Does the Moon go round the Earth?

Everyone knows the answer to this, which is what makes it worth asking. Yes — obviously — and it takes about a month. But ask the same question from the Sun's point of view and you get a different picture, and it is not the picture in most books. The Moon's path around the Sun never loops, never doubles back, and never even wiggles the wrong way.

Two answers, both correct it depends where you stand

"Goes round" sounds like a fact about the Moon. It is not. It is a fact about the Moon and the place you are watching from, and if you change the second one you change the answer.

Neither picture is the "real" one. There is no place in the universe that is standing still, so there is no view that counts as the true one — a point the site makes again about the length of a day, which also comes out differently depending on what you measure it against.

The surprising part the path never loops

Almost every drawing of "the Moon going round the Earth going round the Sun" shows a springy, looping, scalloped path — the Moon swinging out ahead of the Earth and then falling back behind it, like a coiled ribbon wound along the orbit.

The real path does nothing of the kind. It never loops back. It never even turns away from the Sun. It is a very slightly wobbly circle that curves towards the Sun at every single point along it, all year, forever.

The looping picture is not a simplification — it is a mistake, and it comes from one place: drawings put the Moon hundreds of times further from the Earth than it really is, because at the honest distance you could not see it. Stretch the Moon out that far and the loops appear. Put it back where it belongs and they vanish.

Why the Sun pulls the Moon harder than we do

Here is the fact that does it, and it is the one most people find hardest to believe:

The Sun is enormously further away, and gravity weakens fast with distance — but the Sun is so much heavier that it wins anyway. It pulls on the Moon about 2.2 times harder than the Earth does. Not slightly harder. More than double.

So if you insist on asking which body the Moon is really falling towards, the honest answer is the Sun, and it always has been. The Earth is a passenger it happens to circle on the way round.

The other way to see it: the Moon is never going backwards

Both the Earth and the Moon are carrying along Earth's orbit at about 29.8 kilometres every second. The Moon's own circling motion, the part that takes it round the Earth, runs at about 1 kilometre a second.

Those two numbers are why the loops cannot happen. The circling adds or subtracts about 1 km/s from 29.8 — so the Moon's speed along the orbit rises and falls a little each month and never comes remotely close to zero. To make a loop it would have to stop and reverse.

The gold line is the whole story: the Moon speeds up and slows down a little every month, and it is always, always going forwards. The forward motion beats the circling motion by about 29 to 1.

THE TWO REASONS ARE THE SAME REASON

Work out how big the monthly wobble would have to be before the path started curling back on itself, and the answer comes out at about 2.2 times bigger than the wobble actually is. That 2.2 is not a coincidence — it is the same 2.2 as the Sun's pull against the Earth's, arrived at from the other end. The Moon's path stays smooth by exactly the margin by which the Sun out-pulls us.

✋ Try it yourself — draw it honestly

The loops in all those pictures come from one exaggeration. You can find it with a piece of chalk, and the answer is that you cannot draw the thing at all.

Go outside with chalk, or use the biggest sheet of paper you have. Tie a string to a stick and draw a circle 40 cm in radius. That is the Earth's orbit round the Sun.
Now work out where the Moon goes. The Sun is 389 times further from us than the Moon is, so at this scale the Moon sits 40 cm ÷ 389 from the Earth — about one millimetre.
So the Moon's own path is a circle that wanders between 39.9 cm and 40.1 cm from the middle. Draw it on top of the first one.

You cannot. A chalk line is thicker than a millimetre, so the two paths come out as one line. The Moon's path round the Sun and the Earth's path round the Sun are, at any scale you can actually draw, the same circle.

That is the whole trick exposed. Every looping picture you have ever seen has the Moon drawn tens or hundreds of times too far out, because at the honest distance it would be touching the Earth on the page. The loops are an artefact of the exaggeration. They are not in the sky.

And a second one, indoors, in ten seconds. Walk briskly across the room while swinging one hand round and round in a slow circle. Watch your fingertip — not relative to your body, but relative to the floor.

If you walk fast enough and circle slowly enough, your fingertip never travels backwards across the floor. It surges and slows, but it always goes forward. Circle faster, or walk slower, and you can make it loop — and now you can feel exactly what the Moon is doing and why it does not loop. Its walking beats its circling by about thirty to one, so it is not even close.

One more twist the Earth goes round the Moon too

The Moon does not circle the centre of the Earth. The two of them circle their shared balance point — the spot where, if you could put them on a see-saw, they would balance.

Because the Earth is about 81 times heavier, that point sits much closer to us: about 4,670 km from the Earth's centre. And the Earth's radius is 6,371 km, so the balance point is inside the Earth — roughly three quarters of the way out from the centre to the surface. The Earth does not sit still while the Moon goes round; it swings in a small monthly circle about a point beneath its own crust.

Where it works out differently

Out at Pluto the sums come out the other way. Its large moon Charon is heavy enough, compared with Pluto, that their balance point falls in empty space between them. Neither one goes round the other; they both genuinely circle a bare point, locked facing each other forever. It is the clearest case in the solar system of a "moon" that is really half of a pair.

So what is the answer? all three of them

Does the Moon go round the Earth? Yes. Seen from here it circles us every 27 days, and that is a real motion, not an illusion.
Does the Moon go round the Sun? Yes. Seen from the Sun it sweeps a smooth near-circle once a year, and never once turns back.
Which is the real one? Neither, and that is not a dodge. "Goes round" needs somewhere to stand before it means anything. Physics is perfectly happy to answer the question once you say where you are standing, and refuses to answer it before.

Which makes this a small question with a big habit hidden inside it. Nearly every argument about what is really moving — including the one about whether the Earth goes round the Sun, which people were prepared to be imprisoned over — turns out to be a question about where the person asking has decided to stand.

The Moon's distance is measured, not assumed — see rung 2 of the ladder, where a card with a hole in it gets you most of the way there. The 389 on this page is the only number the whole argument needs.