It sounds like this“Something has to keep pushing the Moon forward, or it would slow down and drop.”
Thinking something must keep pushing the Moon along
Why it feels right
Everything we push on Earth stops when we stop pushing: a bike, a trolley, a ball rolling along the ground. Friction is so constant that it hides itself, and 'motion needs a push' becomes simply how the world seems to work. Aristotle taught it, and it stood for about 2,000 years. So an object circling for billions of years looks as if it must be pushed by something.
Why it’s wrong
In empty space nothing slows the Moon, so nothing has to push it. Once moving, it keeps its speed by itself (Newton's first law). Gravity only changes its direction, not how fast it goes, so no engine is needed.
A worked example
Slide a puck across a table and it slows because of friction. Do it on smooth ice and it goes much further. On an air hockey table, further still. Each time you take away friction, it keeps going for longer. Galileo reasoned that with no friction at all it would keep going for ever.
In space there is almost no friction. At the Moon's height there is no air at all. So the Moon, once moving at about 1,020 metres per second, keeps moving at that speed. Gravity pulls at right angles to its motion, so it turns the Moon but doesn't speed it up or slow it down. No engine is needed, and none has ever existed.
The same is true of a spacecraft. A rocket is needed to climb into orbit and reach speed. Once up there at the right speed, the engines can be switched off, and the craft keeps going round for years.
How to catch yourself
- You ask 'what keeps it going?' about something moving in a vacuum.
- You think engines are needed to stay in orbit.
- You assume every moving thing is being pushed.
What to hold instead
Motion needs no push. An object keeps moving at the same speed and in the same direction unless a force acts on it. In space there is nothing to slow the Moon, and gravity only turns it. Pushing is needed to change speed, such as a rocket getting up to orbit, not to keep an orbit going.
Where you’ll meet it
- Seeing why satellites coast for years with their engines off.
- Understanding why a puck slides further on smoother ice.
- Reading Newton's first law, and why Aristotle was wrong about motion.
Questions people ask
- Why doesn't the Moon slow down?
- Nothing slows it. There's no air at its height, and gravity pulls at right angles to its motion, changing its direction but not its speed. The Moon is in fact drifting away from Earth by about 3.8 centimetres a year, because of tides, but that is a tiny effect.
- What is Newton's first law?
- An object stays at rest or keeps moving in a straight line at constant speed unless a force acts on it. It replaced the older idea, from Aristotle, that things need a constant push to keep moving.