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What Causes Semidiurnal Tides?

Tides are the cyclical rise and fall of ocean levels. They represent very long-period waves that circle the globe. The crests of these waves represent high tides, while their troughs represent low tides. Solar and lunar tides normally occur twice a day, and are therefore called semi-daily or semidiurnal tides. The primary tide-generating force is the gravitational attraction between the Earth, the Sun and the moon.

Gravitational Pull
- Newton's law of universal gravitation states that all bodies attract one another. This gravitational attraction, or force, is directly proportional to the masses of the two bodies, and inversely proportional to the square of their distance apart. This means that both the moon and the Sun exert a gravitational pull on the Earth's oceans. Their forces create oceanic bulges at the point on the globe that is closest to them, while another bulge is created on the direct opposite side of the globe because that is where the gravitational pull is weakest. Apart from all other modifying factors, these bulges are, at the equator, around 3.3 feet high for the moon and 1.65 feet high for the Sun.
Semidiurnal Lunar Tides
- The earth spins once around its axis every 24 hours. However, the moon also orbits the Earth. This means the Earth must spin a little farther to catch up to the moon's new location. This extra spin adds approximately 50 minutes to one lunar tide period. Therefore, a given location will encounter two lunar bulges, or high tides, every 24 hours and 50 minutes, or one high tide every 12 hours and 25 minutes.
Semidiurnal Solar Tides
- While gravitational attraction is inversely proportion to the square of distance, tidal forces are inversely proportional to the cube of distance. This means that for tides, distance is much more important than mass. The sun is 27 million times more massive than the moon. However, it is also 390 times farther away. Therefore the tide-generating force of the sun is reduced by 59 million times that of the moon. By dividing 27 million by 59 million, it turns out that the Sun's total tide-generating force is roughly half that of the moon. Thus, solar tides are half the intensity of lunar tides, but occur every 24 hours, or one solar high tide every 12 hours.
Other Tide-Shaping Factors
- While gravity is the primary tide-generating force, there are three major factors that can impact semidiurnal tides. The shape of the shoreline, such as bays and estuaries, can amplify tides by funneling them. The gradient of the shoreline also has a large impact on tides. For example, continental margins amplify tides, while mid-ocean islands typically have tides around the base height, which is 3.3 feet. Finally, local wind and weather patterns can amplify tides.

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