Big Questions

1 · Why don't the planets fall into the sun — or simply fly away?

Take both halves of the question seriously, because both are good physics. The sun really is pulling every planet toward itself, constantly, and nothing is holding them up — so falling in is a fair expectation. And a planet really is moving fast enough that, with no gravity, it would sail off in a straight line and never come back — so flying away is a fair expectation too.

The answer is that both things are happening at once, and they exactly cancel. A planet is falling toward the sun at every moment, and moving sideways at every moment, and by the time it has fallen the sun is no longer where it was falling toward. It falls, and misses. Forever. The shape traced out by falling and missing without end is an orbit — not a balance between gravity and some outward force, because there is no outward force. Mercury, pulled about 6.7 times harder than Earth, is not in more danger; it simply moves faster — 47.9 km/s against Earth's 29.8 km/s — which is exactly what the stronger pull demands.

The full answer, with Newton's cannonball Try to make a planet fall in Watch them all not falling

If falling and moving sideways make a circle, what exactly decides the shape — and could you build an orbit yourself? Next question ↓

2 · How do gravity and motion work together to create an orbit?

Two arrows, one rule. At every instant a planet has a velocity — where it is already going — and gravity adds a small pull straight toward the sun. Gravity never pushes the planet along its path and never slows it on a circle; it only bends the direction. Bend a path by the same small amount at every step and it closes into a circle. That is all an orbit is: motion, endlessly redirected.

The balance is exact and it is not a coincidence — it is a filter. At any distance from the sun there is one speed at which the bending exactly matches the curve of a circle (the square root of the pull times the distance). Too slow, and the path dips closer to the sun, speeding up as it falls; too fast, and it swings wide, slowing as it climbs; either way it traces an ellipse and comes back. Anything that was moving far too slowly hit the sun billions of years ago, and anything far too fast left — so what remains, from Mercury at 47.9 km/s to Neptune ambling at 5.4 km/s, is exactly the set of things whose speed fits their distance. The asteroid belt obeys the same rule rock by rock: it stays up for the same reason Jupiter does.

Set a distance and a speed, and watch Why the asteroid belt stays up too The Earth and Moon doing exactly this

A system where everything's speed fits its distance sounds fragile. So what happens if you change one part of it? Next question ↓

3 · What happens when you change one part of the system?

The satisfying, surprising answer: the orbit changes shape, not just speed. Slow a planet down at one point and it does not spiral into the sun — it drops onto a longer ellipse that swings closer in and then climbs right back out to where you touched it, arriving at exactly the speed it had before. Speed it up and the far side of the orbit lifts away instead. To actually drop something into the sun from Earth's distance you would have to strip away almost all of its 29.8 km/s, which nothing in nature is able to do.

Real changes are gentle and slow — tides towing the moon outward a few centimetres a year, drag on low satellites, sunlight itself nudging small asteroids — with one violent exception: an impact, which changes an orbit all at once but only by as much momentum as it delivers. That is why NASA's DART test mattered, and why the gaps in the asteroid belt exist at all: Jupiter, tugging the same rocks at the same points over and over, is the solar system's longest-running demonstration of what changing one part of the system does to the rest.

Slow a planet down and watch the shape The gaps Jupiter carved in the belt What could really slow a planet down

All of that is the view from above. Seen from where you are standing, the same motions become something much more familiar: the seasons. Next question ↓

4 · Why do we have seasons?

Not because the Earth gets closer to the sun — that is the most common wrong answer in science, and it fails an easy test: when it is summer in the United States it is winter in Australia, on the same planet, at the same distance. The real cause is that the Earth's axis is tilted by 23.4°, and keeps pointing the same way all year while the planet goes round. For half the orbit the northern half leans toward the sun — long days, high sun, summer — and half a year later it leans away, and the light falls on the other hemisphere instead.

Nothing about the Earth changes between June and December; only which end leans toward the light. You can watch the whole mechanism run: the tilt holds still through the entire orbit in the system view, the day/night line leans and straightens through the year on the map, and your own town's daylight stretches and shrinks on its sun page — the same 23.4° seen three different ways.

Watch the tilt hold still for a whole orbit The day/night line leaning with the year Your own town's daylight, month by month The seasons lesson, ready to teach

The sun is half of your sky. The other half is the moon — which refuses to look the same two nights running. Next question ↓

5 · Why does the moon change phases?

The moon makes no light of its own. It is a ball of rock with the sun shining on it, so — like every ball in every light — one half of it is always lit and one half is always dark. The phases are nothing happening to the moon: they are your changing view of that lit half as the moon goes round the Earth. Between the Earth and the sun, its lit side faces away from you: new moon. On the opposite side of us from the sun, the lit side faces you square on: full moon. Everything between is between.

One lap of that cycle, full moon to full moon, takes about 29.5 days — which is where "month" comes from. And a fair follow-up question dissolves a common fear: if the moon passes between us and the sun every month, why isn't there an eclipse every month? Because its orbit is tilted about 5° — most months it passes a little above or below the exact line, and the phases carry on undisturbed.

Tonight's moon, for your own town Scrub a whole month of phases Why not an eclipse every month

The moon does one more thing to you every day — you just have to stand on a beach to notice it. Next question ↓

6 · How are the sun, moon, Earth and tides connected?

The moon's gravity pulls on the Earth — and pulls hardest on whatever is nearest. The ocean on the moon's side of the planet is tugged a little more than the Earth's centre, and the centre a little more than the ocean on the far side, so the water stretches into two bulges: one toward the moon and one away from it. The Earth then turns underneath both bulges once a day, which is why most coasts see two high tides and two low tides in roughly 25 hours.

The sun does the same at a little under half the strength, and the two either team up or argue. When sun, moon and Earth line up — full moon and new moon — the bulges stack and the tides swing hard (spring tides). When the moon sits at right angles to the sun — the half moons — they partly cancel, and the tides go quiet (neap tides). Check it against reality: the tide chart for any station swings biggest a day or two after full and new moon, and the moon page for a coastal town says which regime you are in tonight.

A real tide chart for a real coast Sun, Earth and Moon in one picture How tide predictions are worked out

Our one moon raises the ocean twice a day. So what is it like to be a planet with dozens of them? Next question ↓

7 · Why does Jupiter have so many moons?

Because gravity's reach grows with mass, and Jupiter has more of it than everything else that orbits the sun put together — about 318 times the mass of the Earth. Around every body there is a region where its own pull beats the sun's, and Jupiter's is enormous: room enough to keep the four great moons Galileo spotted in 1610 and to trap passing objects that wandered in too slowly to leave. That is why its moons come in two families — large, orderly ones that formed there in a disc, the way the planets formed round the sun, and small, far-flung captured ones on tilted, backwards orbits that still remember being asteroids.

Every one of them obeys the rule from the top of this page: each moon moves sideways at exactly the speed its distance from Jupiter demands, faster close in and slower far out. The simulator draws 97 of them, each on its real orbit at its real pace — and the innermost visibly lapping the outermost is question one, answered again in miniature.

Jupiter and its moons, moving Every planet's family, compared The captured ones' cousins: the belt

Which brings the chain back to its start — everything up there is falling and missing. The only question left is yours.

The eighth question is yours

These seven have answers. The best question is the one you ask next — the one that starts "but wait, what about…" halfway through a simulator. If it can be computed from real motions and real measurements, there is a good chance it can be built and shown.

If a class asks it and we build it, the class is credited on the page — that is a standing offer, and how it works is written down.

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