What causes Tides? If tidal forces were based only on mass, the Sun should have a tidegenerating

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1 What are Tides? Tides are very long-period waves that move through the oceans as a result of the gravitational attraction of the Moon and the Sun for the water in the oceans of the Earth. Tides start in the oceans and move toward the coastlines where they appear as the regular rise and fall of the sea surface. When the highest part, the crest of the wave, reaches a particular location, high tide occurs; low tide occurs when the lowest part of the wave, its trough, reaches the coastline. The difference in height between the high tide and the low tide is called the tidal range. As this long period wave approaches a coastline water level begins to rise and water moves horizontally into a location, the motion of water into a particular place is called a flood current. As the crest of the wave reaches the location, high tide occurs and water then begins to recede. Water then moves horizontal away from a particular location and water level falls. The movement of water away from a particular location is called an ebb current. When the water level reaches its lowest point the location experiences a low tide. This relates to the trough of the long period wave. The speed of the flood and ebb currents a particular location experiences depends upon many factors including size and shape of the estuary, bay or inlet. The slope of the bottom of the area the tide affects among other things, such as weather and atmospheric pressure, also have an effect on the strength of the tidal current. These factors can be quite complex and have an effect on how fast the current changes and the ultimate depth of the tides. This combination of factors is beyond the scope of this tutorial and will not be discussed in detail. 1

2 What causes Tides? There are several factors that act together to cause tides. The factors that we will discuss here are Gravity and Inertia. Gravity is one major force that creates tides. In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the Sun and Moon on the oceans of the Earth. Newton s Universal Law of Gravitation states that there is a force of attraction among all objects in the Universe. The strength of that attraction depends upon the mass of the object and the distance between them. The greater the mass of the objects and/or the closer they are to each other, the greater the gravitational attraction between them. Tidal forces are based on the gravitational attraction between the water in the oceans and the Sun and Moon. With tidal forces on the Earth, the distance between two objects usually is more important than their masses. The effect of distance on tidal forces can be seen in the relationship between the Sun, the Moon, and the Earth s waters. Our Sun is millions of times more massive than our Moon! Based on its mass, the Sun's gravitational attraction to the Earth is almost 200 times greater than that of the Moon to the Earth. If tidal forces were based only on mass, the Sun should have a tidegenerating force that is millions times greater than that of the Moon. BUT the Sun is almost 400 times further from the Earth than the Moon so its tide-generating force is reduced, the Sun s tide-generating force is about half that of the Moon! 2

3 Gravity, Inertia and Tides The gravitational attraction between the Earth and the moon is strongest on the side of the Earth that happens to be facing the moon, because that side of the Earth is closer to the Moon. This attraction causes the water on this near side of Earth to be pulled toward the Moon. This gravitational force is strong enough to pull the water toward the moon slightly, causing a bulge of water on the near side toward the moon. This bulge is a high tide and will occur at the location that is directly under the Moon Gravity is only one of the major forces responsible for creating tides. Another is Inertia. Newton s first Law of Motion (the Law of Inertia) states that an object in motion will stay in motion in a straight line at a constant speed until acted upon by an outside force. Since the Earth is rotating on its axis the water is in motion and according to the Law of Inertia an object in motion will stay in motion in a straight line, so as the Earth rotates inertia tries to sling the water off of the planet! The result is a bulge of water on the far side of the Earth at the same time and on the exact opposite side of the Earth as the bulge caused by gravity. The combination of gravity and inertia creates two bulges of water. One forms on the side nearest the Moon and one on the side furthest. Locations along the coastline experience High Tide as the bulge reaches it peak. Over the rest of the planet the forces are in balance and water is either rising (flood current) or falling (ebb current). Locations one half way between the tidal bulges experience Low Tide. Since water is fluid, the two bulges stay aligned with the moon as the Earth rotates. The gravitational attraction of the Sun for the Earth is considerable but the interaction among the gravitational forces of the Sun, Moon and Earth is very complex. 3

4 How often do Tides Occur? Most coastal areas on Earth experience 2 high tides and 2 low tides every day, these are called Semi-Diurnal Tides. There are a few exceptions to this. The reasons have to do with the size, location and shape of the Continents and are fairly complex. They will not be discussed further in this tutorial. You are probably familiar with the idea that a 24 hour day is caused by the rotation of the Earth on its axis. It is the time it takes a particular location on the Earth to rotate once on its axis to the same exact spot under the Sun. We call this a day! Just as the Earth rotates under the Sun, believe it or not it also rotates under the Moon! We call the time it takes a particular location on the Earth to rotate to the exact same spot under the Moon a Lunar Day. There is one small catch though, while the Earth rotates through one day, the Moon moves in its orbit around the Earth. The combination of these two motions causes the Earth to have to rotate a little longer to catch up to the same exact spot under the Moon. The additional amount of time is 50 minutes in one Lunar Day. As a result the tide in most coastal areas of the Earth changes every 6 hours 12 minutes 30 seconds. For example: If the first high tide of the day at Indian River Inlet occurred at 9:00 AM then a low tide would be expected at 3:12:30 PM. The second high tide would be expected at 9:25 PM with the next low tide expected at 3:37:30 AM. The first high tide of the following day would be expected at 9:50 AM. This is a very predictable pattern! The diagrams below illustrate the position of a particular location on Earth relative to the location of the Moon and the time of the associated tide. Notice that as the Earth rotates on its axis for 24 hours the Moon has moved in its orbit. This causes the Earth to have to rotate an additional 50 minutes until a particular location is under exactly the same spot under the Moon. 4

5 Effect of the position of the Earth, Moon and Sun on Tides: The gravitational attraction between the Moon and the waters of the Earth is the main cause of the tides, but the Sun certainly plays a part. The tide generating force of the Sun is about half of the Moon. When the Earth Moon and sun are in alignment, the force is additive. The gravity of the Moon and the Sun work together causing extra-high high tides and very low low tides. These tides are called Spring Tides and occur twice per lunar cycle. Spring Tides occur during the Full and New moon phases. The highest high tides occur during the New moon phase since the Moon and Sun are on the same side of the Earth and their gravity is pulling in the same direction. When the Earth, Moon and Sun are oriented to form a right angle they pull of gravity from the Moon and Sun fight each other producing lower than average high tides. These are called Neap Tides and occur twice per lunar cycle during the First and Third Quarter phase. 5

6 The gravity of the Moon has more of an affect on tides than the Sun because the Moon is closer. BUT!! The Sun DOES have an effect! During New Moon and Full Moon phases high tides are higher than other phases because the gravity of the Moon and Sun work together. These are called SPRING TIDES. During First Quarter and Third Quarter phases high tides are lower than other phases. These are called NEAP TIDES. Resources: 6