Earth Science: 16.2B Tides

1. Earth Science: 16.2B Tides Tides

2. Tides • Tides are regular changes in the elevation of the ocean surface. • Their rhythmic rise and fall along coastlines has been noted throughout history. • Other than waves, they are the easiest ocean movements to observe. Mid tide in Harborville Nova Scotia

3. Tides • The cause of tides was not understood however until Sir Isaac Newton applied his law of universal gravitation to the problem. • Newton showed that there is a mutually attractive force, gravity, between any two bodies, such as the earth and the Moon. Mid tide in Harborville Nova Scotia

4. Tides • Recall that the strength of the force of gravity between any two bodies decreases as the distance between the bodies decreases. • At any given time, different areas of Earth’s surface are different distances from the moon. Gravitational pull of the Moon On Earth’s oceans

5. Tides • The pull of the moon’s gravity field will be strongest at the points on Earth closest to the moon and weakest at the parts farthest away. • Ocean tides result from these differences in the gravitational attraction exerted upon different parts of Earth’s surface by the moon (and to lesser degree, the sun). Gravitational pull of the Moon On Earth’s oceans

6. The Cause of Tides • The primary body that influences the tides is the moon, which makes one complete revolution of the Earth every 29 and a half days. • The sun, however, also influences the tides. Gravitational pull of the Moon On Earth’s oceans

7. The Cause of Tides • The sun is far larger than the moon, but because it is so much farther away it’s affect upon the tides is considerably less. • In fact, the sun’s influence on the Earth’s tides is only about 46% of the moon’s influence. Gravitational pull of the Moon On Earth’s oceans

8. The Cause of Tides • To illustrate how tides are produced, consider Earth as a rotating ball covered with a uniform depth of water. • Think about the gravitational forces in the Earth-moon system, ignoring the influence of the sun for now. • This gravitational pull is strongest on the side of the Earth facing the moon and weakest on the far side away from the moon. • This difference causes the shape of the Earth to be affected slightly. Gravitational pull of the Moon On Earth’s oceans

9. The Cause of Tides • The shape of the solid Earth is not affected much by this difference in pull. • However, because water is fluid, it can easily flow from location to location in response to this pull from the moon’s gravity field. • On the side of the Earth closest to the moon, the pull of the moon on the earth’s oceans is stronger than it is on the solid Earth itself. • Ocean water flows toward this pull, producing a tidal bulge also know as a high tide. Gravitational pull of the Moon On Earth’s oceans

10. The Cause of Tides • On the side of the Earth farthest away from the moon, the pull of the moon is weaker than it is on the solid Earth. • As a result, water also flows toward this area and creates a second equally large tidal bulge on this side opposite the moon. Gravitational pull of the Moon On Earth’s oceans

11. The Cause of Tides • Because the position of the moon changes only moderately in a single day, the “tidal bulge” remains in place while Earth “rotates” through these tidal bulges. • For this reason, if you stand on a shore for 24 hours, Earth will rotate through alternating high and low tides. • The ocean stays pulled toward the moon as the solid Earth rotates beneath it. Gravitational pull of the Moon On Earth’s oceans

12. The Cause of Tides • As the Earth rotates and you are carried into each tidal bulge, the tides will rise. • As you are carried into the troughs in between the tidal bulges, the tides will fall. Gravitational pull of the Moon On Earth’s oceans

13. The Cause of Tides • Most coastal locations will experience two high tides every day because of this and two low tides. • High tides are spaced 12 hours apart because the Earth rotates a full revolution in 24 hours. • In 24 hours, you would pass through the tidal bulge 2 times. Gravitational pull of the Moon On Earth’s oceans

14. The Cause of Tides Tidal Cycle: • Although the sun is farther away from the moon, the gravitational attraction between the sun and Earth does play a role in producing tides. • The sun’s influence produces smaller tidal bulges than the moon does. • These bulges are the result of the same forces that produce the tidal bulges created by the moon. • The influence of the sun on the tides is most noticeable near the times of the full and new moons. Gravitational pull of the Moon On Earth’s oceans

15. The Cause of Tides Tidal Cycle: • During a full or new moon, the forces of the moon’s pull and the sun’s pull are aligned. • Their forces are added together and result in a higher tides as the Earth’s oceans are pulled farther by the combined forces. • This combined gravity produces a higher tidal range. • The tidal range is the difference between the successive high and low tides. Gravitational pull of the Moon On Earth’s oceans

16. The Cause of Tides Tidal Cycle: • These times when the gravitational forces of the moon and sun combine produce what we call Spring tides. • Spring tides are tides that have the greatest tidal range due to the alignment of the Earth-moon-sun system. • Spring tides are experienced during new and full moons when the combine pull of the moon and sun is greatest on the Earth. Gravitational pull of the Moon On Earth’s oceans

17. The Cause of Tides Tidal Cycle: • Conversely, at the time of the first and third quarters of a moon, the gravitational forces of the moon and sun work at different angles against each other. • The sun and moon partially offset each other’s influence, cancelling out part of each other’s affect. Gravitational pull of the Moon On Earth’s oceans

18. The Cause of Tides Tidal Cycle: • Because of this, the influence of each body is less and the result is lower tides. • We call these tides Neap tides. • Each month there are two spring tides and two neap tides as the moon goes through it’s cycle about the Earth. Gravitational pull of the Moon On Earth’s oceans

19. The Cause of Tides Tidal Patterns: • We now know the basic causes that shape the tides and when they are at their highest and lowest. • Many other factors, including the shape of the coastline, the configuration of ocean basins, and water depth, all greatly influence the tides. Gravitational pull of the Moon On Earth’s oceans

20. The Cause of Tides Tidal Patterns: • Because of this, tides at different locations respond differently to tide producing forces. • Because of this, the nature of the tides at any actual location can only be determined by actual observation. Gravitational pull of the Moon On Earth’s oceans

21. The Cause of Tides Tidal Patterns: • Three main tidal patterns exist worldwide: • Diurnal tides • Semidiurnal tides • Mixed tides Highest tides in the world are in the Bay of Fundy

22. The Cause of Tides Tidal Patterns: • Diurnal tides patterns are characterized by a single high tide and a single low tide each tidal day. • Tides of this type occur along the northern shore of the Gulf of Mexico.

23. The Cause of Tides Tidal Patterns: • A semidiurnal tidal pattern is characterized by two high tides and two low tides each tidal day. • The two highs are about the same height and the two lows the same low point. • This type of tidal pattern is common along the Atlantic coast of the United States.

24. The Cause of Tides Tidal Patterns: • A mixed tidal pattern is similar to a semidiurnal pattern except that it is characterized by a large inequality in the high water marks and low water marks of the tides. • There are usually two high tides and two low tides each day. • The high tide marks are at different points however as are the low tide marks. • These tides are found on the western coast of the US.