# 3: Gravity

• 3.0: Introduction to Gravity
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.
• 3.1: Orbital Mechanics
Humans have been studying orbital mechanics since 1543, when Copernicus discovered that planets, including the Earth, orbit the sun, and that planets with a larger orbital radius around their star have a longer period and thus a slower velocity. While these may seem straightforward to us today, at the time these were radical ideas. Johannes Kepler elaborated on Copernicus' ideas in the early 1600's, stating that orbits follow elliptical paths, and that orbits sweep out equal area in equal time.
• 3.2: Moments of Inertia
Now that we've covered some planetary motion and mass basics, let's move on to planetary structure. Knowing the mass of a planet is the first step to learning more about its internal structure.
• 3.3: Geoid
We've been talking a lot about calculating the gravitational potential field. Now let's dive into what exactly that is.
• 3.4: Isostasy
• 3.5: Satellite-derived Gravity and Geoid
• 3.6: Summary