Exploration (people being able to explore unknown continents and countries) was aided by astronomy (the stars showed direction) and the development of what instrument? Is it the compass?

Answer 1

Since the compass is magnetic and has nothing to do with astronomy, I think the instrument you want is explained here:

http://www.astrolabes.org/

Answer 2

http://en.wikipedia.org/wiki/Sextant

(Compasses have to do with Earth's magnetic fields: http://en.wikipedia.org/wiki/Compass )