The tides were once one of the great mysteries of our planet — and to some, they are apparently as perplexing as ever. But once Isaac Newton laid out his theory of gravity near the end of the 17th century, the pieces of this scientific puzzle started coming together.
Gravity is what pulls the ocean's waters to create the two high tides and two low tides that most coastlines around the world experience daily. In particular, the moon's gravity is a key player. Gravitational force depends on both mass and distance, so even though the sun is vastly bigger, the moon is so much closer to Earth that the gravitational pull it exerts on our planet is still stronger.
The moon's gravitational pull isn't strong enough to change much on land, but liquid water is more responsive to changes in gravity. As the Earth rotates, the distance between the moon and any particular spot on Earth changes, which in turn changes the amount of gravitational pull the moon exerts on that spot. During Earth's daily rotation, the moon's pull is the strongest at whichever spot is closest to the moon — so the water there bulges toward the moon.
That explains one of the daily high tides. But each spot on Earth only becomes this close to the moon once a day, of course, so the other daily high tide must be caused by something else. That something is inertia — the force responsible for Newton's famous observation that "an object at rest will remain at rest unless acted on by an unbalanced force."
As the image above shows, there is also a bulge of water on the side of Earth opposite the moon. This second bulge forms because the moon's gravitational pull isn't strong enough to move the water all the way on the other side of the planet. Inertia causes this most distant water to stay put.
The sun's gravity does play a supporting role in dictating the tides. When Earth, moon and sun align – which occurs during new and full moons – the sun's gravitational pull lines up with that of the moon, making these effects stronger.
High and low tides are more dramatic in some parts of the world because Earth isn't perfectly round and the continents aren't evenly distributed across the planet. Weather can make a difference, too. But in general, the tides are a fairly predictable phenomenon.
Photo: Chris Combe | Flickr