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Dr Rip's Science of the Surf Dr Rob Brander calls on cosmic forces to explain tides. Most people don’t think too much about the tide. It comes up, it goes down and if you grab a set of timetables you can find the tide times and heights not only for that day, but 10 years in advance. However, while that may be simple, tides can be quite complicated and some readers have asked me to explain how they work. Well, it’s all rather cosmic. The tide is a wave with a crest (high tide) and trough (low tide), but is created by the gravitational pull that both the moon and the sun have on the water in the oceans. The moon orbits the earth creating a moving bulge of water by “pulling the water” towards it. So now there’s a moving bulge of water on one side of the earth and due to the centrifugal force of the earth’s rotation (we’re spinning), there’s another bulge on the opposite side of the planet. So if you were standing on an island in the middle of the ocean, you’d experience two high tides and two low tides each day. Our coast has two tides a day, but others don’t because things like continents and coral reefs get in the way, messing up the path of the tidal wave. You may also notice that the timing of the tides changes
each day, usually by about 50 minutes, which is due to the time difference between the earth’s rotation and the moon’s orbit around the earth. Then there’s the tide range, which is the vertical difference between high and low tide. You may have noticed that the high tide creeps a little higher on the beach each day until it starts retreating on a daily basis. This is where the sun comes in. Both the earth and the moon orbit around the sun. When the moon and the sun line up, their gravitational pull on the earth is combined and we get big tides called spring tides. The high tide comes up higher and the low tide goes out further (a big tide range). This happens during a full moon and a new moon, so about every two weeks. When the moon and sun are lined up at right angles to the earth, their gravitational pull acts against each other and we get neap tides. The high tide doesn’t come up very far and the low tide doesn’t go out very far (a small tide range). So every lunar month (about 29 days), we will get two spring tides and two neap tides. King tides are just a bigger spring tide that occurs when the moon and sun happen to be particularly close to the earth and that happens about twice a year (around Christmas and late May/early June). While our tide range varies from about 1m at neaps to almost 2m at king tides, tide range varies around the world with places like Broome getting tides of 9m. Again, it’s all about the shape of continents, offshore reefs and islands squeezing and amplifying the tidal wave itself. Don’t worry, if all this cosmic stuff has you bamboozled, just read the South Coaster tide tables on page 44-45! Send questions to rbrander@unsw.edu.au.
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