Earth Science: Earth and the Universe
Earth and space items divide roughly evenly between 天体 (the Sun, stars, moon, solar system) and 大地 (strata, fossils, earthquakes).
Rotation and revolution
- 自転 runs west → east once a day, which makes celestial objects move east → west at 15° per hour.
- 公転 takes one year around the Sun; the 23.4° tilt of the axis produces the seasons.
- 南中高度 = 90° − latitude + solar declination. At Yamaguchi (about 34° N): summer solstice 90 − 34 + 23.4 = 79.4°; winter solstice 90 − 34 − 23.4 = 32.6°.
The Moon
- 朔望月 is about 29.5 days. The Moon rotates and revolves in the same period, so the same face always points at Earth. 日食 occurs at new moon, 月食 at full moon.
Earthquakes and strata
- P波 is fast; S波 is slower. 初期微動継続時間 grows with distance, so it gives 震源距離.
- マグニチュード is the size of the quake; 震度 is local shaking. They are not interchangeable.
- In strata the older layer lies below; 示相化石 indicates environment, 示準化石 indicates geological age.
Worked example. A seismogram shows 12 s between the P and S arrivals. The 初期微動継続時間 grows by about 1 s per 8 km, so the 震源距離 is roughly 12 × 8 = 96 km. Show the proportional reasoning before the number.
Traps. Explaining the seasons by distance from the Sun instead of axial tilt; saying the Moon does not rotate; swapping 震度 and マグニチュード; forgetting that 南中高度 depends on latitude, so a Yamaguchi answer differs from an Okinawa one.