 The gravity of the moon and the sun pulls off the water in the oceans. And this is the cause of tides. The moon influences the tides the most. So let's first look at the moon. When the moon pulls the water up, it creates two bulges of water. One bulge directly under the moon. And one bulge on exactly the opposite side of earth. The moon causes these two bulges. And it would do so even without the sun and even without the rotation of earth. Because earth turns, these bulges of water move around earth. Or rather, earth moves underneath them. When there's no bulge, it's low tide. And when there is a bulge, it's high tide. Earth takes 24 hours to rotate. So it would be logical to think that all over earth there's a high tide and a low tide every 12 hours. So twice a day. But this is not the case. The shape of the sea bottom also influences this pattern. We call this the topographical features of the sea bottom. For instance, in the Pacific Ocean there's no land. So the difference between a high and low tide is only a few centimeters. But in the small bit of sea between France and England, all the water coming from the open ocean is forced through a narrow space. This causes a huge difference between a high and low tide in Calais and Dover. This can be around 15 meters or even more. The rotation of earth also has a direct effect on the tides. The rotation of earth causes the water in the oceans to move in big, slow whirlpools around certain points of the oceans. So together, the shape of the sea bottom and the big whirlpool currents disturb the simple two high tides to low tides picture. In fact, there are three different patterns of tides. There are semi-diurnal tides. Semi-diurnal means half day, so twice daily. This is a pattern of two tides a day. Two high tides and two low tides. And they're both equal in height. There are diurnal, daily tides. So this is a pattern of only one tide a day. One high tide, one low tide. And there are mixed tides. This is a pattern of two tides a day. Two high tides and two low tides. But the tides are of unequal heights. Now let's look at the sun. The gravitational pull of the sun also creates two bulges in the water on either side of earth. But because the sun is much, much further away than the moon, the bulges it creates are much smaller. At certain times in the month, the bulges created by the moon and by the sun line up with each other. This happens at full moon. When the sun and the moon are on opposite sides of earth and we can see the whole moon. And at new moon. When the moon is directly in front of the sun and we can barely see the moon. This means that at full moon and new moon, the high tides are extra high and the low tides are extra low. We call this a spring tide. When the sun and the moon are located at 90 degree angles to each other, you can see this in the sky as half moon. They work against each other and the tides are smallest. We call this a knee tide. When there is a big difference between high and low tide, there is a lot of water movement, which means that there will be strong currents. Full moon and new moon are not the best times to dive.