If you have ever been to a large lake or an ocean, you have probably noticed waves crashing on shore. If you stay long enough and watch the water level, you will either see the water coming onshore or retreating offshore. These movements of the water are known as tides.

Tides are the periodic variations in the surface water level of the oceans, bays, gulfs, inlets, and large lakes that are produced by the gravitational attraction of the moon and sun. Since the moon is closer to the earth, it affects the tides almost two and a half times more than the sun.

Other factors that affect tides are the shape of the coastline and ocean floor, depth of the water, and weather such as hurricanes or winter storms. These factors can influence the height, arrival time, and the interval between high and low tides.

When the highest part or crest of the wave reaches ashore, high tide occurs and the water reaches its farthest point inland. When the lowest part or trough of the wave reaches land, low tide occurs and the water level is at its lowest point.

Tides occur in such a way that when the water is pulled to one side of the earth, on the exact opposite side of the earth the water is pulled at the same amount causing the water level to rise. As the water is being pulled in opposite directions, in the middle of these two locations the water level is being squished causing it to decrease. The earth then rotates as the water stays relatively stationary, so where the water is being pulled those locations are seeing high tide and where the water is being squeezed in is at low tide.

In regards to the location of the sun and moon, there are two types of tides, neap tides and spring tides. When half of the moon is seen which is during the first and third quarter phases, the moon and sun are acting against each other. This is a neap tide which results in smaller variations between high and low tides.

When either a full or new moon occurs, the sun and moon are working together. This causes a spring tide which has larger variations between tides.

The orbit of the moon around the earth and their motion together around the sun are not a perfect circle but an ellipse, which affects the strength of the tides.

The closer the earth is to the sun or the moon the larger the difference between the tides, and vice versa. For the moon, this cycle occurs about once a month. So approximately once every 30 days the moon is closest to the earth and once during that time it is farthest from the earth.

For the sun, this occurs once a year. On approximately January 2 the earth is closest to the sun, and around July 2 the earth is farthest from the sun. If the sun and moon were at their closest points and acting together, the high tides would be exceptional high and the low tides would be exceptional low. The opposite is true if the sun and moon are at their farthest from the earth.

Most coastal locations on earth see two high tides and two low tides each tidal day, which lasts about 24 hours and 50 minutes. Due to the angle of the moon`s orbit in relation to the earth`s equator, some locations only see one high and low tide each day. These locations change as the moon rotates around the earth.

Next time you head out to the beach and stay for a few hours, you should have a better understanding of what is happening to the water level.

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Story Image: WeatherBug User Sara Murray captured this image of Popham Beach in Phippsburg, Maine, at low tide.

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