Year 7 · Science · Earth and space sciences
Seasons, Moon phases and tides
Students model the Sun–Earth–Moon system to explain why Australia has summer in December, why the Moon's appearance changes over a month and what causes tides.
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Learning goals
Learners will
- Explain that seasons are caused by the tilt of Earth's axis as it orbits the Sun.
- Describe and explain the phases of the Moon.
- Explain that tides are caused mainly by the Moon's gravity.
Success looks like
- I can use a model to explain why the Southern Hemisphere has summer when the Northern Hemisphere has winter.
- I can name and sequence the main phases of the Moon.
- I can explain why there are usually two high tides a day.
The big idea
Earth orbits the Sun once a year. Earth's axis is tilted about 23.5°, and it stays pointing the same way in space all year.
For part of the year, the Southern Hemisphere is tilted towards the Sun. The Sun climbs higher in our sky, its light is more concentrated and days are longer, so we have summer (December to February).
Six months later, the Southern Hemisphere is tilted away. The Sun is lower, its light is spread over more ground and days are shorter, so we have winter (June to August). At the same time, the Northern Hemisphere has summer.
Seasons are not caused by Earth's changing distance from the Sun. Earth is actually closest to the Sun in early January, during the Southern Hemisphere's summer, but in the Northern Hemisphere's winter.
The Moon orbits Earth about once every 27 days, and it goes through a full cycle of phases about every 29.5 days. Half of the Moon is always lit by the Sun; the lunar phase we see depends on how much of that lit half faces Earth. The cycle runs: new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter, waning crescent.
Tides are caused mainly by the Moon's gravity pulling on Earth's oceans, making two bulges of water: one facing the Moon and one on the opposite side. As Earth rotates through both bulges, most coasts get two high tides and two low tides about every 24 hours and 50 minutes.
Aboriginal and Torres Strait Islander peoples have long linked the Moon's phases with tides and seasonal events, such as the timing of fishing and the gathering of foods.
Worked examples
Modelling the seasons
Use a lamp and a tilted ball to show why Australia has summer in December.
- Mark the equator and Australia on the ball. Push a skewer through it, tilted slightly from upright.
- Place the lamp in the middle of the room. Keep the skewer pointing towards the same wall at all times.
- Walk the ball in a circle around the lamp.
- At one point, the Southern Hemisphere leans towards the lamp: that is our summer. Halfway around, it leans away: that is our winter.
Answer: Summer happens when our hemisphere is tilted towards the Sun.
Phases of the Moon
A week after a new Moon, what phase would you expect to see?
- A full cycle is about 29.5 days, so each quarter of the cycle is about 7.4 days.
- One quarter of the way through the cycle is the first quarter Moon.
Answer: First quarter (half of the Moon's face appears lit).
Timing tides
High tide is at 6:00 am. Roughly when is the next high tide?
- Most coasts have two high tides in about 24 hours 50 minutes.
- Half of that is about 12 hours 25 minutes.
- 6:00 am + 12 h 25 min = 6:25 pm.
Answer: About 6:25 pm.
Practice check
Have a go, then check your answers. Each answer comes with an explanation.
Answer guide for parents
What causes Earth's seasons?
The tilt of Earth's axis as it orbits the Sun — The tilt changes how directly sunlight hits each hemisphere and how long days are.
When it is summer in Australia, what season is it in Japan (Northern Hemisphere)?
Winter — The hemispheres have opposite seasons.
Which phase comes straight after the full Moon?
Waning gibbous — After full, the lit part shrinks (wanes): waning gibbous comes next.
What mainly causes ocean tides?
The Moon's gravity — The Moon's gravity pulls the oceans into bulges; the Sun has a smaller effect.
Why do we see phases of the Moon?
We see different amounts of the Moon's sunlit half as it orbits Earth — Half the Moon is always lit; phases depend on how much of that half faces us. Earth's shadow only matters during a lunar eclipse.
In which month does winter begin in Australia?
june — Australian winter is June, July and August.
About how many days does a full cycle of Moon phases take? (whole number)
29 — The cycle takes about 29.5 days.
Most coasts have how many high tides in about a day? (number)
2 — Earth rotates through two tidal bulges, so most places get two high tides.
Watch out for
- The most common misconception is that summer happens because Earth is closer to the Sun. Earth is nearest the Sun in January, during the Northern Hemisphere's winter.
- Many think Moon phases are caused by Earth's shadow. That only happens during a lunar eclipse.
- Some think the Moon is only visible at night. It is often visible in daytime too.
Tips for parents
- Keep a Moon diary for a month: sketch the Moon each evening (or morning) it is visible.
- When modelling seasons, the key is to keep the axis pointing the same way the whole way around.
- Safety: never look directly at the Sun, and do not use a bare halogen globe, which gets very hot; an LED lamp or torch is safer.
Go further
- Check local tide times for a nearby coast and compare high-tide times with the Moon's phase over two weeks.
- Use a thermometer and a sunny wall to compare how warm surfaces get with the Sun at different angles.
- Find out how the Emu in the Sky, a dark-cloud shape in the Milky Way, is linked to emu breeding seasons in some First Nations traditions.