ACARA v9 CONTENT DESCRIPTION “model cyclic changes in the relative positions of the Earth, sun and moon and explain how these cycles cause eclipses and influence predictable phenomena on Earth, including seasons and tides”
Builds on earlier observations of the day, the year and the changing sky. Here those familiar patterns are explained by the steady motion of three bodies: the Earth spinning and orbiting the sun, and the moon orbiting the Earth.
Predictable motion, predictable patterns
The sun, Earth and moon move in regular, repeating ways. The Earth spins once a day and orbits the sun once a year; the moon orbits the Earth about once a month. Because these motions are so regular, the patterns they create, phases, eclipses, seasons and tides, can be predicted far in advance.
Phases of the moon
The moon does not make light; it reflects the sun. As it orbits Earth, we see different amounts of its lit half.
The sun always lights the half of the moon facing it (shown on the right). At new moon that lit half points away from Earth, so the moon looks dark; at full moon it faces us. The phases cycle about every 29 days.
When three bodies line up
Most months the moon passes a little above or below the direct line between the sun and Earth, so nothing dramatic happens. Now and then the three line up exactly, and one body casts its shadow on another. That alignment is an eclipse, and which body sits in the middle decides whether it is a solar or a lunar one.
Eclipses are a matter of alignment
Eclipses happen only when the sun, Earth and moon line up. Which body is in the middle decides the type.
In a solar eclipse the moon passes between the sun and Earth, casting its shadow on us. This can only happen at new moon.
Tilt, seasons and tides
The same orbital motion shapes the year. Earth axis is tilted, so as it travels around the sun each hemisphere takes turns leaning toward the light, giving us seasons. The moon plays its own role too: its gravity pulls the oceans into bulges, and as the Earth turns beneath them we get the rise and fall of the tides.
Seasons come from the tilt
Earth's axis stays tilted the same way all year. Whichever hemisphere leans toward the sun has summer.
Seasons are not caused by Earth being closer to the sun. They happen because the tilted axis makes one hemisphere lean toward the sun and receive more direct light, while the other leans away. Half a year later the situation is reversed.
Why the sea rises twice a day
The moon pulls hardest on the ocean nearest it and least on the ocean on the far side. That difference stretches the water into two bulges, one under the moon and one opposite it. As the solid Earth turns through these bulges, any coast is carried into a bulge, then out, then into the other, so most places get two high tides and two low tides each day.
Tides: two bulges, not one
The moon's gravity stretches the oceans into two bulges. As Earth turns, a coast passes through both, giving two high tides and two low tides a day.
Now the coast lines up with a bulge, so the sea is high. There are two bulges, one facing the moon and one on the far side, so high tide comes round twice each day.
One face, always
The moon also spins, but in a special way: it turns exactly once for every orbit of Earth. The two motions keep step, so the same hemisphere always points toward us. This is why every photograph of the moon from Earth shows the same familiar pattern, and why the far side was a mystery until spacecraft flew around to look.
The moon shows us one face
The moon turns once for every trip around Earth, so the same side, the gold marker, always faces us. The far side never turns toward Earth.
Because the moon spins exactly once for each orbit of Earth, the gold near side stays pointed at us the whole way round. That is why we always see the same face of the moon and never the far side from the ground.
Why this matters
Understanding these cycles turned the sky from a mystery into a calendar. Tides, seasons, eclipses and phases all follow from a few simple motions, and once you can model those motions you can explain and predict them, the foundation of astronomy and of timekeeping itself.
Quick self-check
1. Why does the moon shine?
2. At new moon, why does the moon look dark from Earth?
3. A solar eclipse can only happen at which phase?
4. During a lunar eclipse, what falls on the moon?