Astrolabe accuracy - sea AND land?

[Naphtali]

I recently read an article about Columbus' voyage of discovery that included a bunch of information with which I am unfamiliar and ignorant. Apparently Columbus located his ship by using an Astrolabe in conjunction with the sun. But apparently an Astrolable's accuracy/precision is insufficient on its own. A Portuguese astronomer-mathematician, Rabbi Abraham Zacuto, created a "nautical almanac" whose tables either corrected [some sort of] errors or imprecision inherent in Astrolabes. I am marginally familiar with sextants and theodolites as devices that work in concert with chronometers to accurately locate the user.

What the heck is an Astrolabe? How does it work, especially in the absence of precise identification of the time?

What is an Astrolabe's standard of accuracy?

What does a nautical almanac do? Does one nautical almanac [tables] function correctly worldwide, or are distinct almanac-tables necessary for different locations, or longitudes, or latitudes?

Would the nautical almanac Columbus relied upon be functional today?

Columbus used an Astrolabe on the ocean. Since it is keyed to the sun, is it functional on land?

 

[Rathdown]

There are two types of astrolabes-- one for use on land, the other for use at sea. All you will probably ever need to know (or possibly want to know) about astrolabes can be found by looking it up on wikipedia.

 

[Stanley Doble]

They work on land or at sea but it is easier on land as the land does not heave and rock under you. Early explorers used them both places. Some years back in Canada, an astrolabe was found that supposedly belonged to explorer Samuel De Champlain who lost it in the early 16th century.