Tides and Moon's Effect
Overview
This lecture explains how the moon causes tides on Earth, the timing and pattern of tides, and the gravitational mechanisms behind them.
How the Moon Affects Earth
- The moon's primary physical effect on Earth is causing ocean tides.
- There are two high tides and two low tides every 24 hours.
- High tides occur about 50 minutes later each day, just like the moon rises about 50 minutes later daily.
Tidal Patterns and Timing
- High tides do not occur at the same clock time every day, with the timing shifting due to the moon's changing position.
- When the moon rises (on the horizon), it is low tide.
- As the moon moves overhead (at the zenith), it becomes high tide.
- When the moon sets, it's low tide again.
- A second high tide happens when the moon is directly on the opposite side of Earth (not visible).
Gravitational Cause of Tides
- The moon causes tides through its gravity, which pulls on Earth's water.
- The part of Earth closest to the moon feels the strongest gravitational pull, causing the ocean there to bulge outward (high tide).
- The solid Earth and farther ocean regions are also pulled, but to different extents, creating two bulges (high tides) on opposite sides.
- Low tides occur at points 90° away from the line connecting the bulges, as water is redistributed.
Tides Without the Moon
- Without the moon, there would be no significant ocean tides.
- The moon's gravity is essential for the pattern of two high and two low tides daily.
Earth's Rotation and Tides
- The moon stays roughly in the same orbital position for a day, while the Earth rotates once.
- As Earth rotates, locations pass through both bulges (high tides) and low tide regions each day.
Key Terms & Definitions
- High tide — When ocean water is at its highest level due to gravitational pull.
- Low tide — When ocean water is at its lowest level.
- Zenith — The point in the sky directly above an observer.
- Gravity — The force causing attraction between masses, responsible for tides.
Action Items / Next Steps
- Read Chapter 2 for more details on the moon and Earth's tides.
- Review class diagrams illustrating tidal bulges and Earth-moon positions.