1. Tides

2. What are they?
The rhythmic rising and falling of ocean surface levels.

3. Why study them? Affect the coastline
Affect the life of marine organisms in tidal areas
Raise and lower sea level
Affect lives of people
Drive circulation in bays and estuaries

4. What causes them? Gravitational pull of sun and moon
Centrifugal force of earth as it spins

5. 1. Gravitational Pull 2. Centrifugal force
Moon pulls water on earth’s surface wherever it faces earth causing a bulge.
2. Centrifugal force
Opposite side of earth has less pull by the moon so it does not pull toward the moon.

6. Result looks something like this:

7. Because there are two bulges and two “flats”, most places on earth have 2 high tides and 2 low tides each day.

8. But there’s more…
One rotation of earth takes…

9. One rotation of earth takes…24 hours
But there’s more…
One rotation of earth takes…24 hours
The moon advances a little on its own rotation each day & it takes a spot on earth 50 minutes to “catch up”
So one tidal cycle high – low – high again takes 24 hours and 50 minutes
Tide charts predict when high and low tides will occur

10. How does the sun affect tides?
Also has a gravitational pull on earth
But the sun is 400 times farther away so the pull is not really noticed
Except…

11. Spring Tides
New Moon
Earth
Pull from moon combined with pull from sun. Result: exceptionally high HIGH TIDES and exceptionally low LOW TIDES

12. Spring Tides
Full Moon
Earth
Pull from moon on one side and pull from sun on opposite. Result: exceptionally high HIGH TIDES and exceptionally low LOW TIDES

13. Neap Tides First quarter Third quarter
Earth
Range between high and low not significantly great because sun and moon’s gravitational pull “cancel” each other out

14. Types of Tides
Diurnal – One high and one low each day. (very uncommon)
Semidiurnal – two high, two low each day
(most common)
Mixed semidiurnal – successive high tides of different heights (most of U.S. west coast)