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The solar system, gravity and moons

Grades 3–4 · 45 minutes · Describe the pattern. The driving question: Why does everything out there orbit — and why don't the planets fall into the sun? Every step below is a link that opens the exact view — one link puts the projector and every student screen on the same sky. Part of the lesson plans by topic and grade.

Before the lesson — what this assumes

Assumes the class knows the planets go around the sun — nothing more.
Pre-work (10 minutes, alone or in pairs): open the solar system simulator, press Play, and try two zoom buttons. Arriving already knowing the controls buys the lesson ten minutes.
Worth a look for the teacher: the classroom guide — projector setup and what each control does.

The plan — every step carries its minutes

0–5 · HookAsk what they already know: how many planets, which is biggest, what keeps them from flying away? Take guesses, write three on the board to check later. Then open the inner planets, live on the projector: this is where Mercury, Venus, Earth and Mars actually are today — not a poster, a computation from the real orbits.

5–12 · The tour outWalk the zoom ladder outward one rung at a time — to Mars, to the asteroid belt (turn the belt layer on), to Jupiter, to Saturn, out to Neptune. Name the order as you go. At the last stop, point out what happened to the inner planets: all four are now a labelled knot, and Mercury's whole orbit is a few pixels. The evenly-spaced diagram in their heads is the thing this picture corrects.

12–15 · The discStill on the whole system: drag the tilt slider flat, so the class is looking edge-on. Every orbit collapses into one thin line — the solar system is a disc, not a jumble. Ask why that might be (it all condensed from one spinning cloud), then tilt back to the overhead view.

15–20 · Orbits in motionJupiter's view with a year on the slider. Press Play. Mercury laps four times while Earth goes round once and Jupiter barely moves. Ask: who's fastest? Who's slowest? What's different about them? Land the rule: closer to the sun → pulled harder → moves faster. That pull is gravity, and it is steering, not sucking — nothing falls in.

20–27 · The moonsZoom to Jupiter's four big moons with a month on the slider and Play: Io whips round in under two days while Callisto takes over two weeks — the same rule again, with Jupiter now playing the sun's part. Then Saturn's rings and moons. Expect “is that real?” — and give the honest answer: the sizes, distances and speeds are real; where each moon sits along its orbit right now is not solved, and the read-out says so. A picture that tells you what it fakes is how real science behaves.

27–38 · Group activityGroups at their machines, journals open, a 10-minute timer on the projector (Space starts it). Tasks on the board: (1) Find the asteroid belt — whose orbits is it between? (2) Count Jupiter's big moons, then Saturn's, from the moon views. (3) On the year view, decide which planet is fastest and which is slowest — and write the WHY in one sentence. (4) Open the Jupiter page or the Mars page and copy down two facts — including what you would weigh there.

38–43 · Wrap-upChart paper: fastest planet and why; two differences between inner and outer planets; check the three guesses from minute one against what was watched. The rule on the wall in their words: closer means faster, and gravity is the reason.

43–45 · The closerEnd on the Earth and Moon, to true scale — the one view where size and distance are both real. If you brought the marble: Earth is the marble, the peppercorn moon stands 48 cm away, and the sun would be a 1.7 m ball 188 m down the corridor. Send them out the door with that walk.

The minutes are there for the teacher's pacing — and so that any single step can be handed to a student as their own five-minute lesson. See students teach the class.

What they should find — the teacher's key

Order of the eight planets, walked rather than memorised; rocky inner vs gas-giant outer.
Edge-on, the system is a disc — one spinning cloud's leftover shape.
Closer means faster, at the planets and again at Jupiter's moons: one rule, two scales.
Materials: projector, machines in pairs, journals, chart paper; optionally a marble and a peppercorn.
Younger classes: stop after the moons and make the tasks find-and-point. Older: add the flight path from the high-school lesson.

Standards: Builds toward 5-ESS1-2; the same ground sits under MS-ESS1-2/3 for older classes.

What the picture fakes: On every system view the planet dots are drawn far larger than scale, and where each moon sits along its orbit is illustrative — sizes, distances, speeds and directions are real, positions are not solved, and the pages say so.

Questions to chase on your own

For the student who wants more — each answerable with the tools, no teacher required:

How many moons does Saturn have big enough to be drawn? Count them on the Saturn view, then find how many are known in total on Saturn's page.
What is between Mars and Jupiter? Turn the belt on and name the four biggest things in it.
Which planet has the shortest year, and how short? Check your answer on its page.

Questions the curious actually ask

Real questions, mostly from real kids — the kind that sound simple and open trapdoors. Worth raising in class before someone raises them for you:

If gravity pulls everything, why doesn't the moon fall on us?
Here's the secret: it is falling — right now, continuously — and missing. The moon moves sideways so fast that by the time it has fallen toward Earth, the Earth's surface has curved away beneath it. An orbit is a fall that never lands. Newton figured this out by imagining a cannon on a mountain firing faster and faster until the cannonball fell all the way around the world — and that thought experiment is every satellite, the space station, and the moon.

Why don't the planets ever crash into each other?
Mostly they can't — each is locked in its own lane by its own speed, and the lanes are separated by distances that make the planets themselves like grains of sand miles apart. But here's the part worth knowing: early on, they did crash. The leading theory for where the moon came from is a Mars-sized world hitting the young Earth. The quiet, orderly system in the simulator is the survivor of a demolition derby — the orbits you see are the ones that lasted.

How does the sun keep burning? There's no air in space!
Because it isn't burning — fire needs air, and the sun would have burned out in a few thousand years if it were a bonfire. The sun is crushing: its own gravity squeezes its core so hard that hydrogen atoms fuse together, and fusion releases millions of times more energy than fire. It has run this way for 4.6 billion years and is about halfway through its fuel. Nothing is on fire. Something much stranger is happening.

Go further — beyond this site

NASA Space Place: All About the Planets — per-planet reading at this band's level.
NASA Solar System — the reference the fact-hunt can be checked against.

Same question, other grades

Each grade band re-asks this topic's question one level deeper — observe it, describe the pattern, measure it, explain the mechanism, quantify it and question the model. This page is the describe the pattern rung.

Grades K–2 Grades 5–6 Grades 7–8 High school & up All lesson plans

Teachers: make this lesson better

You are the one standing in front of the class, so you will see what we cannot: a task that runs long, a question that lands better another way, a grade level pitched wrong, a topic we should build next. Tell us — improvements go into the page, and if we use yours, your class gets the credit on it, the same promise the classroom request form makes.

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