Tides

Split view of the intertidal zone on the Schoodic Peninsula

NPS Photo/Crystal Lewis

An 8 to 12 foot rise and fall of the seas happens twice a day every day at Acadia National Park. This natural phenomenon is known as the tides. At high tide, coastal features are covered with sea water and access to certain islands disappears. At low tide, the seas recede revealing coastal features and ocean life that lives in this intertidal zone.

But how do tides occur? The story of tides starts between 225,623 and 252,088 miles away - on our Moon.

 

The moon from earth.  NPS Photo/Sardius Stalker.

How the Moon's Gravity Impacts the Earth

It might sound like "lunar-sea", but the Moon’s gravitational force on the Earth is so powerful that it causes 8-12 foot tidal fluctuations of here and upwards of 20 foot tides further north along the eastern seaboard. Even though the Moon has 80 times less mass than the Earth, the its gravitational force changes the shape of our planet as it orbits the earth. This force is stronger on the side of Earth closer to the Moon and weaker on the side of the Earth farthest away from the Moon.

 

This animation shows the tidal force in a view of Earth from the North Pole. As regions of Earth pass through the bulges, they can experiences a high tide. Credit: NOAA

How Tidal Forces Change the Earth’s Shape

As the Earth rotates on its axis and revolves around the Sun, the side of the Earth that faces the Moon bulges outward. It's almost as if the Earth were reaching towards the Moon. The bulge doesn’t just appear on one side, but on both, turning our world into a giant egg shape, or oval. We call this phenomenon tidal force.

A few reputable scientific theories explain the bulges in different ways. One theory suggests that the tidal force is a kind of pulling exerted by the Moon on the Earth. Land doesn’t move a whole lot - only a few centimeters or so - but oceans can be sucked out quite far. As the Moon’s gravitational pull ultimately creates bulges on the ocean water closest and farthest away from it, the Earth spins on its axis. High tides are experienced where the water balloons out and low tides occur in between these bulges. Most coastlines experience two high tides, two low tides, and everything in between as the Earth rotates through the two oceanic bulges each day, which are always pointed exactly towards and exactly away from the Moon.

A second theory suggests that instead of a pulling action toward the Moon, the mechanism producing the tides is more of a squeezing toward Earth’s core. As the tidal force bares down on the Earth, the ocean on the sides closest to and furthest away from the Moon bulge out. Either way, the result is that our Earth always looks like a football.

 

What About The Sun?

Though the Sun is about 400 times larger than the Moon, it exerts much less tidal force on the Earth than the Moon. Why? The answer has to do with distance. The Sun is about 400 times farther away from the Earth than the Moon, and this great distance weakens the Sun’s tidal force. If the Moon were to disappear tomorrow, the Sun would still cause tides on Earth. However, the tides would only be about 1/2 of the size they are today.

But when the Sun and the Moon align, they do create extra-large tidal ranges known as spring tides. Spring tides have nothing to do with the seasons, and occur twice a month at the new and full Moon. Because the Sun and Moon are aligned during a spring tide, it’s like they are on the same tug-of-war team, pulling the Earth’s oceans towards them.

But the Sun and the Moon are not always aligned. During the first and third quarter Moon, the Sun is at a right angle to the Moon. A slightly lower high tide, and a slightly higher low tide, result. These milder fluctuations are called neap tides.

 

Life Among The Tides

Life on the coast relies on the rhythmic flow of salt water as it surges out and drifts in. People have relied on the food sources revealed by the tides for thousands of years as they work the waters at the lands edge.

But humans aren't the only ones seeking the animal life that appears as the tide recedes. Rock crabs wave their claws to ward off hungry herring gulls during the low tide buffet, and breadcrumb sponges soak up plankton and oxygen as the water rushes back in. Seaweed floats up and spreads out with the incoming tides, and blue mussel shells snap closed as the tide ebbs in and out, in and out, all day long. A multitude of marine animals and plants call this intertidal zone home.

 

Exploring Acadia's Tidepools

Exploring these protected resources of Acadia provides an opportunity to peek through a window to the sea. A variety of marine animals and colorful algae species become exposed on the rocky shores of Acadia at low tide. Exploring this area at low tide is known as 'tidepooling.' Tidepooling is a fun and fascinating activity if done safely and in a way that leaves tidepools unharmed for future generations.

To ensure the health of our resources and the safety of visitors, please visit our Tidepooling page to find out more about tidepooling in Acadia and how you can protect these fragile places.