Daily Journal

July 6, 2010

By David M. Lawrence

I've been slowly working toward achieving something I've long wanted to do – plot a celestial fix on a chart.

I have long been interested in celestial navigation, but have had little opportunity to learn or practice it. Two decades ago, I took a celestial navigation class. We spent most of our time on learning to reduce star sights to a plotted position. I thoroughly enjoyed it, and kept all of my course materials, but what little skill I had fizzled with disuse.

This expedition has given me a chance to do something I've long dreamed of being able to do. Last week, I shot sun lines – which act as a control on a DR (dead reckoning) position. In my first shots, my work corresponded well with those of Jenan Kharbush, an SEA volunteer who was much more experienced than I. Not bad for my first attempt at using a sextant.

A couple of days ago I plotted my first DR positions. The concept is fairly simple, but making sure you place your marks on the right spot is not so simple. Nevertheless, I did it, and found myself feeling confident that I can do so again without hesitation (and I have done so since).

The use of a sextant at sea is a challenge. You need it to be dark enough to be able to see the planets and navigational stars, but you need enough light to be able to see the horizon. We generally begin shooting at the mid-point between nautical twilight and civil twilight.

The sextant itself has a small telescope that you use to see the stars, a set of filters to protect your eyes (especially when shooting the sun), mirrors that you use to track the image of the object you're shooting, and a graduated scale that you use to measure the height of the object in question above the horizon. It is a challenge for many – but especially for me as I wear progressive lenses (yes, I am getting long in the tooth). I find it hard to figure out where to look out of my glasses to focus on the image in the sextant. On some sextants, the sliding action is smooth. On the one I used tonight, the action was a bit jerky, which made it hard for me to get the image to line up with the horizon. (Maybe it was operator error.)

Once you sight the object, such as Venus, you yell "Standby Venus!" The recorder repeats the command. Then you adjust the sextant so that the image of the object is in line with the horizon. Once you're satisfied that you have successfully done that, you yell, "Mark!" and the recorder writes down the time. You read the elevation angle of the object off the sextant's scale and that is also recorded.

It is better to shoot five or six targets to get a good celestial fix – the more targets, the less likely you will get an erroneous sighting that renders your fix useless.

Once the stars or planets have been shot, you then need to reduce the sights, which can be a relatively complex mathematical problem. Mariners have long used aids such as nautical almanacs to help them obtain the astronomical information that tells them when to expect a particular object at a particular location in the sky. For each object you shoot, you get a line of position. In the traditional way, you'd plot each line of position on a chart. Your position is where they overlap.

We have nautical almanacs, plotting sheets, and the like, but in the interest of time today, I used a computer program to help reduce my sights for me. This morning – my first attempt at obtaining a celestial fix, I shot Jupiter and the stars Vega and Altair. I obtained a fix that was about 15 nautical miles off our actual location according to our global positioning system. It wasn't bad – especially for a novice like myself, but I wished I could have done as well as my watchmate Steve Putnam, whose position was less than one nautical mile off our GPS location.

Tonight, I shot Venus, Saturn, and the stars Antares, Spica, and Vega. By the time I was done, I had a position that was between five nautical miles and 10 nautical miles off of our GPS position. In the open ocean, that's not too bad.

But I want to get better, and I don't have a lot of time to do so.

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For more from sea, listen to an interview with Chief Scientist Giora Proskurowski on the July 2, 2010 Science Update podcast, produced by AAAS.