Eastern Pacific sea surface temperature (SST) and mean equatorial Pacific thermocline depth are key variables in El Niño - Southern Oscillation (ENSO). A linear fit to observations leads to a remarkably simple picture: ENSO can be represented by a classical damped oscillator, with SST and thermocline depth playing the roles of momentum and position, respectively. An independent fit of observed relationships between western and eastern thermocline depth, central wind stress and eastern Pacific SST yields the same picture and supports a recharge oscillator interpretation. The oscillation arises from the interaction between the recharge time of the Warm Pool and the time delay between east and west Pacific. Both finite Kelvin wave speed and SST dynamics contribute to the time delay. Including seasonality in the description, we find two periods of relative instability: boreal spring, with a large phase progression, and autumn, with nearly stationary phase.