2.2 · Why Planets Stay in Orbit

From Ohio 5th Grade Science Power Workbook · Chapter 2 · Lesson 2.2.

Big idea: Gravity between the sun and a planet keeps bending the planet's path, so it travels around instead of flying off.

Learning goals: Explain what keeps a planet in orbit

Describe the shape of a planet's path around the sun

Gravity does the holding

Gravity is a pull between objects. There is a pull between the sun and every planet, and between a planet and each of its moons.

A moving planet would travel in a straight line out into space if nothing pulled on it. The sun's gravity keeps changing its direction a little at a time, so the straight line becomes a loop.

The path is almost a circle

Earth's path around the sun is very close to a circle. It is not a long stretched loop.

Comets are the ones with long stretched paths. A planet's path is nearly round, which is why Earth stays about the same distance from the sun all year.

Worked examples

Question: A model shows a planet moving around the sun. What would happen to the planet if the sun's gravity suddenly stopped pulling on it?

Steps: Ask what gravity is doing in the model. It is bending the planet's direction toward the sun.

Remove that pull and nothing is bending the path any more.

A moving object with nothing pulling it keeps going in a straight line.

Answer: It would stop curving and travel in a straight line out into space.

Common mistakes

Gravity is not only on Earth

Because we feel gravity here, it is easy to think it is an Earth thing.

There is gravity everywhere there is matter. The sun's gravity reaches every planet, and a planet's gravity holds its moons.

Check your thinking: If something is orbiting, gravity is doing it. That works everywhere, not just here.

Orbits are nearly round, not long ovals

Lots of drawings stretch orbits into long thin ovals to fit them on a page. Real planet paths are much closer to circles.

That matters: because Earth's path is nearly round, Earth stays about the same distance from the sun all year.

Check your thinking: If a picture shows a planet swinging far in and far out, it is a comet's path, not a planet's.

Remember

A ball on a string

Swing a ball on a string and it goes in a circle. Let go and it flies off straight. The string is doing what the sun's gravity does — always pulling inward, always bending the path.

Practice tip

Remember: Say what gravity does, not just that it exists.

Tip: Full-credit answers on this topic name the pull and then say what it does to the path: it keeps bending the direction, so the object goes around instead of straight.