Astronomy and the Solar System
Astronomy and the Solar System
This domain tests solar system formation, planetary characteristics, orbital mechanics, the Earth-Moon-Sun system, stars, galaxies, and cosmology.
Core concepts
- The nebular hypothesis explains the solar system's formation from a collapsing, rotating cloud. The frost line separated rocky inner planets from icy and gaseous outer planets.
- Kepler's laws: orbits are ellipses with the Sun at one focus; equal areas are swept in equal times, so planets move faster near perihelion; and P² = a³ when P is in years and a is in astronomical units.
- Newton's law of gravitation, F = G m₁m₂/r², explains orbital motion and weight.
- Earth's seasons result from its 23.5° axial tilt, not changing distance from the Sun. Moon phases depend on orbital position; eclipses require alignment near the Moon's nodes.
- Stars fuse hydrogen on the main sequence. The H-R diagram relates luminosity, temperature, and spectral class (OBAFGKM). Stellar evolution depends on mass.
- Hubble's law and cosmological redshift support an expanding universe and the Big Bang; the cosmic microwave background is relic radiation.
Worked example
An asteroid orbits the Sun at a = 4 AU. Using Kepler's third law, P² = 4³ = 64, so P = √64 = 8 years. Reasoning: cube the semimajor axis in AU, then take the square root to get the period in years; the relationship is nonlinear.
Traps
Seasons are not caused by Earth's distance from the Sun. Lunar phases are not Earth's shadow falling on the Moon; that is a lunar eclipse. A light-year measures distance, not time. Apparent magnitude depends on distance and intrinsic brightness, while absolute magnitude is intrinsic. Kepler's second law means orbital speed varies, not that planets move at constant speed.