Question

This specifies a locations distance north or south to the equator.
a - longitude
b - latitude

Answer 1

i think longitude 
i hope this is correct

Answer 2

the latitude defines the position as being north or south while the longitude defines the position as being east or west I don't quite understand your question but if you are asking which one defines the location as north or south then the answer is latitude, hope you understand it by reading my answer