Lecture Notes: Moon's Orbit and Tidal Effects

Moon's Orbit

• Shape: The moon orbits Earth in an elliptical path, although it appears nearly circular.
• Key Terms:
  • Perigee: The point where the moon is closest to Earth.
  • Apogee: The point where the moon is farthest from Earth.
• Impact on Gravitational Pull:
  • At perigee, the gravitational pull is stronger because the distance (r) is smaller, leading to larger tidal ranges (20% increase).
  • At apogee, the gravitational pull is weaker with less significant tidal ranges.

Earth's Orbit Around the Sun

• Shape: Also elliptical, similar to the moon's orbit.
• Key Terms:
  • Perihelion: Earth is closest to the Sun, occurs around January 3rd.
  • Aphelion: Earth is farthest from the Sun.
• Tidal Effects: Larger tidal ranges occur during perihelion due to stronger gravitational influences.

Declination Effects

• Earth's Tilt: 23.5 degrees tilted toward the sun affects sunlight distribution.
• Moon's Tilt: 28.5 degrees affected by declination changes tidal bulges, ranging between 28.5 degrees north and south.
• Seasonal Tidal Variations: As Earth orbits, these angles change affecting tidal bulges based on declination.

Case Study: January 1983 Event

• Astronomical Factors:
  • Near perihelion and perigee.
  • Full moon leading to spring tides.
• Meteorological Factors:
  • Large storm coincided with high tide and El Niño conditions, leading to an 8-inch rise in sea level.
• Impact: Resulted in severe coastal damage, notably to Crystal Pier, with $100 million in damages.

Tidal Prediction and Influences

• Factors: Over 400 factors influence tides including astronomical alignments, coastal shape, and friction with the sea floor.
• Planetary Influence: Jupiter and Saturn's large masses also affect tidal patterns.
• Tide Prediction: First tide predicting machine by Kelvin in 1872; tides can be accurately forecasted.

Spring Tides and Perigee

• Spring Tides: Occur when the Earth, moon, and sun align, usually during new or full moons.
• Effect of Perigee: When the moon is near perigee during spring tides, the gravitational force is greater, leading to the highest spring tides.

Important Concepts to Remember:

• Understand perigee and apogee in relation to tidal forces.
• Recognize perihelion and its effects on Earth's tides.
• Grasp the influence of declination and Earth's tilt on seasonal tidal changes.
• Recognize factors enabling tidal predictions and the historical context of tide prediction machines.