Astronomy and the Solar System
Domain 3 tests motions and cycles of Earth, the Moon, and the Sun; solar system formation and planetary comparisons; stars and stellar evolution; galaxies; and cosmology. Items often require a cause-and-effect explanation, not a name recall.
Orbits, seasons, phases, tides
- Kepler: orbits are ellipses; equal areas in equal times; P² = a³ when P is in years and a in AU.
- Newton explains orbits as free fall under gravity; orbital speed depends on central mass and distance.
- Seasons result from axial tilt changing solar angle and day length, not distance.
- Moon phases are the changing Earth-facing illuminated fraction. Solar eclipses occur at new moon; lunar eclipses at full moon.
- Spring tides occur at new and full moons; neap tides at quarters.
Solar system and stars
- The nebular hypothesis: collapsing cloud, rotating disk, accretion, and clearing. The frost line separates rocky inner planets from icy/giant outer planets.
- Asteroids lie mainly between Mars and Jupiter; the Kuiper Belt and Oort Cloud supply comets. A meteoroid becomes a meteor in the atmosphere and a meteorite on the ground.
- The H–R diagram plots luminosity against temperature or spectral class. Main-sequence stars fuse hydrogen; giants and supergiants are evolved; white dwarfs are stellar remnants.
- Spectral classes OBAFGKM run hot to cool. Apparent magnitude depends on distance and luminosity; absolute magnitude is intrinsic.
Worked example
An asteroid has a semi-major axis of 4 AU. Using P² = a³: P² = 4³ = 64, so P = 8 years. If another body has P = 27 years, then a³ = 27² = 729 and a = 9 AU. The equation assumes a solar-mass central body; for another star, use the mass-dependent form.
Traps that cost marks
- Explaining seasons by Earth's distance from the Sun.
- Saying phases are caused by Earth's shadow; that is an eclipse.
- Mixing sidereal and synodic periods.
- Confusing apparent and absolute magnitude.
- Treating redshift as motion through space without cosmological expansion.
- Assuming all planets have moons or rings.