Astronomy and the Solar System
This domain covers Earth's motions, seasons, moon phases and eclipses, solar system formation, Kepler's laws, gravitation, stellar evolution, galaxies, and Big Bang evidence. Items often require spatial reasoning.
Core rules and concepts
- Earth's 23.5° axial tilt causes seasons; distance from the Sun is not the primary cause.
- Moon phases depend on the Moon's orbital position relative to Earth and Sun; eclipses require alignment and occur near new or full moon.
- Kepler's laws: orbits are ellipses, equal areas are swept in equal times, and P² = a³ when P is in years and a is in astronomical units.
- Solar system formation began with a collapsing nebula; the Sun formed at the center and planets accreted from the disk.
- Stars evolve according to mass; massive stars end as supernovae and may leave neutron stars or black holes.
- Big Bang evidence includes cosmic microwave background, redshift of galaxies, and light-element abundances.
Worked example
An asteroid orbits at a = 4 AU. Using Kepler's third law, P² = a³ = 4³ = 64, so P = 8 years. The result assumes the asteroid orbits the Sun and that units are AU and years. A planet at 1 AU has P = 1 year; at 9 AU, P = 27 years.
Traps that cost marks
- Explaining seasons by Earth's changing distance from the Sun.
- Confusing moon phases with eclipses; Earth's shadow causes lunar eclipses only at full moon.
- Reversing lunar and solar eclipse geometry.
- Forgetting that Kepler's simplified P² = a³ applies to objects orbiting the Sun, with P in years and a in AU.