Astronomy and Space Science
This domain covers Earth's motions, the Moon–Earth system, the solar system's origin and mechanics, and the life cycles of stars and galaxies. Items test the causes of seasons, phases, eclipses, and tides, Kepler's and Newton's laws, the H-R diagram, and evidence for the expanding universe.
Earth's motions and the seasons
- Rotation takes 23 h 56 min (sidereal) and produces day and night; revolution takes 365.24 days.
- The 23.5° axial tilt changes the angle and duration of sunlight, producing seasons. Earth is actually nearest the Sun in early January, so distance is not the cause.
- Solstices mark maximum tilt toward or away from the Sun; equinoxes occur when the Sun is over the equator and day and night are equal everywhere.
- Long-term wobbles (Milankovitch cycles): precession ~26,000 yr, obliquity ~41,000 yr, eccentricity ~100,000 yr.
The Moon
- Synchronous rotation (27.3-day sidereal period) keeps the same face toward Earth; there is no permanently dark side.
- Phases come from the changing angle between Earth, Moon, and Sun, and repeat every 29.5 days (synodic month).
- Solar eclipses occur at new moon, lunar eclipses at full moon, and only when the Moon is near a node of its tilted orbit.
- Spring tides follow new and full moons; neap tides follow the quarters. Most coasts get two high tides and two low tides daily.
Solar system and stars
- The nebular hypothesis explains a flattened, rotating disk, coplanar orbits, and rocky inner versus icy/gassy outer planets.
- Kepler's third law: P² = a³ with P in years and a in AU. Newton explained it through gravity.
- On the H-R diagram, stars plot by luminosity and temperature; the main sequence fuses hydrogen to helium. Sun-like stars become red giants then white dwarfs; massive stars become supernovae leaving neutron stars or black holes.
- Distances come from parallax, Cepheid variables, and redshift; a light-year is about 9.46 × 10¹² km.
Worked example
An asteroid orbits the Sun at a = 4 AU. Using P² = a³, P² = 64, so P = 8 years. Larger orbits take disproportionately longer.
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).
- Confusing sidereal and synodic months, or apparent and absolute magnitude.
- Treating a light-year as a unit of time.