Earth and Space Physics
Earth, Sun and Moon
The Earth orbits the Sun once per year and rotates on its axis once per day, producing day and night; its axial tilt causes the seasons. The Moon orbits Earth roughly every 28 days, producing phases and eclipses. Solar eclipses occur when the Moon passes between Earth and the Sun; lunar eclipses occur when Earth's shadow falls on the Moon. A frequent trap is confusing the order of bodies or the cause of the seasons (tilt, not distance from the Sun).
The solar system and gravity
Know the order of planets from the Sun and the difference between planets, moons, asteroids and comets. Gravitational force keeps planets in orbit; the force depends on mass and distance. Weight varies between planets because g differs. Satellites in orbit are in free fall, which explains apparent weightlessness. Orbital speed increases for closer orbits.
Stars and the universe
Stars form from collapsing gas clouds; the Sun is a main-sequence star powered by nuclear fusion of hydrogen into helium. Know the life cycle: nebula, main sequence, red giant or supergiant, then white dwarf or supernova and neutron star or black hole. Light-years measure distance, not time. The universe expands; redshift of distant galaxies supports the Big Bang model. Cosmic microwave background radiation is evidence for it.
Worked example
A light-year is the distance light travels in one year: 3 × 10⁸ m/s × (365 × 24 × 3600 s) ≈ 9.5 × 10¹⁵ m. So a star 4 light-years away is about 3.8 × 10¹⁶ m away. When a question asks for the time light takes, divide the distance in metres by the speed of light, not by a year.