Space Systems
The Space Systems domain examines Earth's motions and their observable consequences, the Sun-Earth-Moon system, the solar system, and the origin and evolution of stars and the universe. Items emphasize geometry, reference-table data, and distinguishing real motions from apparent ones.
Apparent motions and coordinates
- Earth rotates west to east, so the Sun, Moon, and stars appear to move east to west at about 15° per hour.
- In the Northern Hemisphere, the altitude of Polaris above the horizon equals the observer's latitude.
- Noon Sun altitude = 90° − |latitude − solar declination|.
- Seasonal constellations reflect Earth's revolution, not stellar motion.
Seasons, phases, and tides
- Seasons are caused by the 23.5° axial tilt, which changes the angle and duration of insolation-not by distance from the Sun.
- Moon phases follow Sun-Earth-Moon geometry over a synodic month of about 29.5 days.
- Eclipses require alignment near the Moon's orbital nodes; solar eclipses occur at new moon, lunar at full.
- Spring tides occur at new and full moons; neap tides at quarter phases.
Solar system, stars, and universe
- Kepler's laws: elliptical orbits, equal areas in equal times, and period squared proportional to average distance cubed.
- Eccentricity = distance between foci ÷ major-axis length; 0 is a circle, values near 1 are highly elongated.
- On the H-R diagram, main-sequence stars run diagonally; red giants are cool and luminous; white dwarfs are hot and dim.
- Big Bang evidence includes galactic redshift, the cosmic microwave background, and primordial hydrogen/helium.
Worked example
At 40°N on the June solstice, declination is +23.5°, so noon altitude = 90° − (40° − 23.5°) = 73.5°. At the December solstice it is 90° − (40° + 23.5°) = 26.5°; at an equinox it is 50°. Higher noon altitude and a longer day deliver more intense, longer-lasting insolation in summer.
Traps that cost marks
- Blaming seasons on Earth's changing distance from the Sun.
- Explaining moon phases as Earth's shadow falling on the Moon.
- Placing a solar eclipse at full moon.
- Claiming Polaris is visible from the Southern Hemisphere.
- Treating eccentricity 1 as a closed orbit.