"Tides" refers to the alternating rise and fall of sea level within a day. What causes the sea level to change? It is actually the gravitational attraction of the sun and moon that cause waters of the ocean to swell and recede at different parts of the earth.

The Moon Tide
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 centre of the spin. This is due to inertia. The water on the far end of the earth, away from the moon is always being pulled out from the centre of the spinning earth-moon unit.

The gravitational and inertial forces are constant, always pulling water towards the moon and directly away from the moon. The forces in either direction are equal to each other. 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.

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 from an area of high tide (where there is a force pulling water outward), through an area of low tide, through an area of high tide again (the opposite pull), and through another area of low tide, before it returns to the point of origin at high tide. This results in two high tides and two low tides in a day (called semidiurnal tides).

The Tidal Day
The moon does not stay put, but rotates around the earth at a rate of about 12° a day, or one rotation a month. The rotation is in the same direction as the earth’s spin, so by the time the earth has done one rotation, the moon has shifted 12° further, and it takes an extra 50 minutes for the moon to be in the same position relative to a point on the earth. Therefore, the tidal cycle is not 24 hours long, but 24 hours and 50 minutes. Because of this, high and low tides are about 50 minutes later every day.

The Sun Tide
The tides are caused mainly by the gravitational attraction of the moon and the earth, but there is also a gravitational attraction between the earth and the sun. The effect of the sun upon the tides is not as significant as the moon’s effects. Basically, the sun’s pull can heighten the moon’s effects or counteract them, depending on where the moon is in relation to the sun.

In one month, the moon rotates around the earth. When the moon is between the sun and the earth (at new moon), the sun’s gravitational pull is in the same direction as the moon’s. During these days the high tides are higher and the low tides are lower than they'd be with just the moon’s pull alone. This is called spring tide.

The same thing happens when the moon is on the direct opposite side of the sun (full moon). The two gravitational forces work together to make high high tides and low low tides.

When the moon is in its first quarter or its last quarter, the sun’s gravitational pull is in perpendicular direction to that of the moon. The sun pulls water away from the areas of high tide to the areas of low tides, resulting in lower high tides and higher low tides. These are called neap tides.

The moon does not rotate around the earth’s equator, but follows an orbit that is inclined in relation to the earth’s axis. Because of this, northern and southern latitudes commonly face only one high tide and one low tide in a day, called diurnal tides. The inclination of the moon changes in relation to the earth on a 19 year cycle.

The earth’s inclination in relation to the sun also effects the tides. The sun’s inclination follows a year-long cycle, and is in highest inclination in the summer and winter months. During these months the "bulges" in the ocean are offset the most from the equator, and it is most likely to encounter only one tide cycle per day, or diurnal tides.

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Duxbury and Duxbury (1994) An introduction to the World's Oceans, Wm. C. Brown Publishers,4th edition: Dubuque: Iowa.

Pinet, Paul (1998) Invitation to Oceanography, Jones and Bartlett Publishers: Sudbury, Massachusetts.

Check out this great site:

NOAA's tide page - explanation of the tides, and a great glossary of terms related to tides.

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