When were the Alt-az system and the RA-dec system first used?
I remember answering a question on the origin of declination last year (Why is declination positive in the northern hemisphere?) which led me to discover this paper on the origin of celestial co-ordinate systems. In it they seem to indicate that the ancient Greeks were the first to use Right Ascension to describe locations in the sky around 300 B.C. Whereas Alt-Az co-ordinates (or variants of) have been around much longer because the horizon is an easy point of reference to start from. The first documented use comes from Babylonian times but the ancient Egyptians are also thought to have used a similar system as far back as 2000 B.C.
Astrolabe, somehow can be taught as recent sky apps in which you can anticipate the exact position of stars, a very older version of course.But sextant is actually a tool for measuring angles (altitude of sun or any other star).
These definitions should not be considered so strict. Since ancient astronomers probably have used astrolabe as a tool for their research as well.