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The King of Tides

By Dr Rob Brander on January 21, 2011 in Other

Every year around Christmas there’s talk of mysterious ‘king tides’ that supposedly rush up the beach inundating hapless sunbathers and swamping coastal properties. It’s a bad rap. There’s nothing mysterious or dangerous about king tides. They are, however, very cosmic.

The tide is a wave that has a crest (high tide) and a trough (low tide) just like any other wave. The big difference is that the tidal wave is not created by wind, like most of the waves we see, but by the gravitational pull that both the moon and the sun have on the water in the oceans.

The moon orbits around the Earth and creates 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.

Sydney does indeed have two tides a day, but other coasts don’t because things like continents and islands 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 fifty minutes. That’s because the Earth’s rotation and the moon’s rotation around the Earth are a little out of whack.

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, about every two weeks. When the moon and sun are lined up at right angles to the Earth, their gravitational pull sort of cancels one another out 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).

The good news is that all this stuff is totally predictable. We know exactly what the timing and height of a tide will be years in advance. The king tide is just a bigger spring tide that occurs when the moon and sun happen to be particularly close to the Earth. It only happens about twice a year and the tide range in Sydney is close to two metres, so don’t freak out. If all this cosmic stuff has you bamboozled, don’t worry, just check out the tide table in the back half of The Beast and read the numbers.