I'm Dr. Rob Brander and I'm a surf scientist from the University of New South Wales. And in this episode we're going to look at a different type of wave and believe it or not, that wave is actually the tide. Now the tide comes up and you can see the high tide mark and it goes back down the and you can see it's low tide now, and it does that every single day.
The tide is a wave. High tide is the crest, low tide is the trough. So there's a lot of water moving up and down the beach, and we need to know what is forcing that water.
And there's two other things about the tide that are interesting. One is that the timing of the tide changes every day. High tide today is going to be different than high tide tomorrow. And the other thing is the amount that the tide goes up and down also changes every day.
So why is that happening? Now the timing of the tide is all about the relationship between the Earth and the Moon. So the Earth is pretty big and the Moon is pretty small.
But the Moon is big enough to exert a gravitational force on the Earth. And that causes the water on the Earth to bulge up towards the Moon. Now if you were standing somewhere on the Earth, remember, the Earth is rotating around.
And what happens is you would experience two of those bulges each day. And that means that you'd have two high tides and two low tides every day. The other thing to remember is that... the Earth is rotating at a certain speed, the Moon is also orbiting around the Earth and orbits at a different speed and there's a lag of 50 minutes between the two.
So the high tide every day is 50 minutes later and the low tide every day is 50 minutes later because of the difference in the Earth's rotation and the Moon orbiting around the Earth. Tide range is the vertical elevation difference between the water level at high tide and the water level at low tide. The reason it changes every day not only is related to the Earth and the Moon, but now the Sun as well. Now, remember that the Earth and the Moon both orbit around the Sun, and sometimes the Sun and the Moon are lined up so that their gravitational forces act together and cause a greater tidal bulge on the Earth, and that creates bigger tides which would be more likely to occur. we call spring tides.
Now, when the moon and the sun are at right angles to each other, their gravitational forces cancel each other out and we get smaller tides, and we call those neap tides. So every two weeks we get really big tides called springs, and every two weeks we get smaller tides called neap tides. You may have heard of something called a king tide, and a lot of people think the king tide is a dangerous sudden surge of water. Well, it's not.
A king tide is just a bigger tide. In fact, it's a bigger spring tide. Because sometimes the moon and the sun are particularly close to the earth.
And when that happens, which is two times a year, your tides are going to be bigger. So they're nothing to worry about, they're totally predictable, and they're not dangerous at all. Well, we've just talked about what tides are and some of the characteristics of tides that you might find on beaches.
But what we'll do in the next episode of Science on the Surf is something different. We're going to actually focus on the different types of beaches and how they behave.