David’s Voyage Blog

Publish Date: Saturday, April 10, 2010
We are quite spoiled in this modern age of GPS:  We always know our exact position on the surface of the earth, and we never have to pick up a calculator to figure it out. I recently counted the GPS devices aboard Three@Sea and discovered that there are eight of them! Except in extraordinary circumstances, I have confidence that one of our on-board devices will be able to tell us (or a search party) where we are.

But what about extraordinary circumstances? What if we experience a catastrophic lightening strike that takes out all of our electronics? What if a massive solar flare disables the global GPS system for awhile? And there are probably a few other scenarios under which GPS might become temporarily unavailable. If one of these extreme scenarios occurs while we’re in the middle of the Pacific Ocean, we would like to be able to figure out where we are, primarily so we can get to where we’re going.

Okay — I doubt any of those catastrophic things will happen. But I’m a geek with a scientific curiosity about celestial navigation, as well as a certain nostalgia for the ways of the ancient mariner. So about six months ago I decided to learn the basics of using a sextant to determine our position, and to teach Ayla how to do it as well (the poor girl). I haven’t been able to spend much time on it, so progress has been slow, but today Ayla and I took our first actual hands-on sights to determine our position, and we had a blast!

To learn how to use a sextant to find our position I’ve read three books on the subject, and I’ve also read the manual that came with the sextant (Davis Mark 25). The first two books were complete treatments of celestial navigation, full of spherical trigonometry and lots of calculations. Let me tell you, these books could cure even the most severe case of insomnia. Although I could follow along with a lot of it, I found myself saying, “This is too hard, and not very fun. I’ve got GPS devices that can tell me where I am, so why am I putting myself through this!?!” But alas, I kept coming back to it.

I recently put my ego in a drawer and bought the third book called Celestial Navigation for the Complete Idiot, and this little book with the belittling title offered a breakthrough: I didn’t have to learn all the spherical trigonometry and do all the calculations; there is a relatively straightforward way to determine your daily position with a single observation, and some basic arithmetic. This method is commonly called “The Noon Sun Shot,” and it reignited my interest in figuring out how to do this. I’ll explain briefly how it works, but don’t worry: there’s no spherical trigonometry involved.

Determining Longitude: Before you leave port you need to set an accurate timepiece to Greenwich Mean Time. GMT is the local time in Greenwich, England (the prime meridian — longitude of zero degrees — also runs north/south through Greenwich). I’m using my cheap Timex digital watch for GMT, which is accurate to within a few seconds per month. Armed with GMT, each day at sea you use the sextant to figure out exactly when the Sun is at its local apex (called “local apparent noon”), and you record that exact time in GMT. The Earth is spinning on its axis at a speed of 15 degrees per hour, so by calculating the time difference between GMT noon and local noon, you can then calculate your exact longitude using basic arithmetic. Pretty simple, and very cool!

Determining Latitude: At local apparent noon (i.e., at the moment you determined longitude above) you use the sextant to also measure the angle between the horizon and the Sun. Because you’re doing this at exactly noon, you know that you’re on the same north/south meridian as the Sun, so the angle you’re measuring represents only the difference in latitude between yourself and the Sun (a few twists and turns aside). You can then refer to the Nautical Almanac to find out the latitude (called declination) of the Sun on this date at noon. With your angle measurement and the Sun’s pre-computed declination you can compute your latitude with simple arithmetic. Also pretty cool!

So off we went to the beach today to take our first sights (we can’t see the sea’s horizon from our marina, so we had to go to a beach with a southwest orientation). Earlier in the week I had explained all of this to Ayla, and she was as excited to try it as I was. While we were standing there on the beach with our sextant in hand, we were joking that the other kids walking by were probably saying, “Look at that poor girl: her dad is making her learn celestial navigation.”

So we took our sights at noon, and we recorded everything meticulously. I’ve been reading about it for so long, and practicing with the sextant on the boat, that taking the sights was actually easier than I thought it would be. When we got back to the boat we did the calculations, and we surprised ourselves with how well we did on our first try. Our latitude was almost exactly correct, and our longitude was off by less than five miles. The longitude error was caused by us determining local apparent noon to be 20 seconds too late (which we figured out by doing the reverse calculations from our known longitude). Wow!  That’s not very much error to be that far off.

I’m sure we can get better with practice, but mostly we were thrilled that we were able to take the measurements and execute the arithmetic procedures to get a reasonable answer. Of course, I know it will be more difficult to do this on a moving boat in the middle of the ocean, so we might not be able to get much more accurate. But even if we don’t get any better, if we can get within 5-10 miles of our target island, I think we’ll be able to see it, either with our eyes or our radar. And that’s the goal.

There are several other celestial bodies that this method will work with in case it’s cloudy or rough at noon (the Moon, Venus, Jupiter, several stars, etc.), and there’s a lot more we can learn about celestial navigation. For now, though, I’m happy to be able to determine our position in at least one way, and we’ll all practice this for awhile before moving on to some more difficult methods.