Astronomy and the Solar System
What to know
The solar system: four inner terrestrial planets (Mercury, Venus, Earth, Mars), an asteroid belt, four outer gas/ice giants (Jupiter, Saturn, Uranus, Neptune), and the Kuiper belt beyond. The Sun is a main-sequence star powered by hydrogen fusion in its core, producing light and heat. Gravity and inertia keep planets in orbit; Kepler's laws: orbits are ellipses, planets sweep equal areas in equal times, and orbital period squared is proportional to distance cubed.
Earth's rotation causes day and night; its axial tilt causes seasons; its revolution around the Sun defines the year. The Moon causes tides and shows phases as its illuminated fraction changes relative to Earth's view; eclipses occur when the Sun, Earth, and Moon align. Lunar phases are not caused by Earth's shadow.
Beyond the solar system: stars vary in mass, temperature, and color (blue hottest, red coolest). The Hertzsprung–Russell diagram relates luminosity and temperature. Stars form from nebulae, fuse elements, and end as white dwarfs, neutron stars, or black holes depending on mass. Galaxies contain billions of stars; the Milky Way is a spiral galaxy. The Big Bang model explains cosmic expansion, redshift, and the cosmic microwave background.
Traps
Questions confuse rotation with revolution, blame seasons on distance from the Sun, or attribute lunar phases to Earth's shadow. Watch for the claim that the Sun is a planet or that the solar system's outer planets are rocky.
Rule of thumb and worked example
Seasons result from the 23.5° tilt: when the Northern Hemisphere tilts toward the Sun, it receives more direct sunlight and longer days - summer - even though Earth is not closest to the Sun then. Use tilt, not distance, to explain seasons.