GED Science
Astronomy & the Solar System
The sky's clockwork comes from three motions: Earth spinning (day and night), Earth orbiting the sun on a tilted axis (seasons), and the moon orbiting Earth (phases and tides). Get the three motions straight and every question is just asking which one is showing.
The words first
- rotation
- Earth spinning on its axis — one turn = one day
- revolution / orbit
- Earth circling the sun — one lap = one year
- axial tilt
- Earth's 23.5° lean — the actual cause of seasons
- moon phases
- the lit fraction of the moon we see, cycling every ~29.5 days
The method
THE THREE MOTIONS
spin (24 h) → day and night
orbit (365 d) + tilt → seasons
moon's orbit (29.5 d)→ phases, tides
SEASONS — the tilt, NOT the distance
your hemisphere tilted TOWARD the sun:
steeper sunlight, longer days → summer
tilted AWAY: shallow light, short days → winter
proof: hemispheres have OPPOSITE seasons at the same
distance — and Earth is actually closest to the sun in January.
PHASES — where's the moon in its orbit?
new (invisible) → waxing crescent → first quarter →
full → waning → back to new
The sun always lights half the moon; phases are about
how much of that lit half faces US.
SUN = a star · planets orbit it · moons orbit planets
gravity holds the whole arrangement together
Worked all the way through
The problem
A student says summer happens because Earth moves closer to the sun. Give two pieces of evidence that the tilt, not distance, causes seasons.
- Step 1 — Test the distance idea against the hemispheres.If distance caused seasons, BOTH hemispheres would have summer together. In fact Australia has summer during North America's winter — same distance, opposite seasons.
- Step 2 — Test it against the calendar.Earth is actually CLOSEST to the sun in early January — the northern winter. Distance predicts the opposite of what happens.
- Step 3 — State what the tilt explains that distance can't.The hemisphere leaning toward the sun gets steeper light and longer days — which is exactly the hemisphere having summer, and it alternates as Earth orbits.
On the board
opposite seasons, same distance
evidence 1
closest to sun in January
evidence 2 — winter!
tilt → steep light + long days
what really makes summer
tilted toward = summer, away = winter
Answer
Opposite seasons in the two hemispheres, and Earth being nearest the sun in January — only the tilt explains both
A second one, in a different form
Tonight's moon is a thin crescent. About how long until it is full?
full cycle ≈ 29.5 days
crescent → full ≈ half the cycle... roughly
waxing crescent is early in the cycle
about 11–12 days
crescent (day ~3) to full (day ~14.75)
Answer
Roughly a week and a half
The usual mistake
Answering that summer happens because Earth is closer to the sun. It isn't.
Check yourself
Ask which of the three motions the question is about — spin, tilted orbit, or the moon's orbit — before touching the answers. And any answer explaining seasons by distance is wrong on sight.
Practice it
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