Falling and orbiting

If the Moon continued along a straight line, it would move away from Earth. Instead, its path curves. In Newton’s description, Earth’s gravity provides the force that changes its direction.

Newton’s description

Newton’s law says that masses attract one another. Increasing either mass strengthens the force; increasing the distance between their centres weakens it. Double that distance and the force falls to one-quarter.

Earth pulls the Moon, and the Moon also pulls Earth. Both move around a shared balance point, called their centre of mass. Here, the diagrams keep Earth fixed to make the Moon’s motion easier to follow.

Why the Moon keeps missing Earth

Imagine releasing an object with no sideways motion relative to Earth. It falls towards the surface. Give it some sideways speed and it travels farther before landing.

With enough sideways speed, and no atmosphere or obstacles, its falling path can curve around Earth without reaching the ground. That is an orbit.

In this view, centred on Earth without rotating along with the orbit, gravity supplies the inward force. The object’s existing motion carries it forwards as gravity turns its path. An outward force balancing gravity would remove that turn and leave a straight path.

A real rocket builds this sideways speed during launch. It initially climbs away from the ground, then gradually turns so that more of its thrust increases its speed along Earth’s surface. Reaching a great height is not enough on its own: without sufficient sideways speed, the spacecraft’s path brings it back down.

The same law at different distances

The Moon is about 60 Earth radii from Earth’s centre. At that distance, Earth’s gravitational acceleration is roughly 3,600 times weaker than at the surface. It is still sufficient to bend the Moon’s path over the course of its orbit.

Newton’s law is accurate enough for many orbital calculations. Later in the series, Einstein’s theory will give us another way to understand this same motion.