How the Moon Affects the Changing of the Tides
The gravitational pull of the moon and the rotation of the earth, coupled with the centripetal force of the earth, causes two high tides and two low tides throughout the day. Find out why the tides don't occur in the same cycle as the rotation of the earth with help from the chair of a department of environmental studies in this free video on the moon and tides.
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Well the moons control the tides. It's plain and simple. There, around here we have what are called diurnal tides. We have two highs and two lows during the day. Now where do those come from? Well it's a combination of three forces essentially. You have the gravitational pull of the moon. Then you have the rotation of the earth, the centripetal forced of the earth and then you have gravity, alright, counteracting so the water doesn't fly off from the planet which is a good thing. But what happens is you have the earth here and you have the moon here. The moon has a gravitational pull on the earth, the earth has a gravitational pull on the moon, that's why it stays in orbit. Now you don't see that gravitational pull on the land surface so much but you do see it with the water and with the moon over here as it pulls what it does is it causes the water to bulge out towards the moon. There's one of your high tides right there. Now, at the same time you're having a bulge here, the earth and the moon are rotating. So what happens? Well you get a bulge wherever the moon is but if we make this the moon now and here's the earth, alright, we've got a bulge here, but with rotation of the earth we get for every action there's an equal and opposite reaction so you get another bulge over on the opposite of the earth from the rotation. It's just like riding around in one of the carnival rides where you get pushed out and you try to get thrown off. Well there's your second bulge, there's your other high tide. In between over here and over here, well if you're pulling the water out here, you got to take it from somewhere so you take it from here. There's your lows right there. Now what happens, the moon moves around and as it does those high spots, the bulges, and the low spots move around. You're in one spot, you're in one position and they move passed you as the moon goes around. There's your high and low tides that occur everyday. Now they don't occur in the same cycle as the rotation of the earth. We know that the earth makes one rotation every 24 hours. The moon rotates around us every 24 hours and 50 minutes. So everyday the tides are offset by about 50 minutes, they're later by about 50 minutes everyday until they catch back up again and that's what's known as a lunar month, it's about 27 days, alright, that through 27 days losing 50 everyday after 27 days you're back to the same starting point again.