Tides Summary Notes

Tides Overview

  • Definition: Periodic raising and lowering of ocean sea level.

  • Characteristics:

    • Long, regular shallow-water waves.

    • Seen prominently in places like Bay of Fundy, Canada (17 meter tide range).

Cause of Tides

  • Main Forces:

    • Gravitational Force: Moon has a greater gravitational pull on Earth than the Sun due to its proximity.

    • Centripetal Force: Keeps Earth and Moon tethered in orbit around their barycenter.

  • Resultant Forces ("Tidal Forces"): Difference between gravitational and centripetal forces creates bulges of water:

    • Water pulled towards Moon (G > C).

    • Water bulges away from Moon (C > G).

Tidal Phenomena

  • Tidal Period: Time between high tides.

  • Lunar Day: Time between two successive overhead moons = 24 hours 50 minutes.

  • High Tides: Occur 12 hours and 25 minutes apart due to the Earth needing to catch up with the Moon's position.

Sun's Effect on Tides

  • The Sun also influences tides but to a lesser extent (about half the effect of the Moon).

Earth’s Rotation

  • Flood Tide: Water moves toward shore.

  • Ebb Tide: Water moves away from shore.

Monthly Tidal Cycle

  • Phases of the Moon:

    • New Moon: Moon between Earth and Sun.

    • Full Moon: Moon and Sun opposite each other.

    • Quarter Moon: Moon appears half lit.

  • Spring Tides: Occur during new or full moons; highest tidal range due to alignment of Moon and Sun.

  • Neap Tides: Occur during quarter moons; lowest tidal range due to right angles formed between Moon and Sun.

Declination and Tidal Bulges

  • Tidal bulges are affected by the tilt of Earth's axis:

    • Sun: 23.5° north/south of equator.

    • Moon: 28.5° north/south of equator due to a tilt of 5° in its orbital plane.

Tidal Patterns

  • Diurnal: One high tide and one low tide per day (24 hours 50 minutes apart). Common in shallow inland seas.

  • Semi-diurnal: Two high tides and two low tides per day (12 hours 25 minutes apart). Common on Atlantic coast of the US.

  • Mixed: Combination of diurnal and semi-diurnal. Characterized by differing tidal ranges and periods.

Ecological Impact

  • Tides affect marine life, e.g., Grunion spawning is timed with tides to ensure eggs are buried.