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11.2: Phases of the Moon and Tides

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    10321
    • Contributed by Miracosta Oceanography 101
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    Tides are periodic short term changes in the elevation of the ocean surface caused to the gravitational attraction of the Moon and Sun, AND the rotational motion (inertia) of the of the Earth. The gravitational pull of the Moon is slightly stronger than the Sun. However, sometimes the gravitational forces of the Sun and Moon join together to make higher tides (Figure 11.4).

    spring and neap tides
    Figure 11.4. Spring and neap tides are related to the orientation of the Earth, Moon, and Sun (note polar orientation in this view).

    Spring Tides and Neap Tides

    • During full moon or new moon phases, the gravitational forces of the Sun and Moon are maximized, producing very large ranges of tidal highs and lows called spring tides (Figure 11.5). During a full moon, the Earth and the Sun and Moon are approximately aligned, producing very large ranges of tidal highs and lows (spring tides).
    Monthly tidal cycle showing spring and neap tides.
    Figure 11.5. Monthly tidal cycle showing spring tides and neap tides.
    • During the quarter moon phases, the gravitational forces of the Sun and Moon are at their minimum, producing very small ranges of tidal highs and lows (neap tides). A neap tide is the lowest level of high tide; a tide that occurs when the difference between high and low tide is least. Neap tide comes twice a month, in the first and third quarters of the moon. During the quarter moon phase, the gravitational forces of the Sun and Moon are at their minimum, producing very small ranges of tidal highs and lows (neap tides).

    Neap and Spring Tides illustrated (NOAA animation)

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