What causes tides?

At the Minas Basin in Nova Scotia, Canada, the ocean tide rushes in over the course of a day to rise an incredible 50 feet. The landscape of the basin is transformed as water fills the area like a bathtub, as soon as it hits the new shore, it begins to rush right back out again, draining just as quick as it came in.

What causes this incredible phenomenon? Let’s look at how the interactions between Earth, the moon, and the sun can reshape the landscape of the planet over the course of half a day.

A constant pull

Photo of a the moon in a dark sky above a rocky coast
Credit: shaunl/ iStockPhoto

Tides are a result of the gravitational forces of the Earth and the moon. While Earth is the more significant gravitational entity, the moon still exerts a powerful force on the earth and the largest contiguous body of liquid on the planet, the ocean.

The moon pulls on the point that it is closest to with the most force. This creates a sort of gravitational bulge, which acts on the ocean and creates a high tide on the place on Earth that is nearest the moon.

However, there are two high tides every day. How is this possible, considering that it takes a full day for a single location on Earth to complete an entire rotation and be at its closest point to the Moon once again?

Opposite ends of the earth

Photo of the moon
Credit: Kevin Wells/ iStock

This is where things start to get a little more complicated. The moon doesn’t just pull on the earth on the point that is closest to the Earth. The moon exerts a gravitational force on every point, object, and molecule on the Earth at all times.

This means that the moon’s gravity exerts so much force that it pulls the entire Earth slightly towards it. This creates a second gravitational bulge on the side of the planet that is furthest away from the Moon. This means that there is a second high tide every day – 12 hours after the first, when the planet has rotated halfway on its axis.

Tidal complications

Photo of a stormy sea
Credit: shaunl/ iStock

To get a full understanding of what makes tides function however, we need to go a little bit deeper. If tides were only influenced by the sheer force of gravity, then there would be tidal changes in large lakes. These changes might be small in most bodies of water, but in larger ones, such as the Caspian Sea or Lake Michigan, they would be noticeable. Why aren’t they there?

Remember how the moon is always exerting force on every part of the earth? This force is so much smaller than Earth’s natural gravity that it has no influence on objects such as rocks or trees. However, the ocean is a body of liquid that is composed of billions upon billions of molecules of water, which are fluid.

The reason that tides form is because the interaction of Earth’s gravity and the moon’s gravity on molecules that are on the points of the earth that are neither closest nor the furthest away essentially pulls them inwards.

This effect would be negligible on individual molecules of water, but taken as a whole, in the largest system of liquid on Earth, it creates the cumulative effect of pushing water away from the points that are not in line with the gravitational interaction of the Moon, the Earth’s core and the two points where the tides are the highest.

So, to boil it down, tides occur because the moon exerts a small amount of gravitational pressure on everything on Earth, and the ocean is such a massive object that all of those micro interactions create incredible cumulative effects that are so powerful that the water level in Canada can rise and fall 50 feet – every day.

What about the sun?

Photo of the sun setting over the ocean
Credit: Jack Frost/ iStock

While most tidal interactions can be explained by the interaction of the moon and Earth, the sun’s gravitational pull influences things as well. When the three bodies align it creates extra-high spring tides and extra-low neap tides right before and after both a full and a new moon.

So, the next time you watch a sandcastle get taken out by the rising tide just remember – it’s all possible because of the pull of the moon and the vastness of the ocean.