The Solar System, Gravity & Orbits — Print · Grade 6 Science

Chapter: Earth, Moon, Sun & Space Models

The Solar System, Gravity & Orbits

Learning goals

I can state what the strength of gravity between two objects depends on.
I can explain why an orbiting satellite does not fall to Earth even though gravity acts on it.
I can explain why the outer planets take so much longer to orbit the Sun.

Readings for this unit

One Sky, Several Mechanisms

Before this unit

Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.

Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.

Every mass attracts every other mass. The strength depends on the two masses and on the distance between them, and it falls very quickly as distance grows: double the separation and the force drops to a quarter.

That is why you notice only Earth's gravity in everyday life. The attraction between two people standing a metre apart exists and is around a hundred-millionth of a newton, far too small to detect without specialised apparatus, while Earth's mass of about 6 x 10^24 kg produces a force you cannot ignore.

The Sun holds about 99.8 per cent of all the mass in the solar system, so its gravity dominates everything. Each planet moves sideways fast enough that it falls around the Sun rather than into it, which is all an orbit is.

Build your understanding

Where this fits

The final chapter uses the field idea from the magnetism lesson, where a force acts across empty space, and applies it to the largest scales in the course. This lesson explains what holds the solar system together; the next three deal with what Earth's motion and the Moon's produce in our sky.

Use three flexible 40-45-minute sessions: what gravity depends on; why an orbit is continuous falling; the solar system and why outer planets are slower, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.

Part 1 · Mass and distance

Gravitational force grows with each mass and shrinks with distance, and the distance effect is the one that matters most for understanding the solar system. Doubling the distance quarters the force, tripling it leaves a ninth, and so on, which is the same inverse-square pattern as light spreading out from a lamp. The force never reaches zero, however far apart two objects are, which is why distant stars still tug on the solar system very slightly.

Two consequences are worth drawing out. First, your weight is the gravitational force between you and Earth, so it changes if either factor changes: a 60 kg person weighs about 588 N on Earth and about 98 N on the Moon, where the mass is far smaller, while their mass of 60 kg is identical in both places. Mass is how much matter there is; weight is a force, and confusing them is the commonest error in this topic. Second, because the Sun's mass is roughly 330,000 times Earth's, its pull reaches across billions of kilometres and still dominates the motion of Neptune. The inverse-square pattern is worth recognising because it keeps appearing. Light from a lamp, sound from a source and the gravitational pull of a mass all spread over the surface of an expanding sphere, and since that surface grows with the square of the radius, the intensity falls the same way. Magnetism in chapter 4 fell faster still, with the cube of distance, because a magnet has two poles that partly cancel at a distance rather than a single source.

An orbit is falling and moving sideways at the same time, which is the idea that makes gravity's role make sense. The note gives the thought experiment that tests whether a learner has understood it.

Pause and explain

A 60 kg astronaut travels to the Moon. What changes and what does not?

Check your reasoning after answering

Their mass stays at 60 kg because mass measures the amount of matter, which does not change with location. Their weight falls from about 588 N to about 98 N, because weight is the gravitational force between them and the body they stand on, and the Moon's mass is far smaller than Earth's. The astronaut has not lost any matter.

Part 2 · An orbit is continuous falling

Imagine firing a cannonball horizontally from a mountain top. Fire it gently and it curves down and lands a short distance away. Fire it harder and it lands further off, and because Earth's surface curves away beneath it, each faster shot travels further before meeting the ground. Fire it fast enough, about 7.9 km per second near Earth's surface, and the ground curves away exactly as fast as the ball falls, so it never lands. That is an orbit, and the reasoning is Newton's.

This resolves the usual confusion about astronauts floating. They are not beyond Earth's gravity: at the International Space Station's altitude of roughly 400 km, gravity is still about 90 per cent of its strength at the surface. They float because they and their spacecraft are falling together, at the same rate, in the same curved path, so there is nothing pushing them against a floor. The sensation is identical to the moment of a lift dropping, extended indefinitely, and it is why the correct term is free fall rather than zero gravity. The cannonball argument also sets a limit worth knowing: an orbit low enough to graze the atmosphere cannot last. The Space Station at about 400 km still meets enough thin air to slow it gradually, losing roughly 2 km of altitude a month, so it must be boosted back up several times a year or it would re-enter within a couple of years. Orbits are stable only where there is effectively nothing left to push against.

Pause and explain

Astronauts on the Space Station float although gravity there is about 90 per cent of its surface strength. Why?

Check your reasoning after answering

Because they are falling along with their spacecraft. Both are in continuous free fall around Earth at the same rate and along the same curved path, so there is no floor pushing up against them and nothing to produce the sensation of weight. Gravity is acting the whole time; it is what curves the path into an orbit.

Part 3 · Why the outer planets crawl

A planet further from the Sun has further to travel and moves more slowly while doing it, and both effects lengthen its year. Earth orbits at about 150 million km and takes 365 days. Jupiter is about five times further out and takes nearly 12 years. Neptune, about thirty times further than Earth, takes about 165 years, so fewer than one full Neptunian year has passed since the planet was discovered in 1846.

The slower speed follows directly from the weaker pull. Gravity at Neptune's distance is roughly a nine-hundredth of its strength at Earth's distance, so far less force is needed to hold the planet in its path, and a planet moving faster than its orbit required would simply swing outward until it settled. Earth travels at about 30 km per second and Neptune at about 5.4. The pattern is so regular that the orbital period can be predicted from the distance alone, a relationship Johannes Kepler extracted from observations in 1619, long before anyone could explain why it held. Distance in the solar system is hard to hold in mind, so one comparison helps. If the Sun were a ball 1 m across, Earth would be a grain 9 mm wide at 107 m away, and Neptune would be a 4 cm marble 3.2 km distant. Almost everything in the solar system is empty space, and spacecraft journeys reflect it: Voyager 2 took 12 years to reach Neptune after launching in 1977.

Pause and explain

Why does Neptune take 165 years to orbit the Sun when Earth takes one?

Check your reasoning after answering

Because it has a much longer path and travels it more slowly. Neptune is about thirty times further from the Sun, so its orbit is far larger, and the Sun's pull at that distance is roughly a nine-hundredth of its strength at Earth, so a slower speed of about 5.4 km per second is enough to hold it in orbit against Earth's 30.

Part 4 · What else gravity does out there

The same force shapes much more than planetary orbits. It pulled the solar system together in the first place from a cloud of gas and dust, and it is why large bodies are round: beyond a certain size, gravity overwhelms the strength of rock and pulls everything towards the centre equally. Small asteroids are irregular lumps because their gravity is too weak to do this, and a body's shape is therefore evidence about its mass.

Gravity also acts between the planets, not only between each planet and the Sun, and the effect is measurable. In the 1840s astronomers found Uranus was not quite where calculations predicted, and rather than abandon the theory they asked what unseen mass could account for the discrepancy. Urbain Le Verrier calculated a position for a hypothetical planet, and Neptune was found within a degree of it in 1846. A prediction that precise, made from a small mismatch in a table of positions, is one of the strongest demonstrations in science of what a good theory can do. The same method is used today to find planets around other stars. A planet's gravity makes its star wobble very slightly, and that wobble shifts the star's light in a way instruments can detect, so the planet is inferred from its effect rather than seen. Several thousand planets have been identified this way and by watching stars dim slightly as a planet passes in front, which is the indirect reasoning from the Earth-interior lesson applied across light years.

Pause and explain

How was Neptune's existence established before anyone saw it?

Check your reasoning after answering

From the discrepancy in Uranus's orbit. Uranus was not quite where gravitational calculations placed it, and instead of discarding the theory astronomers asked what additional mass would produce exactly that error. Le Verrier calculated where such a planet must be, and Neptune was found within a degree of the predicted position in 1846.

Apply the whole unit · Reason about orbits

A satellite orbits 400 km above Earth's surface and completes one orbit every 92 minutes. A second satellite is to be placed 36,000 km up, where one orbit takes exactly 24 hours so that it stays above the same point on the equator.

Your task

Explain the difference in orbital period, and explain why the higher satellite can be used for television broadcasting.

What your response should show

Explains the longer period using both path length and weaker gravity
Connects the 24 hour period to Earth's rotation period
Draws the practical consequence of a fixed dish
Explains specifically why tracking the low satellite is impractical

Response guidance — after your attempt

Explain why the higher satellite takes longer to complete an orbit.
Explain what staying above one point on the equator means for an observer on the ground.
Explain why the lower satellite cannot be used in the same way.

The higher satellite has a far longer path to travel, and the Sun is not the relevant mass here: Earth's gravity at 36,000 km is far weaker than at 400 km, since the distance from Earth's centre is roughly six times greater and the force falls with the square of distance, so it is about a thirty-sixth as strong. A weaker pull holds the satellite in a slower orbit, and combined with the longer path the period stretches from 92 minutes to 24 hours. Because 24 hours is exactly the time Earth takes to turn once, the satellite keeps pace with the ground beneath it and appears to hang motionless above one point on the equator. That means a dish on a roof can be aimed once and never moved again, which is what makes television broadcasting practical. The lower satellite completes nearly 16 orbits a day and crosses the sky in minutes, so any receiving dish would have to track it continuously and then switch to another satellite as it set, which is far more complicated and expensive.

Scientific references for this unit

Parent / teacher teaching notes

Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.

The key idea

Gravity is an attraction between any two masses, and it keeps planets in orbit for the same reason it makes an apple fall. An orbiting object is falling continuously; it simply moves sideways fast enough that it keeps missing what it is falling towards.

Worked examples

Example 1 · Evaluate an orbit explanation

Explain which model is better and how a no-gravity prediction would differ.

Identify forward motion and changing direction.
Connect the change with gravity.
Name a scale or interaction omitted by the sketch.

Answer: A is the better basic explanation. Gravity changes the moving satellite’s direction rather than vanish outside the atmosphere. Without the modeled force, its motion would not follow the same curved path around Earth. The drawing still omits precise speed, distance scale, and other influences needed for a full numerical prediction.

Example 2 · Transfer to a different case

An illustration shows the Sun and planets at equal sizes. Must their actual diameters be equal?

Identify the distinction and conditions in this new case.
Use the relevant mechanism or quantity; preserve units and the chosen boundary.
Explain why the answer fits and what further evidence would be needed.

Answer: No. It may emphasize order or relationships. Use labeled diameter data and a stated scale rather than infer real sizes from an unscaled sketch.

Example 3 · Explain gravity as an interaction · a focused example

Why can gravity matter in space?

State the literal condition or observation in the example.
Apply the distinction explained in this teaching part.
Check whether the claim stays within the given evidence.

Answer: It acts across distance between masses, so a lack of air or contact does not eliminate the interaction.

Avoid these mistakes

Misleading claim: Gravity stops at the top of the atmosphere.
Misleading claim: A bright reflected object must be a star.
Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.

Check your understanding

Why is brightness an incomplete classification test?

Discuss the answer

Distance, size, reflection, and emission affect appearance. Classification needs the body’s nature and relationship.

Why can gravity matter in space?

Discuss the answer

It acts across distance between masses, so a lack of air or contact does not eliminate the interaction.

What is missing from “an orbit has no gravity”?

Discuss the answer

Gravity supplies the changing direction in the basic orbit model; the statement removes the mechanism it needs.

May the sketch be used to calculate actual distance?

Discuss the answer

Only if its scale and geometry support that purpose. Otherwise use a labeled distance record.

Sustained scientific explanation and revision

These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.

Activity 1 · The Solar System, Gravity & Orbits · Model and explain

Assigned reading and preparation

Read One Sky, Several Mechanisms and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For The Solar System, Gravity & Orbits, study Parts 3–4 and attempt the worked case: Explain which model is better and how a no-gravity prediction would differ. Explain the steps before opening the model.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

A is the better basic explanation. Gravity changes the moving satellite’s direction rather than vanish outside the atmosphere. Without the modeled force, its motion would not follow the same curved path around Earth. The drawing still omits precise speed, distance scale, and other influences needed for a full numerical prediction.

Activity 2 · The Solar System, Gravity & Orbits · Practice, revise and exit

Assigned reading and preparation

Read One Sky, Several Mechanisms and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For The Solar System, Gravity & Orbits, complete practice questions 1–6 and both choice checks, then answer independently: An illustration shows the Sun and planets at equal sizes. Must their actual diameters be equal? Save a correction explaining one changed claim or method.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

No. It may emphasize order or relationships. Use labeled diameter data and a stated scale rather than infer real sizes from an unscaled sketch.

Words to know

orbit: A curved path arising from motion and gravitational interaction around a system.
star: A body that emits light through its internal energy processes, such as the Sun.
evidence: A relevant observation, measurement, or record used to evaluate a claim.
system boundary: The chosen limit specifying what is included and which exchanges cross it.
uncertainty: A stated limit in measurement or knowledge, distinct from a careless mistake.

In the real world

Exit check

An illustration shows the Sun and planets at equal sizes. Must their actual diameters be equal?

Check your explanation after answering

No. It may emphasize order or relationships. Use labeled diameter data and a stated scale rather than infer real sizes from an unscaled sketch.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.

Problem 1

Why is brightness an incomplete classification test?

Answer and explanation

Distance, size, reflection, and emission affect appearance. Classification needs the body’s nature and relationship.