Gravity and Orbits

Gravity is the force of attraction between any two objects that have mass. Every single thing in the universe — you, this page, every planet, every star — pulls on everything else with gravity. The more massive an object is and the closer you are to it, the stronger its gravitational pull.

It is worth clearing up a common confusion between mass and weight. Mass is how much 'stuff' you are made of, and it never changes. Weight is how hard gravity pulls on that mass, so it changes from place to place. You have the same mass on Earth, the Moon, and Mars — but you would weigh different amounts on each, because their gravity is different.

Gravity is what keeps the planets orbiting the Sun, the Moon orbiting Earth, and satellites orbiting our planet. An orbit is a curved path caused by a balance between two things: the forward motion (speed) of the orbiting object and the inward pull of gravity. If the speed is just right, the object curves around without spiralling in or flying away.

A clever way to picture an orbit comes from Newton himself. Imagine firing a cannon from a very high mountain. Fire gently, and the ball lands nearby. Fire harder, and it lands further away. Fire hard enough, and the ball curves around the whole planet and never lands — it is in orbit! That is exactly how satellites stay up.

Albert Einstein showed in 1915 that gravity is not just a force — it is actually a curvature of space itself. Massive objects like stars bend the fabric of space around them, and planets follow these curves. This idea, called General Relativity, is one of the most important in all of science, and it has been confirmed many times by experiments and observations.

Gravity even shapes the largest things in existence. It pulled together the gas that formed the Sun and planets, it holds billions of stars together in galaxies, and at the extreme it creates black holes. From an apple falling to the birth of galaxies, the same simple force is at work everywhere.