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A tide occurs when the level of the ocean rises or falls due to tidal forces. The moon and the sun both exert tidal forces on the earth and the earth's ocean. The movement in the solid body of the earth caused by tidal forces is called the earth tide; it is not readily perceptible because of its minimal magnitude and because the ocean tide is larger and in the same direction. The ocean tide is a movement of the level of the ocean relative to the solid earth of about 2 meters (6 feet). The movement of water towards the shoreline is called the flow of the tide, while the movement of water away from the shoreline is called the ebb of the tide.

As the tidal forces caused by the moon cause elongation of the ocean in both directions, a tide occurs approximately when the moon is at the meridian and when the moon is opposite the meridian. The rotation of the earth relative to the position of the moon takes approximately 24 hours and 50 minutes, thus a high (or low) tide is experienced about every 12 hours and 25 minutes.

While the "average" tide has a magnitude of about 2 meters, the actual timing and amount of change in water level experienced is dependent upon the relative position of the sun and the moon and the shape and orientation of the coastline.

The tidal force the sun exerts on the earth is about 45% that which the moon exerts. When the sun is approximately 90 degrees away from the moon, the conflicting tidal forces result in a neap tide, which is noticeably smaller than the average tide; when the sun and moon are approximately in the same or opposite positions in the sky, the tidal influences combine to create a spring tide which is significantly larger than average, with both a higher high tide and lower low tide. Spring tides thus occur at new moon and full moon, i.e., twice a month.

The tide acts to cause the surface of the ocean to become an ellipsoid with its long axis pointed at the moon, but as the earth rotates beneath the moon, the water of the ocean must move in response to tidal forces. As the ocean does not cover the entire body of the earth, the movement of the water will interact with the coastline.

Bay of Fundy

When water flows into bays which become narrower, the increase in the height of water will be magnified. The most extreme example of this is in the Bay of Fundy between New Brunswick and Nova Scotia, where the tidal change reaches an extreme of 16 meters at the Minas Basin at the narrow end of the bay. This effect is amplified because the length of the Bay of Fundy is similar to the natural wavelenth of the tide, which significantly increases the tide.

Other phenomena with "tide" in the name

The phenomenon known as a rip tide is not necessarily a tide phenomenon, though it can be caused or strengthened by the ebb of a tide. A tsunami is also known as a "tidal wave" in colloqiual english, even though the cause is localized large earth movements (earthquakes or landslides), not any tidal force.