NavList:

Dale, you wrote:
"I'm somewhat surprised not to find much discussion on Navlist about the astrolabe. Did it play no role in celestial navigation throughout history?"

Astrolabes (of a sort!) were used for some celestial navigation, but they were in the same category as cross-staffs. Results were mediocre, and they were not easy to use at sea.

The baroque, ornate astrolabes that sell for vast fortunes today were intended in earlier centuries as expressions and demonstrations of wealth. Although they had a variety of practical purposes built into their frequently "complex" designs including some fascinating mathematical details, it's clear from the intricacy of the engraving and the high-quality metalwork often included that these were not "tools". An astrologer, wishing to be paid big money for a detailed astrological horoscope by an emir or a king or other wealthy patron, might have preferred a beautiful instrument to impress and convince the "mark"... er, I mean, the "customer"... of the great value of the horoscope and the astrological expertise of the sidereal forecaster who wielded its powers. And a wealthy patron might also wish to acquire a finely-crafted astrolabe, too, in order to impress other "royalty" and suggest a "celestial" measure of approval on the regal reign. Suffice to say, astrolabes were beautiful because beauty sells.

What of navigation then? Enter the "mariner's astrolabe". That's how they're known to historians, at least, but it's an insult to the magnificent terrestrial astrolabes! These were tools, and very simple tools at that. Stick a standard scale of angles, like a common protractor on a disk. Mount an arm on a point of rotation at the center of the disk. Affix sighting peeps on that arm. Hang the disk from some sort of loose mounting point so that it may swing free enough to seek a gravitational vertical but not so freely that it oscillates with the motion of the ship. And that's about it! You now have a mariner's astrolabe.

What can you do with a mariner's astrolabe? You can measure the angular height of any star by direct observation, sighting through the peeps. Or you can measure the height of the Sun byt letting its light shine through the upper peep to the lower. This is a low-quality angle accurate to perhaps a degree or half a degree on a good day. Given the limitations of navigation and the limitations of astronomical data before about 1700-1750, that was good enough, and the mariner's astrolabe was competitive with the cross-staff and, a bit less so, the back-staff. You could get a rough latitude and nothing more.

There's a brief article on the concept of the mariner's astrolabe on the website of Royal Museums Greenwich which includes a photo of an example of the instrument. The descriptive "technical" section of the article seems incomplete, broken even. Maybe it's a remnant of an earlier text. And the article includes a fairly preposterous recent legend about the astrolabe being invented by Hypatia of Alexandria. Precious little is known reliably about the life or work of Hypatia. Here' the link:
https://www.rmg.co.uk/stories/topics/what-mariners-astrolabe.

So you can probably set aside the "mariner's astrolable" as a minor tool in celestial navigation history. Nonetheless, there's plenty of intriguing math to be found in the construction and engraving of the highly ornate (terrestrial) astrolabes that are more famous and far more valuable to collectors. Though not especially practical, they're worth exploring...

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA