What Are the Daily Changes in the Level of Ocean Water?

Pull of the Moon

• The first primary cause of tides is the gravitational pull of the Moon on the oceans. If the Moon was stationary, the tidal period would be equal to the period of the Earth's rotation, or 24 hours. However, the Moon is also revolving around the Earth. This requires the Earth to spin a little further than one rotation to catch up to the Moon's new location, adding approximately 50 minutes to the tidal period. This is why high and low tides do not occur at the same time every day. Instead, they arrive 25 minutes later each time.

Pull of the Sun

• The second primary cause of tides is the gravitational pull of the Sun on the oceans. While the Sun is 27 million times more massive than the Moon, it is also 390 times further away. As distance is more influential to gravitational pull than mass, this distance results in the Sun producing solar tides that are half the level of the lunar tides. When the Moon and Sun are aligned on the same side of the Earth, their gravitational pulls are combined, resulting in the largest high tides, known as spring tides. When they are aligned on opposite sides of the Earth, their gravitation pulls offset each other, resulting in the smallest high tides, known as neap tides.

Types of Tides

• The complicated geometrical relationship of the Earth, Moon and Sun results in three types of tides. Which type of tide occurs depends upon the location. Diurnal tides produce one high and one low tide per tidal day. This type of tide occurs in the Gulf of Mexico and Southeast Asia. Semi-diurnal tides produce two high and two low tides per tidal day. This type of tide occurs on the Atlantic coasts of the US and Europe. Mixed tides produce two high and two low tides; however, the level of each is different. In other words, a high-high tide, low-high tide, high-low tide and low-low tide is produced. This occurs on the west coast of the US and Canada.

Why Are There No Tides in a Lake?

• Tides are formed by the difference in gravitation pulls on a given body of water. For there to be a difference, a large distance is required. For example, the Moon's gravitational pull on a lake is the same at any point, producing no variation in water level. However, for a large body of water, the gravitational pull is different, creating a bulge, or higher level, at the strongest point of gravitational pull.