degree p<0: exterior resonances (|p+q|<|p|) degree p>0: interior resonances (|p+q|>|p|) order q>=0 Examples: resonance 2:3 is an exterior resonance given by p=-3, q=1, then |-3+1|:|-3| = 2:3 resonance 3:2 is an interior resonance given by p=2, q=1, then |2+1|:|2| = 3:2 trojans 1:1 are given by p=-1, q=0 resonance 1:2N means Neptune makes 2 revolutions and the particle 1 revolution. resonance 2:1N means Neptune makes 1 revolution and the particle 2 revolutions. critical angle: sigma = (p+q)*lambda_planet - p*lambda - q*longper lambda = longper + mean anomaly Minima of R(sigma) give the libration centers.