Star Fix

A few nights ago we were sailing among the Northern Marianas Islands in the North Pacific. The ship was doing a routine independent steaming exercise and it was another boring night as we passed close to the island of Rota, the southernmost CNMI island, which is a bit north of Guam.

As is usual when i am bored on the bridge, staring at a radar screen devoid of ships, and only showing the occasional rain squall and Rota in the distance, I pull out the sextant and try my luck.

That night was a particularly good night. I knew even before the sun set that there would be a good horizon and few clouds to obscure the stars. I would need both if I hoped to fix my position with any accuracy.

My AB Justin feels a compassion for my obsession with Celestial Navigation as he is equally obsessed with rigging. This was the man who spent most of his lunch and both breaks the other day splicing and eye in wire with what is known as a Liverpool Splice. To make matters more challenging he used the method that his rigging book labeled "the hard way."

As Justin kept a sharp look out for any small craft ahead of us, I pulled out the only Celnav book I use anymore: Pub HO 249 which give a convenient list of 6 stars that are both bright and optimally placed for a good fix for anywhere in the world.
I was not going to go overboard, so I picked 4 stars and grabbed the ship's sextant and headed out to the gyro repeaters on the bridge.

To make things a bit clearer, to reduce your position from the stars, you start with a big assumption, and then use a sextant, a chronometer, and a method of turning the angle from the sextant in to lines on the chart that are crossed to give your position. The assumption is where you are in the world. In the days before GPS this could be a sketchy proposition. Fortunately today you are just basically checking to make sure that the GPS is working properly, so I use the GPS fix (accurate to mere feet) as my “assumed position.” From there I figure out where the stars should be from the aforementioned HO 249. Once I know the direction and height of each star, I pre-set the sextant to the correct angle and use the compass on the bridge wing to point it in the correct direction. Nine times out of ten if there are no clouds the star and horizon appear in the sextant as I point it in the right direction. I then fine tune the sextant so that the star is superimposed on the horizon and then mark the time. While this sounds simple, this is the “art” of Celnav. Getting the sextant properly held and taking an accurate reading is tougher than it sounds, especially with the ship rolling. Accuracy here is measured in teths of seconds of arc. A second of arc is 1/60th of a degree, so we are talking very fine measurements.

This night I was able to gather my four stars in short order, and for the heck of it, I shot the moon, which was about 20 degrees above the Western horizon. I was feeling good and went inside and took a GPS fix at exactly 7PM, less than 10 minutes after I started shooting the stars.

The math involved in reducing stars is a matter of spherical trigonometry, which is quite beyond my willingness to explain here. When I was in school, and in my Coast Guard tests, I had to use tables and books to reduce all of these numbers and triangles. Scientific calculators make things simplier, but for my money, I like using computers. There is a great program that allows me to plug in my sextant numbers, the time of the sight, and the approximate position, and it automatically calculates for me my “most probable position.” Basically instead of doing lots of math and then plotting the lines on the chart, it spits out a latitude and longitude. This night was my golden hour. I entered my 5 sights into the computer and it spit out the exact latitude from the GPS (especially impressive since we were heading due South so that the Latitude changes rapidly) and 0.1 minutes of Longitude out. This is accurate to 0.1 nautical miles, or about 670 feet, about the length of my ship. In ten years of shooting stars, this was the most accurate I had even gotten. Needless to say my ego was huge that night. I was a happy camper, although no one else on the ship was sharing my joy. I guess it’s a Celnav geek thing.