Gravity and orbits
Physics · Key Stage 3 · The School
Gravity is always pulling
Every planet is constantly pulled toward the Sun by gravity. That force never switches off and never weakens to zero. If a planet were moving in a straight line with no gravity acting on it, gravity would pull it off that line and curve its path toward the Sun. An orbit is the curved path that results.
Falling, but always missing
Imagine firing a cannonball sideways from a very tall tower. Gravity pulls it down as it travels, so it follows a curved path and lands some distance away. Fire it faster, and it lands further away, because it travels more sideways distance before gravity brings it down the same amount. Fire it fast enough, and the curve of its fall matches the curve of the Earth itself — so it keeps falling, and keeps missing the ground, forever. That is exactly what an orbit is.
Why closer planets move faster
Gravity is stronger the closer you are to the Sun. A planet close to the Sun needs to move faster sideways to keep "missing" it in the same way a farther planet does with a weaker pull. Mercury, the closest planet, orbits in about 88 days. Neptune, the farthest, takes about 165 years — both are doing the same basic thing, at very different speeds.
A satellite launched into orbit is deliberately given huge sideways speed, often around 8 km/s close to Earth. Gravity pulls it toward the Earth the entire time, exactly as it would pull anything else. But the satellite is moving sideways so fast that by the time gravity has pulled it down a little, it has already moved far enough sideways that the curve of the Earth has dropped away beneath it by the same amount. It falls constantly and hits the ground never. Cut the sideways speed in half and gravity wins: the satellite spirals in and eventually falls to Earth for real.