Tides are the rising and falling of sea levels caused by the combined effects of the gravitational pull of the moon and the sun and the centrifugal force of the Earth's rotation. The moon has the strongest influence due to its closer proximity. This creates bulges in the ocean that result in two high tides and two low tides every 24 hours and 50 minutes. Spring tides occur at new and full moons when the sun, moon, and Earth are aligned, creating higher tides. Neap tides happen when the sun and moon are at right angles, resulting in smaller differences between high and low tides.

1. Tides

2. What are Tides? Tides are the rising and falling of the sea. During high tide, the water is deeper and comes further onto the beach. Another name for high tide is flood tide . During low tide, the water is more shallow and does not come as far onto the beach. Another name for low tide is ebb tide .

3. The Global Ocean The Earth is covered by more than 70% water!! All of the oceans are interconnected They can move as a “system”

4. The Sun’s mass is 27 million times that of the moon, but it is also 390 times farther from the earth. So although the sun affects our tides, the moon exerts the greater gravitational attraction because of its proximity to our planet .

5. The oceans are liquid and greatly affected by two forces of nature: The gravitational pull of the sun and moon, and The centrifugal forces the earth applies as it spins. The gravitational force of the moon is one ten-millionth that of earth, but when you combine other forces such as the earth's centrifugal force created by its spin, you get tides

6. The Main Culprit The gravity of the moon “tugs” at the global oceans… … kind of like a vacuum cleaner “tugging” Grandpa Simpson’s face making him look like his son Homer!

7. This is high tide at Douglas (in Juneau, Alaska). Photo from Daniel Cornwall at Alaskan Librarian .

8. This is Douglas at low tide. Photo from Daniel Cornwall at Alaskan Librarian .

9. Tidal Bulges - The pull of the sun and moon creates tidal bulges. The bulge is really a large wave beneath the ocean that moves across the earth. There are usually 2 high and 2 low tides every 24 hours and 50 minutes .

10. Spring Tide When the Earth, moon and Sun are in line twice a month at new and full moons, the gravitational pull on the Earth is increased, and tides are higher then at any other time. Extremely High Tides and Extremely Low Tides (more extreme (about 20%) differences) Hint: “S”pring tides = “S”traight line up

11. Neap Tide When the sun and the moon are at 90 degrees or right angles to the earth the gravitational pull of the sun and moon are competing. High tides are not as high and low tides are not as low (less extreme differences- 20% lower) Hint: “N”eap Tides = “N”inety degree angles

12. Spring and Neap Tides - At what moon phases do these occur?

13. Look for the phases!