Astronomy and the Earth–Moon–Sun System
Domain 2 tests the Earth–Moon–Sun system and broader astronomy: motions, seasons, phases, eclipses, tides, solar-system structure, stars, and evidence from light.
Core rules and concepts
- Earth rotates west-to-east once per sidereal day (23 h 56 min); the solar day is 24 h. Revolution takes 365.24 days.
- Seasons result from axial tilt (about 23.5°), not changing distance. Solstices mark maximum tilt toward or away from the Sun; equinoxes have equal day and night.
- Moon phases depend on the Sun–Earth–Moon angle. The synodic month (phase cycle) is 29.5 days; the sidereal month is 27.3 days.
- Solar eclipses occur at new Moon and lunar eclipses at full Moon, but not monthly because the Moon’s orbit is inclined about 5°.
- Tides are mainly gravitational, with spring tides at new and full Moon and neap tides at quarter phases.
- Starlight carries spectrum, temperature, composition, and motion information; redshift indicates recession.
Worked example
A flagpole 2.0 m tall casts a shadow when the Sun is 50° above the horizon. tan(50°) = height/shadow, so shadow = 2.0/tan(50°) = 2.0/1.19 ≈ 1.7 m. This is a solar-altitude problem, not a seasons problem. If the Sun were lower, the shadow would be longer.
Traps that cost marks
Do not say summer happens because Earth is closer to the Sun; perihelion occurs in Northern Hemisphere winter. Do not mix sidereal and synodic months. Eclipses do not occur every month. A full Moon rises near sunset and is highest near midnight, not at noon.