Earth's oblateness and its temporal variations

The oblateness is a fundamental property of the Earth under stable rotation. Different oblateness parameters can be defined, each having its physical significance. We retrace the main developments in concept as well as in measurement toward a qualitative understanding of the oblateness and particularly of J(2), the so-called oblateness coefficient that 'glues' together distinct concepts of oblateness. We examine the numerical values and discuss the dynamical and geometrical consequences of the oblateness. Finally, we report the recent findings of the variations in J(2), including an outstanding multi-year anomaly that occurred since 1998, leading to a discussion of possible cause(s) in terms of large-scale mass transports in the Earth's atmosphere-hydrosphere-cryosphere-solid Earth-core system.