Tuesday, October 20, 2020

Table Top Testing Astrolabe 2.0

My first attempt at crafting an astrolabe was all about creating a paper mockup of a traditional device. For 2.0, I focused on functionality. Here's what I arrived at:

I started with templates from the Astrolabe Project. Instead of making the one double sided device, I opted to laminate two separate pages. And rather than use a traditional rule, I made use of a piece of heavy duty-fishing line. The line, when held taught, is far easier to measure with than the crude rules I previously made.

By removing the rules, specifically the alidade, I removed the ability for the astrolabe to serve as its own inclinometer. I made up for this by hacking together a separate inclinometer out of $0.50 protractor, a straw and more fishing line. I was inspired by tutorials like this one.

While primitive, this device does a far better job of measuring elevation than my first astrolabe did.

But enough talk; how does the new device perform? To determine this, I came up with a five question quiz. Below are my answers, and authoritative ones from timeanddate.com.

Question Astrolabe's Answer TimeAndDate.com's Answer Error
When does the Sun rise on January 18th? 7:15 AM 7:24 AM 9 minutes
How long is the day on August 4th? 14 hours 14 hours, 1 minute 1 minute
When does twilight begin on October 11th? 7:20 PM 7:01 PM 19 minutes
What's the maximum angle of elevation the sun will reach on March 20th? 53° 51.3°
At what time of day is the Sun due West on February 9th? Never. Sun sets at 255° Never. Sun sets at 252°

As you can see, I got off to a slow start by missing sunset by 9 minutes. And the twilight answer is off too, though in hindsight I think this was a bad question. The astrolabe has a single dashed line that represents twilight, but timeanddate.com has 3 different twilight values. So I'm not even sure what I'm measuring there.

The other measurements, to me, are impressive. I predicted a random day's length to 1 minute, a maximum sun elevation to 2 degrees and a sunset to 3 degrees.

This is only a few of the types of questions the astrolabe can answer. There are variations on these solar questions that involve predicting direction, elevation, time and date from known variables. And then there are questions involving stars that I didn't even begin to explore. The same can be said about the scales to measure height and distance of objects.

In short, it's truly remarkable how much value you can get out of two sheets of paper and some string.

Next up are a battery of field tests. I'm curious if the device's usability and accuracy holds up once I've left the cocoon of my kitchen. And from there, I hope to answer the ultimate question: is this device a practical tool, or just an impressive historic footnote?

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