 Alternating rise and fall of sea level within one day.  They are caused by gravitational attraction of sun and moon on Earth.  Causes water of ocean to swell and recede at different parts of the Earth.

Earth and Moon Interaction In order to better understand the nature of tides we must first study the objects we know best. In this case we mean Earth…. and her Moon.

The earth and the moon are two great masses that have a significant gravitational pull on each other. This is what keeps the moon in orbit around the earth, and it is also what causes tides to occur in the ocean. Picture the earth with a uniform level of water all around it.

 The moon ’ s gravity pulls on the earth, and pulls the water towards it.  The water moves up into a slight bulge on the side of the earth that faces the moon. At the same time, there is a force pulling water out in the opposite direction of the moon.

To understand this force, you need to picture the earth and the moon as one unit. Picture two unequal balls on the ends of a stick.  If you spin this stick around, you can imagine the force that a particle might feel if it were on the far end of either the moon or the earth. It would feel a force outward, away from the center of the spin.

 This is called the centrifugal force. The water on the far end of the earth, away from the moon is always being pulled out from the center of the spinning earth-moon unit.  The bodies of water that feel these forces change constantly as the earth rotates within these forces, but the force directions are always toward and away from the moon  These 2 forces are constant and equal to each other.

Semidiurnal tides As the earth turns upon its own axis in about 24 hours, a point on the earth moves through areas with these different forces acting on it. In one rotation (one day), a point on earth travels through 1.Area of high tide (where there is a force pulling water outward), 2.through an area of low tide, 3.through an area of high tide again (the opposite pull), 4. and through another area of low tide, before it 5.returns to the point of origin at high tide. This results in two high tides and two low tides in a day 6.called semidiurnal tides. Semidiurnal Tides

 The moon rotates around the earth once a month, in the same direction as Earth ’ s ’ spin  Result: high and low tides are about 50 minutes later every day.  By the time the earth has done one rotation, the moon has shifted further,  It takes an extra 50 minutes for the moon to be in the same position above a point on the earth. The Tidal Day

1. When the moon is between sun and Earth (new moon), sun ’ s gravitational pull is in same direction as the moons Earth & Sun Interaction 2. During these days high tides are higher and low tides are lower than with just moon ’ s pull alone. 3. And is called spring tide When moon is directly on opposite side of sun (full moon), two gravitational forces work together to make high high tides and low low tides.

 When moon is in first or last quarter, sun ’ s gravitational pull is perpendicular to moon ’ s pull  Sun pulls water away from areas of high tide to areas of low tide, resulting in lower high tides and higher low tides…….  Called neap tides

Inclination The moon does not rotate around the earth ’ s equator,  Result: high latitudes usually have only 1 high and 1 low tide in a day  Called diurnal tides

 Result: it is most likely to have only one tide cycle pr day (diurnal)  Has highest affect in summer and winter months  During these month ” bulges ” in the ocean are offset most from the equator  Sun ’ s affect follows year-long cycle  Earth ’ s orbit in relation to sun also effects tides