Kepler’s Laws and Orbital Motion
- Earth & Space
- Ages 14–18 · Grades 9–12
What you’ll find out
- Planets move in ellipses with the Sun at one focus, sweeping equal areas in equal times.
- The square of a planet’s period is proportional to the cube of its distance from the Sun.
- Newton’s law of gravitation explains Kepler’s laws and governs satellites and spacecraft.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Kepler’s three laws
In the early 1600s, the astronomer Johannes Kepler analysed years of precise observations of Mars. He discovered three laws of planetary motion. First, each planet travels in an ellipse, a slightly squashed circle. The Sun sits at one of two special points inside it, called foci. Second, a planet speeds up as it approaches the Sun. It slows down as it moves away. Third, the farther a planet is from the Sun, the longer its orbit takes, following a precise mathematical rule. Kepler’s laws allowed astronomers to predict planetary positions far more accurately than before. They took him many years of calculation.
Newton explains the orbits
Later that century, Isaac Newton showed why Kepler’s laws work. Every object attracts every other with a force of gravity. The force gets weaker with distance. A planet is moving sideways very fast while the Sun’s gravity pulls it inward. The result is a curved path that keeps missing the Sun. Newton imagined a cannon on a very tall mountain. Fired faster and faster, a cannonball would land farther and farther away. At a high enough speed, Earth’s surface would curve away as fast as the ball fell. Then it would orbit. Satellites and moons orbit in exactly this way.
Curriculum links: NGSS HS-ESS1-4 · England KS4: Physics · Cambridge IGCSE: Physics · NCERT Physics, Class 11