying to out shine each other. Castor. Pollox. Orion. The Pleiades. The clouds of Magellan, and my favorite constellation: Corvus the Crow. But the stars that I could see 5 hours ago are now in different places. At one time I had thought that this was due to our movement across the earth, but now I know that it is caused by the fact that the earth is rotating inside of a relatively stationary sky, which causes the stars to appear to travel from east to west.

While we crashed through the waters of the equatorial Pacific, Brian and I maintained our watch and chatted. No matter what we talked about, our topic of conversation always turned to that of ships and sea stories. Of the old navigators who crossed oceans with such different navigation equipment and techniques than what is used today. The Polynesians followed the stars and could find islands that were located hundreds of miles away by studying the patterns of refracted waves around those islands. the Vikings, who's compass was little more than a needle in a bowl of water, used the stars to guide them across oceans; and all navigators of the past who have used the stars to steer them around the world. We talked about how this is different than being aboard Odyssey where we rely on radar and GPS, and the discipline of celestial navigation has become a hobby. Yet something remains the same: our GPS, which can tell us our position on the face of the earth to within 30 meters, is guided by a constellation, but this constellation is not of stars, it is of satellites.

The basic process of celestial navigation is to measure the height (or altitude) of a heavenly body (certain stars and planets, the moon and our own star, the sun) above the horizon and record the time at which this measurement was taken. This is a process known as taking a site or shooting a star. The tool that is used to site a heavenly body is the sextant. Shooting a star is done by pointing the sextant at a celestial body and lowering a reflected image onto the horizon. Using the altitude, the time at which it was measured, and several tables, one can calculate a line of position (a line traveling around the world on which the observer is located). If one takes 3 sights of 3 different celestial bodies with in a few minutes of each other, the place where these 3 lines of position cross (known as a fix) is the location of the observer.

At 0500 I took out my nautical almanac and sight reduction tables and began to pre-compute today's stars. Alioth, Spica, Acrux, Canopus, Sirius, Betelguse, and Pollux. The books tell me "shoot these today for they are the brightest." As the eastern sky lightened and my horizon became clear I took my sextant out and pointed it to the stars.

I am shooting, using my sextant to introduce a star to the horizon. This is something that has been done for hundreds of years, and I feel timeless. For just a few moments I have forgotten where I am and what ship I am on. It is just me and the star and the horizon telling a story: a story of an angle of which I am the apex.

After I take my sights I return my sextant to its case and proceed to calculate our position. Under the irritating brightness of the halogen light above the chart table I begin to reduce my sights. Tables and plotting sheets and triangles and pencils, are the tools I use now, for my sextant is put away. After 20 minutes I have found that my position is 5 miles off of the GPS position for the time that I shot. Maybe I will shot better tomorrow morning, now it is time to go to bed.

Log by Jacob Kasper

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