Astronomy: Earth, Moon, Sun, and Solar System
Domain VI covers Earth's motions and their seasonal consequences, the Earth-Moon system, the Sun's structure and activity, and the formation and architecture of the solar system. Items ask you to reason about orbits, phases, eclipses, planetary properties, and the scaling of the system.
Earth's motions
Rotation (23 h 56 min sidereal day) produces day and night; revolution (365.25 days) plus an obliquity of 23.44° produces seasons. Tilt, not distance, controls the angle of insolation and day length. Solstices mark maximum and minimum solar altitude; equinoxes bring roughly 12-hour days everywhere. Axial precession shifts the calendar over about 26,000 years.
The Moon
The Moon rotates synchronously - one rotation per orbit (27.3 days sidereal, 29.5 days synodic) - so the same face always points at Earth. Phases come from the changing Sun-Earth-Moon angle, not Earth's shadow. Solar eclipses occur near new moon and lunar eclipses near full moon, and only when the Moon is near a node. Tides are dominated by the Moon, with spring and neap variation. The giant-impact hypothesis explains the Moon's volatile depletion.
The Sun and solar system
The core fuses hydrogen via the proton-proton chain; energy crosses radiative and convective zones; the photosphere (about 5,800 K) shows sunspots tied to an 11-year magnetic cycle. The solar wind drives aurorae and space weather. The nebular hypothesis yields terrestrial and Jovian planets separated by the frost line, plus asteroids, Kuiper belt objects, and Oort cloud comets.
Worked example
An asteroid orbits at a = 4 AU. Kepler's third law gives P² = a³ = 64, so P = 8 years. Mars, at 1.52 AU, gives P² = 3.51 and P ≈ 1.87 years, matching the observed 1.88 years.
Traps
Attributing seasons to distance from the Sun; explaining phases with Earth's shadow; expecting an eclipse every month; thinking a comet's tail trails behind its motion rather than pointing away from the Sun; and treating the light-year as a unit of time.