How Does the Moon Affect the Tides?

For years, the ocean has fascinated humans. It seems as though the tide takes on a life of its own, especially considering the vast array of life that lives below its surface. However, the water doesn’t just move on its own — a particular body in outer space has quite a significant role to play. How does the moon affect the tides?

Gravity Plays a Large Part

The moon’s gravity is a major component of how the water on Earth moves. When the sea rolls in or out, you’re watching the planet slightly elongate from the pull of the moon. It gets slightly shorter at the poles and longer at the equator, creating tidal bulges.

The water squeezes around the globe, extending on that closer end and the farthest one. The water at the poles pushes down and out, adding to those swelling currents. These create what are known as high tide and low tide.

When a beach or body of water is experiencing high tide, it’s either facing the moon directly or is on the opposite end of the Earth. At low tide, the gravity pulling the water to create a high tide is tugging hard enough that these bodies lose some or all their water.

You can visualize it like an American football with a circle in the middle. When someone standing on that circle is pointed toward the longer end of the ball, they would be experiencing high tide. When that circle rotates to the flatter part, it represents low tide. Going from one high tide cycle to another — as well as a low tide to a low tide — takes around 12 hours.

How does the moon affect the tides? These gravitational pulls create the waves you see at the beach, the flow of a nearby stream and the ripples in a pond. However, because it orbits in the same direction as the Earth rotates, it’s not possible to track the tides exclusively by this natural satellite. It takes about 50 minutes for the moon to catch up with a given point on Earth, and the planet’s shape and weather patterns play a part in the conditions, as well.

The Sun Contributes, Too

Of course, the larger of the two bodies in the Earth’s sky also has a role to play in the tides. Interestingly, if the moon were not around, this planet would still have tides — they would just be much smaller and occur at the same time each day. However, since it is part of the equation, it has some interesting effects depending on the phase.

If the sun and moon are pulling in the same direction when the latter is in its full or new phases, they squeeze the Earth harder. This creates what scientists call “spring tides,” which are a bit stronger than the standard high tide. Alternatively, when the moon is in its first or last quarter phases, it is 90° to the sun, which means the two bodies are pulling and pushing in opposite directions. This weakens the tidal effect, creating “neap tides.”

This happens due to the proximity of the moon versus that of the sun. Were the distance between the moon and Earth one-third what it currently is, the former would have 50 times more of an effect on the waters than the sun. If the moon were three times further away from the Earth, its impact would be just 1/10th of the sun’s.

The moon’s effect on the tides is actually about two and a half times larger than the sun’s, the latter having about 40% of the former’s tidal force. It is also 400 times further away than the moon, but its diameter is about 400 times bigger, so it still has an important, notable effect on the water.

Why Do Tides Vary Globally?

Were the tides an exact science, they would be much simpler to predict. However, since the Earth has multiple influences on its oceans and the planet isn’t an exact sphere, the water’s behavior tends to change depending on the part of the world someone is in.

If the Earth were a perfect sphere with no continents, the whole planet would see two high and two low tides equally proportioned each lunar day. However, those massive swaths of land block some of the water surges as the gravitational pull changes with the rotation of the planet. Thus, the pattern of the movement will be different in a nearby ocean basin, or even within a different region of the same basin.

The major shorelines worldwide experience what scientists refer to as semidiurnal tides. These are the most basic tide patterns, where the highs and lows reach the same heights. When those highs and lows are different, they become mixed semidiurnal tides. In some areas, there is just one low and one high tide a day, which are diurnal tides. These occur in enclosed basins like the Gulf of Mexico, the Java Sea and Bristol Bay.

The Difference Geography Makes

There are also sometimes geographical features at play, the Bay of Fundy being a perfect example. The tide variations here are the highest in the world because of the naturally occurring rocking motion within the water and the shape of the basin. Its funnel-like shape pushes water higher up the shores, while the water within it takes about 13 hours to move from the head of the bay to the mouth and back again.

This coincides with the cycle of the Atlantic Ocean, which floods into the bay every 12 and a half hours. At high tide, the Bay of Fundy can get as high as 52 feet thanks to that extra push. This makes it a perfect candidate for the projects underway attempting to harness the power of the tides as a source of clean electricity.

The Moon’s Effect on the Tides

How does the moon affect the tides? Its gravitational pull tugs the Earth’s water, tugging it at the equator and pulling it from the poles. As the planet shifts, the water shifts, as well, creating the stark low tides and massive waves at high tides. While the sun has some effect, the moon and the Earth’s geography and weather patterns have the largest influence.