Gravity is something that we have been dealing with literally all our lives. At first glance, the concept seems pretty straightforward. In lay terms, it is an invisible force that pulls us towards the center of the Earth. But why does this happen? How do we explain it? How does it affect both the Earth and other objects in the solar system? These are just some of the questions that we will explore in this chapter.
Geophysics has several branches devoted to answering these questions. Geodesy deals with the function of Earth's gravitational field and the shape of the planet. The figure to the right shows the Earth's true oblate spheroid shape. Planetary physics studies both orbital mechanics and planetary structure. Orbital mechanics, which we will discuss in the next section, studies the planet's tilt, precession, and spin. Planetary structure, as the name suggests, deals with the Earth's layers and their thickness, composition, and density.
Through exploring these fields, we will examine the Earth's long term climate cycles, Newton and Kepler's laws, and mass anomalies in the Earth. By the end of this chapter you should feel comfortable with explaining planetary orbital properties, the moment of inertia and its relation to planetary layering, gravitational potential, isostasy, the Earth's shape, and why mass anomalies occur.