16 May 2016

Astrolabe Mark II

I had some exciting news last week: the WoodenBoat Store will soon be carrying my book, "An Unlikely Voyage". The store is run by WoodenBoat Publications -- the same company that brings us WoodenBoat Magazine, the WoodenBoat School, and the WoodenBoat Show at Mystic, CT. I'm really proud they are carrying it. The book is not in stock yet, but will be soon... probably in a few weeks.

Next, I spent a lot of time last week experimenting with the Astrolabe Mark I -- enough to discover some serious flaws that I wanted to fix before moving on. The biggest problem being the shape, diameter and length of the center post.

The most obvious flaw was the post's diameter. It was too wide. Wide enough so its shadow was two degrees wide on the scale. I took my first set of sights by taking readings from where I thought the center of the shadow was, but clearly that was just a guess. I then tried to improve the center post by grinding the end into a point. But that had its own problems, illustrated in the video below...

Note to self: always take videos in landscape mode!

Anyway, after grasping the fundamental flaw in the center post, I decided to build a better astrolabe: the Astrolabe Mark II!

The main feature of the Mark II is a short, thin center post, made from a 3-penny nail. The short post means the astrolabe is pointed closer to the sun when the post's sharp point is positioned on the scale.

Mark II Astrolabe with short, thin center post
The thin nail wasn't strong enough to mount the disk on, so I moved the mounting post to the top of the astrolabe, drilling a hole right on the 90 degree mark.

The weight, again, was mounted on the bottom. The top-mount gives the weight more leverage, so it doesn't have to be as heavy.

Astrolabe is now mounted at the top, rather than the center
The other improvement I mean to try this week was suggested by Philip Sadler: to "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."

To facilitate this double reading, I found a scale that goes from 0 to 90 degrees on both the right and left sides. That way I can take direct readings on both sides, without having to do any math. With a better astrolabe and Philip's improved technique, I expect much better results this week. If only the sun will cooperate!

Next Up: Mark II Astrolabe Results


  1. Such complications that cleverness collects! Can you tie a knot in a piece of string? If so - Enter, stage left, the kamal….

  2. Never heard of a kamal but it sound intriguing. I will google it. Thanks!

    1. As I said, I never heard of a Kamal, so thanks for the tip. I looked it up on the Internet and found some interesting info on it.

      The goal of my building an astrolabe is to to take sightings in my backyard, so I can practice the CelNav calculations. It really doesn't matter if the readings are accurate or not, but once I got involved in building it, of course I wanted to make it better. That's why it's getting complicated (not really.)

      Two problems with a Kamal: 1) you need to be able to see the horizon, which I can't, and 2) you need to look directly at the celestial object, which is fine for stars, but a bit tricky with the sun.

      The horizon problem is why I turned to the Astrolabe, which doesn't need the horizon.

      The Kamal is still very interesting, though. I'll have to remember if for another time. Thanks again.

  3. Hi John. Thank you for sharing your experiences with us. I kindly ask your permission to translate Celestial Navigation posts into Turkish to share with my friends who are looking for some resources to start. Hope it is okay for you.


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