Communications satellites, spy satellites, weather satellites, the moon and the planets are all in orbits governed by the same equations.
Kepler worked it out from the data (empirical) but Newton discovered the underlying law (theoretical).
Gravitational attraction can provide the centripetal force necessary to hold a body in a circular orbit.
Equating the equations for gravity and centripetal force leads to Kepler's third law
\(T^2\over r^3\) = constant
Earth satellites can have many different orbits but all must rotate about the centre. Low orbits have the shortest time period.
Use flashcards to practise your recall.