It sounds like this“The Earth sits still in the middle and the Moon goes round it.”
Thinking the Earth stands still while the Moon goes round
Why it feels right
From the ground, Earth feels solid and still, and the Moon visibly travels across the sky. Diagrams show a big Earth in the middle and a small Moon on a loop around it. Earth is so much heavier that, as a first approximation, it really can be treated as fixed. It is the simplest picture, and it is mostly right, which is why the correction surprises people.
Why it’s wrong
The Moon pulls on Earth exactly as hard as Earth pulls on the Moon, so Earth moves too. Earth is 81 times heavier, so it moves much less: the two circle a shared point about 4,670 km from Earth's centre, inside Earth, and Earth wobbles round that point once a month.
A worked example
Newton's third law says forces come in pairs: if Earth pulls on the Moon, the Moon pulls on Earth, equally hard and in the opposite direction. Equal pulls make the lighter body move more. Earth is about 81 times as massive as the Moon, so it moves 1/81 as much.
Two bodies orbit their common centre of mass. For Earth and Moon, that point is 1/82 of the way from Earth's centre to the Moon's: 384,400 km divided by 82 is about 4,670 km. Earth's radius is 6,371 km, so the shared point is about 1,700 km below Earth's surface.
So Earth doesn't sit still. Its centre circles that point once a month, a wobble astronomers can measure. For a much heavier pair, such as the Sun and Jupiter, the shared point lies just outside the Sun. For Pluto and its moon Charon, which are closer in mass, it lies in empty space between them.
How to catch yourself
- You describe the Moon as orbiting a fixed Earth.
- You think a smaller body pulls on a bigger one less.
- You draw only one of the two bodies moving.
What to hold instead
Earth and Moon pull on each other equally. They both move round a shared centre, about 4,670 km from Earth's centre, inside Earth. Earth moves less because it is far heavier, but it does move: its centre traces a small circle every month.
Where you’ll meet it
- Finding planets around other stars by the wobble they cause.
- Understanding why Pluto and Charon look like a double planet.
- Working out how the Sun wobbles because of Jupiter.
Questions people ask
- Does the Moon orbit the Earth, or the Earth the Moon?
- Both orbit their common centre of mass, which lies inside Earth, about 4,670 km from its centre. Because Earth is so much heavier, it is simpler to say the Moon orbits Earth, but Earth moves too.
- How do astronomers find planets by their wobble?
- A planet's pull makes its star circle a shared centre, so the star's light shifts slightly as it moves towards and away from us. Measuring that shift tells astronomers there is a planet and roughly how heavy it is. It is how the first planets around Sun-like stars were found.