The moon pulling on Earth’s water doesn’t explain how tides work


You’ve probably heard that the moon causes tides by pulling Earth’s water toward it. It’s true that the moon’s gravity tugs all water facing it slightly toward the heavenly body.

This creates a high tide on Earth below the moon. As the Earth spins, the moon appears over different parts of the sky and high tide follows it around.

But that’s less than half the story.

While the moon pulls the water underneath it away from the Earth, water on the opposite side isn’t subject to the same pull; the moon offsets our planet’s center of gravity, making the force slightly weaker on the side of Earth not facing the moon.

That weaker gravity field lets oceans rise higher. It’s more nuanced than that, though. Except the dog in this case is a planet spinning in one direction.

As Earth rotates, the water moves with it. So when the moon weakens the pull of gravity on the side of Earth it is not facing, the momentum of all that water causes the liquid to bulge on the weaker-gravity side. (That momentum helps waters rise on the side facing the moon, too, since the moon’s gravitational pull is millions of times weaker compared to the Earth’s.)

This forms a sphere-like oval of water, where the narrower edges of the oval shape are where the tides are high. The two forces drawing the water in opposite directions leave the long edges, where tides are low.

So at any given moment on Earth, this complex interaction creates two high tides and two low tides.

But it’s not just the moon

The moon’s complex gravitational meddling isn’t the only force acting on the oceans and other bodies of water. Land, wind, and weather can all affect tidal patterns.

Storm surges, for example, happen when a hurricane’s high-speed winds push water toward land, flooding the shoreline and beyond. A high tide during a storm surge, or a storm tide, can raise the water level 20 feet or higher.

Here’s a diagram of how it works when this wall of water comes ashore. Here, a 15 foot surge results in a 17 foot storm tide, with waves atop that:

storm surge

Hurricane Sandy’s storm tide reached 8 feet or more in parts of the Jersey Shore, and the surges eroded at least 98 feet of beach during Hurricane Patricia. Tropical Cyclone Winston, which has killed at least 20 people since making landfall on Fiji yesterday, has seen surges at least 10 feet high.

The relationship between the sun and moon, what we see on Earth as lunar phases, also affects tides.

When the two align during a full or new moon, the combined gravity can make tidal ebbs and flows more intense. A new moon during Hurricane Sandy worsened the floods that caused incredible destruction across the Northeast.

There were similar worries about winter storm Jonas — the astronomical pull of the sun and moon meant that the tides were running a few inches higher than usual.

In normal weather, this might not be noticeable — but the storm surge, high tide, andperhaps even rising sea levels caused record flooding in New Jersey and Delaware.

It’s complicated

All these forces — gravity, geography, weather, physics — are acting on tides basically all of the time, either together or in opposition to one another. It’s why tidal patterns can vary so widely across the globe, or even on a particular beach.

Fun fact: The largest variation in tides anywhere in the world is at the Bay of Fundy in Nova Scotia, where the tidal range can be as drastic as 55 feet.