Question

Explain what causes spring and neap tides, and identify the phases of the moon when these tides occur. Be sure to mention what happens to Earth’s crust, ocean water, and the positions of the Sun, Earth, and the moon.

Answer 1

Tides are the regular rise and fall of sea level caused mainly by the differential gravitational pull of the Moon and the Sun on different parts of the Earth, together with the centrifugal effect from the Earth–Moon orbital motion. Because gravity weakens with distance, the side of Earth closest to the Moon is pulled more strongly than the center, and the center more than the far side; this creates two tidal bulges — one toward the Moon and one on the opposite side. The solid Earth (Earth’s crust) also deforms in response to these forces (solid‑Earth or body tides), but by a much smaller amount than the ocean water.

Spring tides
- Cause: When the Sun, Earth, and Moon are aligned (a configuration called syzygy), the tidal forces of the Sun and Moon combine constructively. The Sun’s gravitational effect adds to the Moon’s differential pull, producing larger tidal bulges.
- Effect on ocean water and crust: Higher high tides and lower low tides (greater tidal range). The ocean bulges are larger; the solid Earth also shows slightly larger vertical deformation (tens of centimeters).
- Moon phases: Occur at new moon and full moon (twice each lunar month).

Neap tides
- Cause: When the Moon is at first or third (last) quarter, the Sun and Moon are at roughly right angles as seen from Earth. Their tidal effects partially cancel each other because the Sun’s pull reduces the Moon’s differential effect on one side of Earth.
- Effect on ocean water and crust: Smaller tidal range — lower high tides and higher low tides. Ocean bulges and solid‑Earth deformation are reduced.
- Moon phases: Occur at first quarter and third (last) quarter (twice each lunar month).

Notes
- The Moon produces the dominant tidal effect; the Sun’s tidal contribution is about 40–50% of the Moon’s (roughly 46%).
- Spring and neap tides alternate about every week, so each type happens roughly twice per lunar month.
- Local tidal ranges are strongly modified by coastline shape, seafloor bathymetry, and resonance, so actual tidal heights vary greatly from place to place.