Tuesday, October 4, 2011

Navigating by the Sun

I have never found our boat's position using our sextant - the odd device
used by navigators for centuries to measure the position of the sun, thus
allowing them to plot their position. Sort of.

We know of a few cruisers who carry a sextant. We know of even fewer
cruisers who know how to use it. After all, it is complicated. We've heard
that some cruisers use it as a hobby - we haven't met them yet. Most carry
it in case of catastrophic failure - the boat is hit by lightning and all
electronic devices are destroyed or multiple GPS failures (because everyone
carries a spare or two...don't they). Same for us. Should some calamity
befall us and our three GPS units are not working, we can get home using the
sextant to navigate. Well. That's the idea, anyway.

I've taken a couple of informal tutorials on how to do this. The first was
from Bob in Mag Bay on the west coast of Baja, Mexico. He showed us how to
use the sextant, and take a noon sight. The second was with Steve in Puerto
Vallarta. He showed me a fabulous way to do all the myriad of calculation on
a calculator, thus eliminating the need for the three volume sight reduction
tables. And, I've read Blewitt's book several times, so I felt I had a clue.
But I'd never tried to put all the pieces together to actually take a sun
sight, do the calculations, and plot my position.

So while we sit here with time on our hands, I've spent the last week
re-reading Blewitt's "Celestial Navigation for Yachtsmen". Then finally, I
took a sun sight. Well, actually I took 5 in a row, and averaged the result.
So far, so good.

Then, I sat down with the Nautical Almanac, my sight reduction tables, and
my trusty Blewitt, and tried to calculate my position. Well. The
frustration!! Most of my problems stemmed from the fact that I was using a
2010 Nautical Almanac. And...with a few alterations, it can be used for
2011. But I goofed it up, didn't add the necessary 87 degrees, and found my
calculated position was an ocean away from my assumed position. Not OK.

I worked at this for a few hours each day for FOUR days. Finally, yesterday,
I figured out my mistake. And it worked!! I found my line of position. But
just one line of position. Which means my boat could be anywhere along that
line. A line of infinite distance, by the way. In theory, you should take
several sun sights throughout the day, and then you'll be able to use the
different lines of position that you generate for each sight to pinpoint
your location. So now, I just need to start all over, take a few more sun
sights, and do a few more calculations...

I'm feeling pretty good about this new found knowledge. By sometime next
month, I might be able to tell you where we are. But of course, by that
time, we'll have moved on.

Labels: Navigation, books


Belinda Del Pesco said...

I've been thinking about this post since I read it a few weeks ago. I'm so inspired that you've tackled this, and that you kept at it, when it appears to be so complicated and frustrating. Four Days of trying, eh? That's my watermark; if I take this on and try to learn it, I won't complain till the fifth day. :)

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