What Are Tides And What Causes Them? Tides are the rhythmic rising and falling of ocean levels. It takes 6 hours and 12 minutes to go from high to low tide. Since the Earth fully rotates every 24-­‐
hours, most locations on Earth experience four tides each day: two high tides and two low tides. The two major factors that influence tides are GRAVITY from the Moon and the Sun and INERTIA from Earth’s rotation. Gravity is a force of attraction between two objects; think of it as two objects pulling on each other. For example our bodies are held onto the Earth by the force of gravity. Every time you jump up, you are quickly pulled back to the ground. The Earth exerts a gravitational pull on your body and, believe it or not, your body exerts a gravitational pull on the Earth. The Earth’s pull, however, is SIGNIFICANTLY larger than your force, because the Earth is so, so, so much bigger. Have you ever been on a ride at an amusement park where you are spun in a circle and feel as though you are stuck to the wall? Or have you ever been on a merry-­‐go-­‐
round at a playground and felt as though you were being pushed off? Inertia is responsible for that sensation. This force describes the fact that objects in motion continue to move in the same direction (in a straight line). When the Earth is spinning, the oceans are also in constant motion and this inertia works with gravity to create tides. We know that the moon exerts a gravitational pull on the Earth. The rock on Earth’s crust is solid and doesn’t bend toward the moon as a result of this pull. Our oceans, on the other hand, are able to bend and react to the pull of gravity. Oceans are made of water and water is fluid, not solid. The gravitational pull of the moon makes the water in our oceans bulge towards the moon (on the side of the Earth facing the moon). Check out the diagram below: On the opposite side of the Earth, or the “far side,” the gravitational attraction of the moon is less because it is farther away. Here, inertia, due to the Earth’s rotation, is greater than the gravitational force. The water tries to keep going in a straight line, moving away from the Earth, also forming a bulge. Remember the sensation of being pushed off the merry-­‐go-­‐round? The same thing happens to the water in the oceans. It is pulled to the outside orbit of the Earth’s rotation thanks to inertia. This creates another bulge of water on the opposite side of the Earth. Check out the diagram below: Force of Inertia Together, the gravitational pull of the moon and the inertia of the Earth’s rotation create two bulges, which are the high tides, of ocean water on either side of the Earth. When the Earth spins under either of these bulges of water, that location experiences a high tide. When a location on Earth is not under one of the bulges, it experiences a low tide. See the diagram below: Explanation: There is only so much water on Earth, so if it is pulled into the bulge, it must be taken away from another location. Low tide locations, areas not under the bulge, have supplied the water for the high tide locations, which are areas under the bulge. When the tide is rising during the transition from low tide to high tide, it is called the Flood Current. When the tide is falling during the transition from high tide to low tide, it is called the Ebb Current. The difference in height between low tide and high tide is called the Tidal Range. It is important to note that the moon is not the only body in the solar system that exerts a gravitational pull on the Earth. The Sun also pulls on Earth and affects the tides as well. Despite the fact that the sun has 27 million times the mass of the moon, it is 390 times farther away from the Earth than the moon. Because the moon is closer, it exerts a greater pull on the Earth than the sun – two times the force as the sun to be exact. Therefore, the bulges produced by the moon (lunar tidal effect) are greater than those produced by the sun (solar tidal effect). There are two specific types of tides that each occur twice a month: Spring Tides and Neap Tides. Spring Tides are when the moon and sun pull together, the bulges are especially large and the flats are especially flat. They occur twice each month at the new moon and the full moon. Neap Tides are when the moon and the sun pull in opposite directions and the bulges are smaller. They occur twice a month at the first quarter moon and third quarter moon. Spring tides produce very high, high tides and very low, low tides. Neap Tides produce very mild high and low tides. There is the biggest tidal range during Spring Tides and the smallest tidal range during Neap Tides. Name _____________________________________________________ Date ________________________ Tab __________________ What Are Tides And What Causes Them? As you read the article, please underline/highlight any information that you feel is important. Then, define the key terms and answer the 4 review questions. Key Term Definition/Description Tides Gravity Inertia Flood Current Ebb Current Tidal Range Spring Tide Neap Tide 1. Which has a bigger gravitational pull on the Earth – the Sun or the Moon? __________________________________ 2. What are the two forces that cause the tides? ______________________________________________ and ________________________________________________ 3. Circle either the words LARGE or SMALL in the sentence below in order to make true statement: Spring Tides have a LARGE / SMALL tidal range and Neap Tides have a LARGE / SMALL tidal range 4. How long does it take to go from high tide to low tide? ______________________________________________