It's tidal. (Critical Thinking Skills).

Everyday, coastlines expand and shrink because the ocean rises (high tide) and falls (low tide). The reason: gravity. Gravity is the attractive force that keeps planets orbiting around the Sun. It's also the force that pulls matter to the center of Earth. Both the Moon and Sun's gravity pull on Earth, causing oceans to periodically bulge. Check out the diagram below to see how.

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Follow this exercise to find out why the times of high and low tides change from day to day.

1. Examine the Tide Time Diagram (FIGURE A). The center circle represents Earth as seen from a point above the North Pole. Earth rotates counterclockwise. Observe the direction of sunlight and the times of day marked in the circumference.

2. Observe the larger circle representing the daily positions of the Moon in relation to Earth during one lunar month (29.5 days rounded to 30 for this exercise).

3. FIGURE B is a diagram of Earth with tidal bulges and the Moon as seen from the same perspective as FIGURE A. Trace the diagram using transparent paper. Place over FIGURE A, superimposing the two diagrams of Earth. Advance the Moon a day at a time. In which direction does the moon orbit around Earth?

ANSWERS

Critical-Thinking Skills: It's Tidal 

3. counterclockwise

4. 12:50 pm

5. 1:40 pm

6. one hour

7. more than 24 hours