19 May 2016

Good As It Gets?

My original motivation for building an astrolabe was to brush up on my celestial navigation. This is a skill that quickly fades if you don't practice, and I was definitely out of practice. But after a week or two of doing the calculations, I'm back to speed. Yesterday it took me about 15 minutes to 'clear' the three sights, and plot the 'cocked hat' on a plotting sheet.

So as a form of practice, building an astrolabe is both fun and useful. Highly recommended.

Now, I started off knowing that I could only expect so much accuracy from such a crude instrument, but naturally I wanted to get the most precision possible. Partly, that involved learning what corrections I should and shouldn't apply to the sights -- the corrections are different from those needed for sights taken with a sextant. But it also involved learning how to use the astrolabe itself in the most effective way possible. I think I've now done that, and yesterday's sights are probably about as good as it's going to get.

As discussed previously, to get the most accurate sights with an astrolabe, you need to keep the face of the instrument as parallel to the rays of the sun as possible. If they are directly parallel, then the gnomon will not cast a shadow on the face, so you must turn the face ever so slightly towards the sun. This allows the gnomon to cast a shadow, but also increases the reading slightly.

I say slightly, but even a quarter of a degree will throw your readings off by 15 miles.

Anyway, I did my best with yesterday's sights. Here are the raw data, and the plotted LOPs:

11:40:17     21° 30'
15:15:47     61° 30'
20:44:50     35° 30'

The date was 18 May 2016. All times in GMT.

Best you can do with a homemade Astrolabe?

This time, the cocked hat was to the north of my actual position, and the 'fix' was roughly 50 nm away.

Reminding ourselves that an error of 1 degree will throw the fix off by 60 miles, and that the resolution of the instrument is about 1 degree, I believe this is about the best you can hope for with such a simple device.

I think Magellan would have been thrilled with such precision, and it is certainly good enough for practicing your CelNav calculations, which are the hard part of doing celestial navigation.

It's also ideal, I think, for someone who wants to learn CelNav, without the expense of buying a sextant.

However, I actually own a sextant, so the next time the Sun cooperates, I am going to try the 'pan of dark liquid' trick of taking a sextant sight in your backyard, just to compare the results.

I hope this series of blog posts has inspired at least a few readers to try it themselves. Let me know if you have any questions about building your own astrolabe, or doing the calculations!

Next Up: Wooden Boat Show

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