Relative gravity is an attempt to identify the structure of universe where we live, using Newtonian gravity, Coulomb's law and special relativity, along with the basic mathematics.
Two co-ordinate system.
A point say (a,b) in X-Y axis, and (a', b') in X'-Y' axis. where X'-Y' axis are rotated an angle θ from X-Y
a' = a * cos(θ) + b * sin(θ);
b' = b * cos(θ) - a * sin(θ);