I haven't gotten to the library yet. Meanwhile, I can't figure this out.
Everyone knows he did something brilliant to the astrolabe but if you look it up online every website says it was something else.
He was the first to make an iron astrolabe. No, he was the first to make a copper astrolabe. No, he was the first to make a mariner's astrolabe. No, he was the first to make a spherical astrolabe.
Astrolabes are beautiful things and I would love to be able to walk into class with one and say, “This is a Zacuto astrolabe, and here is how you use it.” But what was it?
Taking the average, it appears that he was the first to make the mariner's astrolabe out of metal instead of wood, thus preventing it from warping.
I hope to post some final answer to the question, as well as a useful application of the mariner's astrolabe for the use of high school students. Check back.