What causes tides?
Tides are caused mainly by the Moon's gravity pulling on Earth's oceans, with help from the Sun. This creates two bulges of water, so most coasts get two high tides a day.
The Moon pulls hardest on the side of Earth facing it, drawing the ocean into a bulge. On the opposite side, the Moon's pull is weakest, and another bulge forms because the water is effectively left behind. As Earth rotates through both bulges, coastlines experience high and low tides, usually about 12 hours and 25 minutes apart.
The Sun's gravity also affects tides, but less, because it is so far away. When the Sun, Moon and Earth line up at new and full moons, their pulls combine to make extra-high and extra-low spring tides. When they are at right angles, at the quarter moons, the smaller neap tides occur.
The biggest tides
The shape of coastlines can make tides much bigger. The Bay of Fundy in Canada has the highest tides in the world, with a difference of up to about 16 meters between high and low water.
Did You Know?
Tides are slowly lengthening Earth's day by about 2 milliseconds per century, because friction from the moving water acts like a brake on the planet's spin.