Why planets stay in orbit — Astronomy, 14–17 years
An orbit is a continual fall shaped by motion. Gravity pulls an object inward, while its sideways speed carries it onward instead of letting it crash straight into the central body.
An orbit is a curved fall
A planet is always being pulled towards its star by gravity. At the same time, it is moving sideways. The result is that it keeps missing the star and follows a curved path around it.
The problem of falling objects
People needed to explain why the Moon does not fall to Earth and why planets do not fly away. Newton connected the falling of an apple with the motion of the Moon: the same gravity acts in both cases.
A simple orbital calculation
Earth travels about 940 million kilometres in one year. Divide by 365 days: about 2.58 million kilometres per day. Divide again by 24 hours: roughly 107,000 kilometres per hour along its path around the Sun.
It is not a perfect circle
It is tempting to draw every orbit as a circle because that is a useful first picture. In reality, most planetary orbits are slightly elliptical, and a planet moves faster when it is closer to its star.
Orbits in real life
Weather satellites, navigation satellites and the International Space Station all use carefully chosen orbits. Engineers calculate gravity and speed so a spacecraft passes over the right places or remains above Earth.
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