Long Term Timing Observations of PSR B1509-58

We present an updated phase-coherent timing solution for the young, energetic pulsar PSR B1509-58 for twenty years of data. Using a partially phase-coherent timing analysis, we show that the second frequency derivative is changing in time implying a third frequency derivative of (-9+/-1)x10^-32 s^(-4). This value is consistent with the simple power law model of pulsar rotation.