Tides

Tides have been so important for commerce and science for so many thousands of years that tides have entered our everyday language: time and tide wait for no one the ebb and flow of events a high-water mark and turn the tide of battle

High Tide Bay of Fundy

Low Tide

Tides Tides are generated through a combination of gravity and the motions of the Earth, Moon and Sun.

Tides It is gravity that tethers the sun, planets, and moon together and that tugs every particle of water on Earth toward the Moon and the Sun.

Does the moon or sun drive the tides? BOTH! But mostly the moon! Why?

Tides Although the gravitational attraction between Earth and the Sun is more than 177 times greater than between Earth and the Moon, the Moon dominates the tides because the Sun is 390 times farther from Earth than the moon, so its tide generating force is reduced to only 46% that of the moon.

The Earth – Moon System Earth and Moon travel through space as a system You can visualize this by imaging Earth and the Moon as ends of a sledge hammer, flung into space, tumbling slowly end over end.

The Earth – Moon System So, as Earth and the Moon orbit the Sun together, they rotate around the center of mass of the Earth-Moon system, which is called the barycenter.

Gravity Newton’s law of gravitation states:  every particle of mass in the universe attracts every other particle of mass. The greater the mass of the object and the closer they are together, the greater the gravitational attraction

Earth-Moon System In an idealized case, the Moon creates two bulges in the ocean surface: one that extends toward the Moon and the other away from the Moon. As Earth rotates, it carries various locations into and out of the two tidal bulges

Tidal Bulges: The Moon’s Effect Lunar bulges – in an ideal ocean and ideal Earth Tidal periods- time between high tides would be 12 hours Reality – high tides occur 12 hours and 25 minutes. WHY?

Lunar vs. Solar Day During the 24 hours is takes Earth to make a full rotation, the moon has continued moving another 12.2 degrees to the east in its orbit around Earth. The Earth must rotate an additional 50 minutes to “ catch UP” to the Moon Lunar day = 24 hours and 50 minutes Solar day = 24 hours Lunar tides happen (typically) twice every lunar day.

Types of Tides Ideally, we expect two high tides and two low tides of unequal heights during a lunar day.

Types of tides Due to modification from varying depths, sizes, and shapes of ocean basins, tides in many parts of the world exhibit different patterns: 1. Diurnal– have a single high and low water each lunar day Common in the Gulf of Mexico and the coast of SE Asia Tidal period of 24h 50m 2. Semidiurnal– have two high and tow low tides during a lunar day. Common along the Atlantic Coast of the U.S. 12h 25m 3. Mixed tides- Have characteristics of both diurnal and semidiurnal tides Common throughout the world and is the type found along the Pacific Coast of the U.S Tidal period of 12h 25m

How do tides vary during a monthly tidal cycle? Monthly tidal cycle is 29 ½ days.  Moon to complete an orbit around Earth.  This influences the tidal conditions

Combined Effects of Sun and Moon When the moon is at quadrature, in either quarter phase, the tide generating force of the sun is working at right angles to the tide – generating force of the moons. During these times of the month, we experience a minimal tidal range. This is called neap tide The time that elapses between successive spring tides ( full moon and new moon) or neap tides ( first and third quarters) is a little more than 2 weeks.

Combined Effects of Sun and Moon When the sun and the moon are aligned, either with the moon between earth and the sun(new moon), or with the moon on the side opposite the sun(full moon), the tide- generating forces of the sun and moon are added together During these times of the month, we experience the highest and lowest tides. This is called spring tides( because the tide surges, not because of the season.) Syzgy!

Do tides affect marine life? Tidal pools What organisms live in tide pools? How do they survive the area they live in? AD8gJT0

Tide- Generating Force To understand the tide generating force, you need to understand centripetal force. Centripetal force- pulls orbiting body toward the center of its orbit.

Tide- Generating Force The tide generated on Earth is primarily the result of the rotation of Earth and the Moon about their common center of mass, the barycenter. For all particles on Earth, the gravitational attraction of the Moon is directed toward the center of the Moon. The strength of this gravitation attraction is greater for particles closer to the Moon. The more distant from Earth the tide-generating body, the smaller the force. This is why the Moon controls tides far more than the sun.

Equilibrium Theory of Tides Factor that affect the equilibrium theory of tidal bulges: Assume the Moon is aligned with Earth’s equator so that the maximum bulge occurs at the equator on opposite sides of the Earth. Earth requires 24 hours for one complete rotation and so, at the equator we experience two equal high tides and two equal low tides each day. This pattern is call the semidiurnal tides, there are two tidal cycles per 24 hour day.

Equilibrium Theory of Tides Ignore some of the complexities of real tides, but provides good model of gross tide phenomena Some of the simplifying assumptions made by the theory are; 1. Earth has two equal tidal bulges, one toward the Moon and one away from the Moon 2. The ocean covers the whole Earth at uniform depth 3. The continents have no influence

Gravity The gravitational attraction varies with the square of distance, so even a small increase in the distance between two objects significantly decreases the gravitational force between them. When an object is twice as far away, the gravitational attraction is only one quarter as strong.

Lunar vs Solar day - Which is longer?

Lets get some definitions: Syzygy – When the earth, moon and sun are aligned. Tidal range – The vertical difference from high to low tides. Spring tide – Large Tidal range. (Very high highs and very low lows.) Occurs when the moon is in syzygy. Neap tide – Small Tidal range. (Low highs, and high lows.) Occurs when the moon is in the quarter position or “quadrature”

What about the sun? Does the sun have as big of an effect as the moon? Recall:

Size and distance relations.

How does the Sun measure up? Sun is 27 million times the size of the moon. Sun is 390 times farther away from the Earth than the moon. Lot’s of fancy math later… The sun bulge is about 46% of the lunar bulge.

Lunar Tide + Solar Tide What happens when two waves are in the same spot at the same time. Hint it starts with an “I” told you about this stuff last time…

Stupid “Interference”

What causes Neap and Spring Tides? They are the result of interference between solar tides and lunar tides.

Where do we get Neap and Spring Tides?

We are done with notes for now. But I got a fun assignment… You guys rock!!!

Earth-Moon System So the bulge or the tide… Is it that the tides move in and out, or that the Earth rotates through hills of water called tidal bulges? What is the period for tides then?

The Earth- Moon System The barycenter is not located in the space between Earth and the Moon. Instead is it located with in Earth’s mantle 4700km (2900mi) from planet’s center. The tidal pattern we see on Earth primarily results from this “sledge hammer” rotation of the Earth-Moon system around its center of mass.

Earth-Moon System The gravitational forces on objects located at different places on Earth due to the Moon are shown by arrows. The length and orientation of the arrows indicate the strength and direction of the gravitational force.