Gravity, orbits, days and seasons

Learning objectives

By the end of this topic you should be able to:

• explain what the force of gravity depends on and calculate weight on other worlds
• explain how gravity keeps planets, moons and satellites in orbit
• explain day and night, years, leap years and the phases of the Moon
• explain the seasons and changing day length using Earth's tilt
• analyse shadow and gravity data and design models
• discuss satellites and changing the clocks

Before you start

You will use these skills. If any feel shaky, review them first.

• the solar system (see The solar system)
• forces, mass and weight (see Newton's laws of motion)
• ratios and rearranging W = mg

Key vocabulary

Gravity

The force of attraction between any two masses.

Gravitational field strength (g)

The force of gravity on each kilogram, in N/kg.

Orbit

The curved path of one object around another, held by gravity.

Axis

The imaginary line Earth spins around, tilted at 23.5°.

Hemisphere

Half of Earth: northern or southern.

Phase

The shape of the lit part of the Moon that we see.

Understanding the ideas

1. Gravity and weight

   Every mass attracts every other mass. The force is larger for bigger masses and smaller at greater distances. Weight is the force of gravity on an object: W = m × g, where g is 9.8 N/kg on Earth but only 1.6 N/kg on the Moon.

2. Orbits

   Planets, moons and satellites stay in orbit because gravity constantly pulls them towards the object they orbit while they move sideways at high speed; they keep "falling around" it.

3. Days, years and the Moon

   Earth spins once a day, giving day and night, and orbits the Sun once every 365¼ days, so we add a leap day every four years. The Moon orbits Earth about once a month, and we see different amounts of its sunlit half as phases.

4. Seasons

   Earth's axis is tilted at 23.5°. When a hemisphere is tilted towards the Sun, the Sun is higher, its light is more concentrated and days are longer: it is summer. When it is tilted away, it is winter. Distance from the Sun is not the cause.

5. What does it connect to?

   This topic links to forces and Newton's laws, the solar system, climate and weather, and later to satellites and space exploration.

Seasons and day length

• Cause: Earth's 23.5° tilt, not its distance from the Sun.
• June: north tilted towards the Sun, so northern summer, long days, high Sun and short shadows.
• December: north tilted away, so northern winter, short days, low Sun and long shadows.
• Opposite hemispheres have opposite seasons.
• Weight: W = m × g; mass stays the same everywhere.

Gravity and seasons in the real world

Weather and communication satellites orbiting Earth help forecast monsoons and floods. Farmers plan planting around the seasons. Architects design buildings to let in low winter sunshine but shade out the high summer Sun.

Worked examples

Assessment tips

Questions on this topic often use diagrams of Earth's orbit, shadow data and planet data. Expect to:

• Explain seasons, day length and Moon phases correctly.
• Calculate weight and mass using W = mg.
• Explain orbits using gravity.
• Analyse data and design models.

Common mistakes: saying seasons are caused by distance from the Sun; saying there is no gravity in space; saying the Moon's phases are caused by Earth's shadow; and confusing mass with weight.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.