The Boat coll

Plotting the Sights

If youve stayed with me this far, you are probably ready for your first set of sights!

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

For a noon sun sight we want to start taking measurements about 1/2 hour before the estimated time of solar noon.

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

Since I was fairly close to the edge of the water, the height of the sextant above the water was approximately 6 feet. Ideally, you want to your sextant to be 0 feet above the water, because that way you have the most accurate view of the true horizon. I.e., you should be in the water with your eye half-submerged.

Thats 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. Ill 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.


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 sextants index error, and in my case, it was 1.2 off the arc. 

Ill discuss next time what this strange but helpful off the arc or on the arc terminology means, and how to correct for this error.

At any rate, with the sextant to my eye, and Time Signal in my ears, here are the sights I took:

Time	Hs
16:30:00	70° 4.2
16:35:00	70° 25.8
16:40:00	70° 37.2
16:45:00	70° 48.4
16:50:00	70° 57.2
16:55:00	71° 1.6
16:56:00	71° 1.8
16:57:00	71° 2.4
16:58:00	71° 2.6
16:59:00	71° 2.6
17:00:00	71° 2.0
17:01:00	71° 1.6
17:05:00	70° 59.8
17:10:00	70° 51.6
17:15:00	70° 41.0
17:20:00	70° 28.2
17:25:00	70° 13.2

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.



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.

Thats okay! Dont let that stop you. As Capt. Pete Culler famously said, experience starts when you begin. Just do the best you can.


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.

Im 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. Youre going to need it for the next step.


Speaking of next time... if youd 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, its just an email, but Automatic Notification Process sounds better. I wont spam you, and you can de-sign up at any time.

>>> Next Episode: Latitude - Correcting the Sight

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