12 May 2016

First 'Fix'

Last time, I built a very simple astrolabe to take some sun sights I could use to practice my celestial navigation (CelNav) calculations. So how did it perform?

I took four sights at 08:37, 13:25, 15:32, and 16:42 EDST. Since it was a partly cloudy day, I had to run out whenever I noticed the sun beaming brightly. Pretty much the same way you'd have to grab your sights onboard a small boat.

On a small boat, you could use these readings -- taken at different times of the day -- to generate a running fix. Since my backyard was seriously becalmed, I was saved the labor of 'advancing' the fixes from one time to the next, so the plotting was fairly simple.

To calculate the lines of position from the sights, I used the StarPath forms. All CelNav calculation forms are more or less the same; they differ in how they are organized. The StarPath forms are well organized, which helps when you aren't doing the calculations every day.

Here are the calculations for the first sight:

calculations for 08:37 sight
And here are the Lines of Positions (LOP) plotted for all four sights:

Plotted lines of position
As always, you can click on the images to get a closer look.

So, what are we looking at? First, the LOPs are labeled with their times, like 'LP 0837'. To make them easier to pick out, I've colored them red.

You can see that LPs 0837, 1325, and 1642 create a rather large 'cocked hat'. If you look closely inside the triangle, you will also see a dot surrounded by a circle. That is the plotted location of my backyard. I used geometry to locate the center of the cocked hat, and discovered it was a mere 8 nautical miles from my home.

Wow, right? From a crude, home-made astrolabe? Amazing!

Not so fast.

The good news: Yes, all the lines of position were in fact in the general vicinity of my actual position. That is, they were not in Kansas or Kyoto. Two of them (0837 and 1642) passed within five miles. But the 1325 line was almost 30 nm away. Not great.

Also, not surprising. If you look closely at the astrolabe, you will note that the shadow cast by the gnomon (the technical name for the center post) is 2 degrees wide. There are 120 minutes in 2 degrees, and each minute represents a potential error of 1 nautical mile. The only surprise is that the cocked hat is as small as it actually is!

Shadow cast by gnomon is 2 degrees wide!
Adding to the potential error was the fact that I only recorded the time to the nearest minute. If I remember correctly, the sun moves something like 16 miles in a minute.

However, I wasn't going for accuracy with this first set of readings. I was just hoping to see an observed position somewhere in my general neighborhood. I've definitely got that. Now it's time to refine my technique, and maybe even my astrolabe.

Next Up: Astrolabe Mark II


  1. Hi John, given the "crudity" (please forgive me if the term seems harsh) of your instrument and the fact you didn't take the reading to the second (yes, that is critical), I think you did remarkably well. Much better than my first ever celestial site, which put me in the middle of the Sahara Desert. It wouldn't be bad but I was on a ship in the Med, so I knew (and was glad to know) that the error was with me and not the navigation of the ship. I'll be curious to see how you improve the accuracy of your astrolab. Enjoy reading all your blogs! Regads, Jeff

    1. Oh, it's crude, that's for sure ;-)

      We're supposed to have rain over the weekend, but hope to have my Mark II Astrolabe ready for the next sunny day.

  2. You got the method down now all you need is a better instrument for taking your observations with. Go on ebay and search for "bubble sextant" the kind that were used in airplanes. Also check out our discussions among celestial navigators on navlist http://www.fer3.com/arc/


    1. Hi Gary, thanks for the idea. I enjoy taking a DIY approach to these kind of problems, which is why I'm playing around with the astrolabe. I have a sextant that I could use with a pan of oil or some other kind of artificial horizon, but sometimes its more fun doing things the hard way. And you learn more! I will definitely check out the navlist. Thanks for the URL.


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