I said last time that all sights consist of two numbers:
- The date and time of the sight
- The height of the celestial object above the horizon, at that exact time
Therefore, on 12 July 2010, a half hour before estimated Solar Noon, I positioned myself on a beach with an unobstructed view of the southern horizon (my exact location is what you are going to try to discover!) I had my inexpensive Davis 15 sextant, and a chronometer -- set to GMT of course!
Before you take your sights, you want to measure and note down two small errors that you will have to correct for, later:
- the height of your eye above the water
- the index error of your sextant
That's a bit inconvenient, though, particularly while sailing in shark-infested waters, so it is acceptable to take your measurements above the water. That slightly alters your view of the horizon, creating what is called a dip error. I'll discuss next time how to correct for this error.
Then I looked at the horizon through the sextant. Actually, at the two horizons, since you always look at two things at the same time through a sextant. I adjusted the vernier knob on the sextant until the two horizon lines were lined up, and then read the 'distance' between them.
What the two horizons look like through the sextant... sort of
Of course, the measurement should read exactly 0°, since I was lining up something with itself. However, the reading is never exactly 0°. This hopefully tiny error is called the sextant's index error, and in my case, it was 1.2' off the arc.
I'll discuss next time what this strange but helpful 'off the arc' or 'on the arc' terminology means, and how to correct for this error.
Those two errors carefully noted down, I was ready to take some sights. It was coming up on 16:30 GMT. I expected solar noon to occur some time around 17:00 GMT. My plan was to take sights every 5 minutes, until 16:55 GMT, where I would start taking sights every 1 minute. I would do this until around 17:05 GMT, when I would go back to taking sights every 5 minutes until 17:30.
The usual way to take sights is for one crew to handle the sextant, and another to handle the chronometer and notebook. This is the way Helena and I mange it, if there are two of us. We swap positions every day so we both get practice with the sextant.
It's more difficult for a single-hander to juggle both chronometer and sextant, but it can be done. I cheat a bit by using a really useful iPhone application called 'Time Signal'. This app announces the time audibly every 10 seconds. With headphones, I can hear the announcements, even when it's noisy. It makes it very easy to know the time, even when your eye is glued to the sextant.
Here's a video that shows how it works...
'Time Signal' iphone app
Great app, the music is NOT include (thank goodness!)
Great app, the music is NOT include (thank goodness!)
This app allows me to keep my eye on the sextant, while my ears take care of the time. You will note that all my sights are taken at :00 seconds. This simplifies the plotting process and probably makes it a tad more accurate. It's difficult to do this with a hand-held watch, but Time Signal makes it a snap. If you have an iPhone, iPad, or iTouch (the ultimate on-board navigation tools, by the way!), you should check it out.
At any rate, with the sextant to my eye, and Time Signal in my ears, here are the sights I took:
So, those are the sights. Note that each sight is composed of both a time, and an angle. The angle is the height of the Sun above the horizon at that particular moment. Now, what do we do with them?
After carefully putting away your sextant down in the cabin, you pull out a piece of graph paper and your favorite pencil, and you graph all the points, as I have done below.
Note the example below does not use the same sights as above. That would be cheating! You will need to make your own graph. I used a pen for this example because the ink shows up better in a scan. I actually do all my graphs in pencil, for obvious reasons! Click the image for a larger view.
The sights, plotted on graph paper
What you do is plot the time on the X axis, against angle on the Y axis. You should end up with a graph that looks something like the example.
Once you have plotted all the points, you must draw a curve that fits the points as closely as possible.
Neatness and accuracy counts! The more accurate the curve, the more accurate your position will be.
As you draw your graph, you will discover that there is a bit of science and a bit of art to it. The sights you will be plotting are 'smoother' than normal, because I was standing on solid ground when taking them. Yet you will see that drawing the curve still requires a bit of judgement.
That's okay! Don't let that stop you. As Capt. Pete Culler famously said, experience starts when you begin. Just do the best you can.
Plastic French Curves, available at any office supply store
If you are a really crummy draftsman (like me) you can try using a set of French Curves. You can find them in the States at any office supply store for a few dollars. I imagine they are just as inexpensive, elsewhere.
I'm not sure they make the curves more accurate, but they look more accurate, and save a bit of time. Try them, if you like.
Free hand or not, have your curve drawn for next time. You're going to need it for the next step.
Speaking of next time... if you'd like to be notified when I post a new 'lesson' (erratically, about 3 times a week), you can sign up for my super-sophisticated Automatic Notification Process. Actually, it's just an email, but Automatic Notification Process sounds better. I won't spam you, and you can de-sign up at any time.
>>> Next Episode: Latitude - Correcting the Sight
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