What you will learn
- how Earth’s rotation produces day and night, and why the Sun appears to “move” across the sky,
- how Earth’s axial tilt (not its distance from the Sun) causes the seasons,
- the cause of the Moon’s phases and the difference between a lunar and a solar eclipse,
- how the Moon’s gravity produces two tidal bulges on Earth,
- why eclipses do not happen every month.
On 21 December, the Southern Hemisphere is tilted towards the Sun.
- Sunlight hits Melbourne at a steeper angle — the same beam of light covers less ground, so more energy per square metre.
- Days are longer in summer (more hours of sunlight).
- The atmosphere has less time to cool down overnight.
Result: hot summer days even though Earth is actually a little further from the Sun in December than in June.
Key idea: it is the tilt, not the distance, that causes seasons. The tilt causes both steeper sunlight and longer days for the tilted-towards hemisphere.
1. Day, night and the Earth’s rotation
Earth spins on its axis once every 24 hours. At any instant half the Earth faces the Sun (day) and half faces away (night). From the ground, the Sun appears to move east to west because we are rotating west to east.
The axis is tilted at from the vertical, and this tilt stays pointed in the same direction in space all year as Earth orbits.
2. Seasons
Earth takes about days to orbit the Sun once. Because of the axial tilt:
- When the Southern Hemisphere is tilted towards the Sun → Southern summer (Dec/Jan/Feb).
- When it is tilted away → Southern winter (Jun/Jul/Aug).
- When the tilt is sideways to the Sun → spring or autumn (equinoxes around March and September).
At the equinoxes (around 21 March and 23 September), Earth’s tilt is sideways to the Sun.
- The Sun is directly overhead at the Equator.
- The terminator (line between day and night) passes through both poles.
- Every place on Earth gets roughly hours of daylight and hours of night.
The word “equinox” means “equal night”.
3. Phases of the Moon
The Moon orbits Earth about once every days (a “lunar month”). Half the Moon is always lit by the Sun, but we only see the lit side from certain angles.
- New Moon: Moon between Earth and Sun — we see the unlit side.
- First quarter: a quarter-orbit later; we see half lit.
- Full Moon: Earth between Moon and Sun — we see the fully lit side.
- Last quarter: three-quarters of the way around; we see the other half lit.
In the Southern Hemisphere:
- A waxing Moon (getting fuller, first half of cycle) looks like a backwards D (curve on the left).
- A waning Moon (getting thinner) looks like a backwards C.
In the Northern Hemisphere the shapes are reversed because the observer is “upside down” relative to the ecliptic.
Key idea: the phase tells you the angle between Sun, Earth and Moon — not a change in the Moon itself.
4. Eclipses
An eclipse happens when Sun, Earth and Moon line up.
- Solar eclipse — Moon is between Sun and Earth. Moon’s shadow falls on Earth. Happens at New Moon.
- Lunar eclipse — Earth is between Sun and Moon. Earth’s shadow falls on the Moon. Happens at Full Moon.
Eclipses do not happen every month because the Moon’s orbit is tilted about from Earth’s orbit around the Sun. Most months the Moon passes above or below the Earth-Sun line rather than through it.
5. Tides
The Moon’s gravity pulls the water on Earth’s surface slightly towards it. This creates a bulge of water on the side facing the Moon. A second bulge forms on the opposite side (because the solid Earth is pulled away from the water on the far side).
As Earth rotates, each point passes through two bulges and two low regions — so most coasts see two high tides and two low tides each day (every h min).
- Spring tides (largest range): Sun and Moon aligned (New or Full Moon).
- Neap tides (smallest range): Sun and Moon at right angles (First/Last quarter).
At New Moon the Sun, Moon and Earth line up.
- Moon’s gravity pulls water towards it — creates the main tidal bulge.
- Sun’s gravity adds a smaller bulge in the same direction.
- Combined bulge is higher; the opposite (far-side) bulge is also larger.
Result: a bigger high tide and a lower low tide — a spring tide. Spring tides are why king tides happen around a New or Full Moon.
Key idea: tides depend on the alignment of Sun and Moon, not how close they are.
Practice: Year 7
Tier 1: recall and identify
- How long does Earth take to spin on its axis? How long to orbit the Sun?
- Why do we have day and night?
- Explain in one sentence why Earth has seasons.
- What is the angle of Earth’s axial tilt?
- Name the phases of the Moon in order starting from New Moon.
- How long is one lunar month (from New Moon to New Moon)?
- What lines up during a solar eclipse? At what phase does a solar eclipse happen?
- What lines up during a lunar eclipse? At what phase?
- How many high tides does most of the Australian coast see each day?
- What is a spring tide?
Tier 2: explain and reason
- Explain why summer days are longer than winter days.
- Why are temperatures hottest about a month after the summer solstice, not on the solstice itself?
- Explain why a solar eclipse does not occur at every New Moon.
- Sketch the relative positions of Sun, Earth and Moon at (a) Full Moon, (b) First quarter.
- Why does a lunar eclipse turn the Moon reddish rather than simply making it disappear?
- Explain why tides still occur on lakes but are much smaller than ocean tides.
Tier 3: apply to a novel context
- If Earth’s tilt were instead of , describe two ways life would differ.
- On a planet with no moon, would tides still exist? Justify.
- A sailor plans a week-long coastal trip and wants the biggest tidal range to explore tidal pools. Which phase of the Moon should she pick?
- Mars has a tilt similar to Earth’s () but takes about days to orbit the Sun. Predict how seasons on Mars would differ from Earth’s.
Challenge
Harder reasoning
- The Moon is slowly moving away from Earth (about cm per year). Predict what will eventually happen to (a) the length of a day on Earth and (b) the size of tides, and justify each.
- Some years have a “blue Moon” — two Full Moons in the same calendar month. Explain why this is possible, using the -day lunar cycle.
- If the Moon’s orbit were in exactly the same plane as Earth’s orbit around the Sun, how often would solar eclipses occur? Explain.
- Design a classroom model using a torch (Sun), a ball (Earth) and a smaller ball (Moon) that shows why we see lunar phases. State one limitation of the model.
Answer key
Attempt the practice first. When you're ready to check, expand the answers below.
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Year 7 answers
Tier 1: recall and identify
- hours to rotate; about days to orbit the Sun.
- Earth rotates on its axis; at any moment half faces the Sun (day) and half faces away (night).
- Earth’s axis is tilted , so each hemisphere gets more direct sunlight and longer days during its summer.
- .
- New Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter, waning crescent.
- About days.
- Sun, Moon, Earth in a line (Moon between Sun and Earth). Happens at New Moon.
- Sun, Earth, Moon in a line (Earth between Sun and Moon). Happens at Full Moon.
- Two high tides (and two low tides) per day.
- A tide with a larger than usual range, occurring at New or Full Moon when Sun and Moon are aligned.
Tier 2: explain and reason
- The hemisphere tilted towards the Sun sees the Sun rise earlier and set later because more of its latitudes are on the sunlit side of the Earth during rotation.
- The ground and oceans take weeks to heat up, so peak temperature lags the peak of sunlight. This is called seasonal lag.
- The Moon’s orbit is tilted about from Earth’s orbit around the Sun, so at most New Moons the Moon passes above or below the Earth-Sun line, missing the alignment needed for an eclipse.
- (a) Full Moon: Sun - Earth - Moon in a line (Earth between). (b) First quarter: Sun, Earth and Moon form a right angle at Earth.
- Earth’s atmosphere bends (refracts) red sunlight around into the shadow. Blue light is scattered away, leaving the Moon lit by deep red light — hence “blood Moon”.
- Tidal bulges require large bodies of water stretched across distances where the Moon’s pull differs enough to matter. Lakes are too small for significant differential pull, so their tidal change is millimetres.
Tier 3: apply to a novel context
- With zero tilt: (i) no seasons — each latitude’s weather would be roughly the same all year. (ii) Equal day and night everywhere year-round. Agriculture, migration, and biodiversity would all change.
- Yes — the Sun’s gravity produces solar tides too, roughly half the size of Moon tides. Without a moon, tides would still exist but would be smaller and only driven by the Sun.
- New Moon or Full Moon — spring tides occur then, giving the biggest range between high and low.
- Martian seasons last about as long as Earth’s because a Mars “year” is nearly twice as long. The temperature pattern is similar (tilted-towards hemisphere has summer) but the orbit is more elliptical, so seasons have unequal length in different hemispheres.
Challenge
- (a) As the Moon recedes, tidal friction slows Earth’s rotation — days get longer. (b) Weaker pull at greater distance means the tidal bulges shrink, so tides become smaller.
- A calendar month is - days, slightly longer than the -day lunar cycle. Occasionally two Full Moons fit in the same month — the second is called a blue Moon. Happens roughly every years.
- Every New Moon — about once every days. Because the orbit is tilted, the Moon usually misses the alignment; if the tilt were zero the alignment would happen every lunar cycle.
- Stand in a dark room. Hold a small ball (Moon) at arm’s length and slowly orbit around a student (Earth) while a torch (Sun) shines from one side. Observer sees different lit portions as the Moon orbits. Limitation: the scale is wrong — in reality the Moon is far smaller and much further away than this model suggests, and the model cannot demonstrate why eclipses are rare.
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