# Orbiting Cannonballs

An orbit is the curved path taken through space by one body around another due to the gravitational force between them. This path is often regularly repeating, for example the Earth moving around the Sun. A classic way to understand orbits is to consider throwing a ball horizontally on Earth. The ball will accelerate down due to gravity, coming to rest on the ground having covered some horizontal distance. Over small distances, the Earth appears flat, but at larger scales we must account for the curvature of the planet. If we throw the ball fast enough, by the time the ball would have landed on a flat plane, the Earth’s surface will have curved away, leaving the ball stuck in perpetual free-fall; this is a body in orbit. The velocity needed for a small mass to orbit the Earth near the surface is around 8

When we look at an orbit where one body has much more mass than the other (for example the International Space Station orbiting Earth), it may appear like the larger body is unaffected and remains stationary. However, both bodies are in fact orbiting around their shared center of mass. The shape and speed of an orbit can be described using Kepler’s Laws of planetary motion.

**Figure 1:** Imitation of Newton's famous drawing.