09 May 2016

Backyard CelNav

Thinking about the Jester Challenge has renewed my interest in celestial navigation. Not only does CelNav (as I will call it) excite my inner math nerd, it somehow seems to fit well with the whole idea of the voyage. Of course I will bring a GPS with me -- I'm not stupid -- but working up a noon position in the traditional way would not only be satisfying, it will keep me busy for part of the day.

For a large part of the day, at my current rate of calculation!

I've been slowly working my way through a new book called "Hawaii by Sextant", by David Burch and Stephen Miller. David is the director of the Starpath School of Navigation, while Stephen is their lead instructor in celestial navigation. As the sub-title of the book says, it's "an in-depth exercise in Celestial Navigation using real sextant sights and logbook entries."

In a nutshell, you use the logbook data to plot your DR (dead reckoning) track, then use the Sun, Moon, planet, and star sights to calculate a fix, and thereby work your way across the Pacific. I personally find it a fascinating and very challenging way to practice CelNav and -- perhaps more important -- dead reckoning. I thought I was reasonably good at both. I'm glad I discovered just how bad I was before leaving for Plymouth!

Anyway, highly recommended if you have any interest in navigation beyond your chart plotter, but after spending a great deal of time this weekend finding out why my latest workbook problem so very wrong -- and learning quite a lot in the process -- I got the hankering to take some backyard sights of my own.

One problem: unless you have a sextant with an artificial horizon attachment (I don't) or are willing to set up a pool of motor oil (a method of creating an artificial horizon... nope, not for me), there's no way to take a sight in your backyard, unless your backyard happens to contain a beach.

Or is there?

This line of thought lead me to whip together a simple astrolabe in the shop last night.

The first step was to print out a suitably nautical looking compass, and glue it to a board. I used a spray-on adhesive and MDF, but anything will do.

Compass glued to board

After letting the glue dry, I cut out the compass. You can use your bandsaw if you have one, or a jigsaw, or a coping saw. I used a coping saw and got a little exercise as a bonus.

Cut out compass
Next I drilled a hole in the center, a bit larger than the #6 machine screw that I found in my junk box. I wanted the disk to be able to rotate freely on the screw, without any wobble. The screw must be long enough to stick out an inch or two from the center of the compass.

I found a stick in my junk wood box, drilled a hole in one end, and mounted the compass to it using the aforementioned screw, a few washers, and a nut. I hand-tightened the nut so the compass turned smoothly.

Compass mounted on stick
Then I drilled a pilot hole for a small wood screw, exactly on the 90 degree mark. I then screwed in the round-head screw, leaving enough of the shaft out of the hole to hang a weight from a string.

The idea is to use gravity to align the astrolabe so 90 degrees is straight down. In effect, this creates your artificial horizon, automatically, even if your house is surrounded by trees, as mine is.

That's it!

This morning I went outside to take my first morning sun sight. Lucky for me, it was a nice bright morning.

First morning sun sight
As you can see, when you line up the astrolabe with the edge pointing towards the sun, the tall center post casts a shadow across the compass, allowing you to read the sun's altitude (Hs, in CelNav lingo) directly from the scale.

If you use your phone's camera to take the shot, you will not only record the angle, you will also record the time. In my case, 31.4 degrees (my estimate) at 8:37 watch time (EDT.)

This afternoon, weather gods permitting, I will take an afternoon sight and thus be able to work out a running 'fix' on my home's position.

Obviously, accuracy is going to be limited, but the calculations required are the same whether the sight is accurate or not, so the practice is the same.

Next time, I'll show the plotting and calculations required to work the sight.

Next Up: First Fix


  1. John,
    this is great :)
    Really love your approach ! I am either too lazy or maybe just too old to write a blog, but your writing helps me in my own preparations for trans Atlantic voyage and (maybe) a Jester chalenge as well. So there is a chance to shake hands perhaps one day :)
    Have a look at following two publications:

    1 ) "Pencil paper and stars" by Alaistar Buchan. A lot of good stuff there.

    2) "Thoughts, Tips, Techniques & Tactics For Singlehanded Sailing" by Andrew Evans. Also an excellent source of ideas & tips for singlehandlers.

    If you do not find them on www for download, let me know and I will send you a pdf copies.

    Montreal, QC

    1. Hi Zoran, I was able to find both PDFs on the web (thank you, Google.) I will take a look at them at the earliest opportunity.


  2. Woohoo! Nice one. Sort of a cross between an astrolabe and a mason's level.

  3. Nice looking, but as in all things navigational, the devil is in the details. The real measure of your celestial prowess is how close you get to your known position, otherwise why use an instrument at all? Squeezing the optimum accuracy out of your device requires that you:
    1. Make sure your "astrolabe" is round. Printers have a different mechanism for moving the paper (rollers) and the printing (inkjet head or laser beam on drum). They can be off quite a bit. Measure the circle with a ruler and adjust scaling so that it comes out round when you print.
    2. With your method, the position of the pivot and plumb mount have to be dead on. I would use the smallest diameter possible (think pin or needle) and place them at the center and on the actual scale.
    3. Being able to read with the greatest scale resolution (i.e., precision) would be helped by using a scale with ½ degree markings and also using a 180 degree scale instead of 360 (so the scale can be printed larger in your printer, maybe using legal paper). That way, you could measure the sun's shadow by turning your instrument around to face the other direction and take another reading, averaging the two. This should remove some of the systematic error due to placement of the pivots.

    Good luck on a fine project.

    1. Hi Philip,

      1. it never occurred to me that the printer might distort the circle. I will check it out.

      2. Getting the pins dead on isn't difficult if you work accurately. My center pin is clearly too thick -- the shadow covers 2 degrees. It's possible to roughly estimate the center of the shadow, so not as big a problem as it might seem at first glance, but a thinner pin would help.

      3. I wonder about bigger vs smaller scale... a bigger scale makes it easier to have finer gradations, but I wonder if larger distance will make the shadow spread more, offsetting the accuracy. Will have to experiment to find out.

      I do like your idea about reversing the instrument to take two readings. Excellent idea.

      Thanks for your very useful input.

  4. Very interesting! I look forward to your next installments on this.

    Best Regards,

    Kerrville, TX


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