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Astronomy & space · Grades 5–9

Solar-system structure and scale

The Sun is a star at the centre of the planetary system. Planets, moons, small bodies, gas and dust form a gravitationally interacting system.

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Prerequisites

Fractions, ordering and basic gravity.

Study notes

Architecture

The Sun is a star at the centre of the planetary system. Planets, moons, small bodies, gas and dust form a gravitationally interacting system.

Categories

A planet, a moon and a star are different kinds of object. A moon orbits a larger nonstellar body; stars sustain fusion during their main-sequence phase.

Distance units

An astronomical unit is a defined length close to the mean Earth-Sun distance. Light-years also measure distance, not time.

Scale models

One scale rarely shows both body sizes and orbital distances conveniently. State what is scaled and what has been exaggerated.

Worked examples

What kind of astronomical object is the Sun?

The Sun is the star of the solar system.

A star

What does a light-year measure?

It is the distance light travels in a year under the defined convention.

Distance

Check your understanding

  1. In a model where 1 AU is 2 cm, how far is an object at 5 AU from the origin?

  2. Two model orbit radii are 3 AU and 9 AU. Find the larger-to-smaller radius ratio.

  3. Which statement is correct about a moon?

    1. It must orbit Earth
    2. It must make its own visible light
    3. It can orbit a planet
    4. It must be larger than its planet

  4. Why can a solar-system poster be misleading if sizes and distances use different scales without labels?

Answers and marking guidance

Answers and marking notes
  1. 10 cm

    Apply the stated scale: 5×2.

  2. 3

    9/3=3.

  3. It can orbit a planet

    Moon is a natural-satellite category, not a synonym for Earth's satellite alone.

  4. Viewers may infer false size-to-separation relationships. Explicit scale labels and exaggeration notices prevent that confusion.

    Credit the two scales and the misleading inference.

Further reading and review references

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