Chapter 9 Notes on Tides

Chapter 9: Tides

What Are Tides?

• Periodic raising and lowering of ocean sea level.

• Occur daily with varying patterns in different regions.

• Explained by Newton's gravitational laws, showing the relationship between gravitational pull from the sun and moon.

Tide Generating Forces

• Combination of gravity and motion between Earth, Moon, and Sun creates tides.

• Gravity and motion impact water in oceans.

• Barycenter exists between the Moon and Earth, acting as a common center of mass or balance point beneath Earth's surface due to Earth's greater mass.

Gravitational Forces

• Gravitational force based on Newton's Law of Universal Gravitation:

  • Every object with mass attracts every other object.

  • Force is proportional to the product of masses.

  • Inversely proportional to the square of separation distance:

  • Increased mass = Increased force.

  • Increased distance = Decreased force significantly.

• For each point on Earth, gravitational attraction from the moon can be calculated:

  • Greatest at zenith (closest to the moon).

  • Least at nadir (farthest from the moon).

Centripetal Force

• Center-seeking force linking orbiting bodies to their parent bodies (e.g., Earth orbits the Sun).

• Maintains the orbits of planets through gravitational attraction.

• Gravity provides the centripetal force that holds the moon in orbit.

• Each particle on Earth experiences identical centripetal force to maintain its path.

Resultant Forces

• The difference between gravitational force and centripetal force is relatively small:

  • Around 1/1,000,000 of Earth's gravity.

• No tides occur where the resultant force is vertical (zenith, nadir, and equator).

• Significant horizontal resultant force produces tidal bulges.

Tidal Bulges

• Lunar Bulges:

  • Result from forces pushing water into two simultaneous bulges:

  • One directed toward the Moon.

  • One directed away from the Moon.

• Tide-generating forces reach maximum at points 45° relative to the equator, situated between nadir and zenith.

Tidal Phenomena

• Tidal Period: Time between high tides.

• Lunar Day: Time between two successive overhead moons, approximately 24 hours and 50 minutes.

• Solar Day: 24 hours long.

• High tides occur 12 hours and 25 minutes apart, as the moon rises approximately 50 minutes later each night.

Sun's Effect on Tides

• Solar Bulges:

  • Caused by the sun, resulting in tidal bulges on either side of Earth, roughly 46% the size of lunar bulges.

  • The moon, being closer, exerts about twice the gravitational force as the sun.

Earth's Rotation and Tides

• Flood Tide: Water moves toward shore.

• Ebb Tide: Water moves away from shore.

• Tidal bulges remain fixed relative to the positions of the sun and moon, while Earth's rotation brings different geographic areas into these bulges.

Monthly Tidal Cycle

• The monthly tidal cycle spans about 29.5 days, corresponding with the moon's orbit around the Earth.

• Tidal Range: The difference between high and low tides.

• Syzygy: Occurs when the moon, Earth, and sun align.

• Quadrature: Occurs when the moon is in the first or third quarter phase.

Spring Tides

• Occur during new or full moons (twice per month).

• Greatest tidal range due to constructive interference, producing high high and low low tides, caused when the moon and sun align.

Neap Tides

• Occur during quarter moons.

• Least tidal range due to destructive interference, where the moon and sun are out of alignment.

Phases of the Moon

• New Moon: Moon is positioned between Earth and the sun (not visible).

• Full Moon: Earth positioned between the moon and the sun (fully illuminated).

• Quarter Moon: Half-lit appearance.

• Waxing Crescent: Phase moving from new to first quarter.

• Waxing Gibbous: Phase from first quarter to full.

• Waning Gibbous: Phase from full to last quarter.

• Waning Crescent: Phase moving from last quarter to new moon.

Complicating Factors Influencing Tides

• Declination: The angular distance of the moon or sun above/below the Earth's equator (23.5° for the sun; 28.5° for the moon).

• Water envelope oscillates with these shifts, causing unequal tides (diurnal and semidiurnal).

Earth's and Moon's Orbits

• **Elliptical Orbits: **

  • Earth to Sun:

  • Perihelion: Tidal range is greatest in January when closest to the sun.

  • Aphelion: Tidal range is least in July when farthest.

  • Moon to Earth:

  • Perigee: Tidal range is greatest when the moon is closest.

  • Apogee: Tidal range is least when farthest.

• Perigee-apogee cycle lasts ~27.5 days.

• Proxigean Tides: Occurs when spring tides coincide with perigee, resulting in exceptionally high tidal ranges (occurring every ~1.5 years).

Predicted vs. Real Tides

• Predicted idealized tides are two high and low tides per lunar day, with roughly six lunar hours between high and low.

• Real tides are modified by continents and seafloor friction, which complicate idealized tidal bulges.

• Tides behave as shallow-water waves, affected by the depth of the water.

Amphidromic Point

• A point where there is no tidal range; crests and troughs rotate around this point.

Cotidal Lines

• Connect points experiencing simultaneous high tides and reflect the rotation of tidal waves, occurring every 12 hours.

• Tides experience low tides 6 hours after high tide in the same cell.

Tidal Patterns

• Diurnal Tides: One high tide and one low tide per day (less common).

• Semidiurnal Tides: Two high and two low tides per day with similar tidal ranges (common along the U.S. Atlantic Coast).

• Mixed Tides: Two high and two low tides per day with differing tidal ranges (most common, e.g., US Pacific Coast).

Tides in Coastal Waters

• Predicting tides accurately in coastal waters is difficult due to the impact of local bathymetry and bathymetry shapes.

• Standing Waves: Tide waves reflected by coasts can enhance tidal ranges, as seen in the Bay of Fundy, with record tidal ranges of 21.6m.

• Tidal Bores: A tide-generated surge of water moving upstream in certain rivers under specific conditions.

Coastal Tidal Currents

• Rotary Current: Accompanies the slowly turning tide crest in ocean basins.

• Reversing Current: Changes direction as tides move in and out.

• Flood Current: Water rushes into bays or rivers as the tide comes in.

• Ebb Current: Water drains from bays or rivers with outgoing tide.

• Slack Water: Periods of no current at high or low tide, indicating a temporary balance between inflow and outflow.

Tides and Marine Life

• Grunion Spawning: Small fish that come out of the water in California to spawn after the higher high tide peaks following the highest spring tide.

Imagine the ocean as this big, dramatic stage where tides are the daily performances—raising and lowering like a breathy sigh, intricately choreographed by the Moon and Sun's gravitational pull. It's a soap opera of celestial bodies affecting everything around them! Let’s spill some tea about the forces behind these tidal dramas!

What Are Tides?

Tides are these mesmerizing cycles of the ocean—every day they rise and fall, but not in a boring way! Each region has its own pattern, like signature moves in a dance. It all comes down to the magical attraction of the Moon and Sun, drawing water in and out like it's flirtatiously teasing our shores.

Tide Generating Forces

The real scoop? It's a wild mix of gravity and the motion of Earth, Moon, and Sun that creates this whole tidal affair. Think of it as a love triangle with the barycenter, that sweet spot between the Earth and Moon, being the gossip hub—where all the massive pulling happens!

Gravitational Forces

Newton’s Law of Universal Gravitation plays a crucial role in our tidal tale: every object with mass draws in every other object! It's like the gravitational equivalent of "you scratch my back, I’ll scratch yours"—the closer the Moon, the stronger its pull, and it gets really intense as we get closer to it (that’s zenith, darling) but falls off as we drift away (nadir vibes).

Centripetal Force

Now, here comes the heavy-hitter! Imagine centripetal force as the ultimate glue keeping our orbital buddies together. It’s like gravity is throwing a party and it’s the host. Every little particle on Earth feels this force, making sure that no one drifts off into space!

Resultant Forces

But not everything is rosy—there's a catch! The difference between gravitational forces and centripetal forces is sneaky small (only about 1 part in 1 million of Earth’s gravity), which means no drama in certain spots! No tidal mess at zenith or nadir—only thrilling action where horizontal resultant forces create those stunning tidal bulges.

Tidal Bulges

Now, let’s talk about those bulges, not the kind you see at the gym but the lovely, rounded mounds of water known as tidal bulges. There are two in a row—one facing the Moon (major crush) and one on the opposite side (the friend zone!). They flaunt their love for the Moon and their envy for the Sun, who tries hard but manages only 46% of the Moon's charm.

Tidal Phenomena

And here’s where it gets technical: tides have their own rhythm! We have tidal periods (the intervals between high tides) and lunar days—24 hours and 50 minutes of lunar antics! You can set your watch by it—high tides are 12 hours and 25 minutes apart because, spoiler alert, the Moon rises about 50 minutes later each night!

Sun's Effect on Tides

Solar bulges are also in the mix, driven by the Sun’s influence, but it’s the Moon who truly wears the crown in this tidal romance, being that irresistible force of nature!

Earth's Rotation and Tides

As Earth spins on its axis, the tides rise and fall—it’s like a water ballet! We have tidal floods (water rushing in) and ebbs (water retreating), both happening while the tidal bulges stay loyal to the Moon and Sun, patiently waiting for Earth to shuffle through.

Monthly Tidal Cycle

The entire drama unfolds over a month (about 29.5 days) as the Moon does its round, causing tidal ranges (the difference between high and low tides) that are nothing short of riveting! Don’t forget syzygy when the cosmic trio aligns—those are the moments for spring tides!

Spring & Neap Tides

Spring tides—oh, they’re the sensational peaks during new or full moons, boasting the grandest tidal ranges thanks to the Moon and Sun’s perfect alignment. On the flip side, we have neap tides during the quarter moons, where it feels like a quiet lull in the action due to not-so-friendly alignment.

Phases of the Moon

Each moon phase adds to this cosmic gossip! From the shy new moon hiding between Earth and the Sun, to the glamorous full moon shining bright—there’s a whole spectrum of appearances that the Moon flaunts!

Complicating Factors Influencing Tides

But it’s not all easy sailing—factors like declination and the Moon’s eccentric orbit keep things spicy, causing unexpected and juicy twists in tide heights, leading to diurnal or semidiurnal tides that can vary in their emotional ranges!

Tides in Coastal Waters

In coastal waters, predicting tides is like trying to solve a mystery—every little change in bathymetry (the underwater landscape) can flip the script! You might even encounter standing waves in areas like the Bay of Fundy, where tidal ranges reach record-breaking heights and surprise everyone!

Tides and Marine Life

Finally, let’s sprinkle in some gossip about marine life! Have you heard about the grunion? These little fish cheekily birth their babies right after high spring tides, adding to the tidal drama—and that, my friends, is nature’s most captivating love story!