An astrolabe was a horrible device to use. The navigator hung it from his left thumb to get it vertical. Then he looked directly at the sun over the sighting arm and moved the arm up or down to point directly at the sun and get a reading. What with the rolling and pitching of the ship, there was little hope of an accurate sighting. Add to that the direct sun viewing burned out many navigators’ corneas and it was a horrible job.
“The navigator hung it from his left thumb to get it vertical. Then he looked directly at the sun over the sighting arm and moved the arm up or down to point directly at the sun and get a reading. “
I don’t buy it. Any navigator with half a brain would flip it over and use the shadow of his thumb to do the same thing. No need to look at the sun. Sounds like the sort of speculated usage some historians came up with by totally guesswork. Experimental archeology is disproving a lot of that sort of junk these days. Reconstruct a device and give it to some non-morons to figure out and they usually can come up with something that makes more sense than the ‘conventional wisdom’.
That is what I was thinking, with the pitching and rolling of a ship in the ocean how do you really get an accurate reading? I mean dead calm, okay. But with the bobbing up and down, I would think difficult.