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ELI5 Yahaaa

Falling forever

Why Planets Orbit the Sun

A planet is not held by a string. It is falling — and always missing.

A planetSideways speedThe Sun's pull
Falling and moving sideways at once traces a circle.

  1. Step 1 of 5

    The Sun pulls on everything near it.

    Enormous mass Feels the pullGravity pulls straight toward the Sun

    The Sun holds 99.8% of all the mass in the solar system. Its gravity reaches out past every planet and tugs each one toward the middle.

  2. Step 2 of 5

    So why doesn't Earth just fall in?

    EarthPulled toward the SunRacingsideways 30km/s

    Because Earth is also travelling sideways — about 30 kilometres every second. It is moving across the pull, not into it.

  3. Step 3 of 5

    Falling plus sideways speed makes a curve.

    1 Moving straightahead2 Pulled inward3 Path bends intoa loop
    An orbit is a fall that keeps missing the ground.

    Every second, the Sun's gravity bends Earth's path a little toward it. Every second, Earth's speed carries it past. The result is a permanent curve — an orbit.

  4. Step 4 of 5

    Closer planets have to hurry.

    Mercury's year88 daysEarth's year365 daysNeptune's year165 years

    Near the Sun the pull is stronger, so a planet must move faster to keep missing. Mercury races round in 88 days. Neptune plods round in 165 years.

  5. Step 5 of 5

    Nothing keeps it going — that's the point.

    On Earth Things stop — air andground rubIn space Nothing rubs, so motionnever stops

    Space has no air to slow things down. Once a planet is moving, it just keeps moving, so the orbit runs for billions of years with no engine.

The short version

An orbit is falling sideways fast enough that you keep missing what you are falling toward.

Try it yourself

Swing a ball on a string over your head, then let go. It flies off straight — the string was doing gravity's job.

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