Astronomy and Earth’s Place in the Solar System
Astronomy and Earth’s Place in the Solar System
This domain tests Earth’s motions, the celestial sphere, Moon phases and eclipses, solar-system organization, and stars.
Core rules and concepts
- Earth rotates west-to-east, causing apparent daily motion of celestial objects from east to west. Earth revolves counterclockwise around the Sun.
- Seasons result from Earth’s 23.5° axial tilt, not changing distance from the Sun. The noon Sun is highest in New York at the summer solstice.
- Moon phases depend on the Sun-Earth-Moon angle. Eclipses require alignment near the Moon’s orbital nodes; they do not occur every month.
- Tides are caused mainly by the Moon, with the Sun modifying spring and neap tides.
- The solar system has terrestrial inner planets and Jovian outer planets. Kepler’s laws describe elliptical orbits; eccentricity e = distance between foci ÷ major axis.
- Stars are classified by temperature and luminosity on the HR diagram; evidence for the Big Bang includes redshift and the cosmic microwave background.
Worked example
An orbit has a major axis of 2.0 AU and foci separated by 0.1 AU. Eccentricity e = 0.1 ÷ 2.0 = 0.05, so the orbit is nearly circular. Earth’s eccentricity is about 0.017.
Traps that cost marks
- Explaining seasons by distance from the Sun; tilt is the cause.
- Saying Moon phases are caused by Earth’s shadow; that causes lunar eclipses.
- Claiming eclipses occur every month; the Moon’s orbit is tilted.
- Thinking higher eccentricity means a larger orbit rather than a more elliptical shape.