Why Do We Have Tides?

Every day, all over the world, the oceans experiences two high tides and two low tides. If you surf, snorkel, boat, paddleboard, or spend any time at the beach, you might have even checked tide reports for the day so you know what to expect (a great practice by the way). As the day drags on, the waters come and go. But why twice a day? Well, in short: it all has to do with the moon.

Our Earth has gravity, and therefore a massive gravitational field; it’s what keeps you from floating right off the ground into space. The moon also has a gravitational pull and, though smaller than ours, it effects how the ocean acts. The Earth rotates on its axis every 24 hours, like those toy globes with a pole through the top and bottom. Since the moon moves as well, tides are based on the “lunar day”, which is 24 hours and 50 minutes. This is why, occasionally, tide reports will only have 3 tides listed for the day rather than the normal four. The last tide of the day prior happens right before midnight, and the first tide of the next day occurs right after. The gravitational pull from the moon essentially squeezes the water around the globe, with the highest tide of the day facing the moon, the low tides on the sides, and the lower high tide away from the moon (image below).

Image Source: Maximum Weather Instruments, 2022

While gravity pulls the highest high tide toward the moon, physics and the laws of inertia create that lower high tide and the ocean bulges up. This bulge will always be smaller than the moon’s side. As the Earth rotates, the tides remain in the same formation (highest high tide facing the moon, and so on). This moves the tides around the world, which is why high and low tides in different locations always occur at different times. Part of an animation from NOAA is attached below, because I believe this concept is understood best when visualized.

Earth’s tides are one of the most consistent things we experience each day, like how the sun rises and sets. Tides are essential for near shore ponds and tidepools (hence the name) and all the organisms that live there. They carry nutrients and small, drifting marine life like phytoplankton. Many animals also use the tidal patterns in different aspects of their life; dolphins have been seen moving with the tide, crabs learned to burrow into the sand before drying out, and epaulette sharks have even evolved to walk out of tidepools after their mid-day snack if the waters get too low. It’s interesting to think that something outside of Earth is affecting them all, but it goes to show how everything is truly tied together (or should I say tide?).

References

NOAA. (2022). What causes tides? NOAA SciJinks – All About Weather. https://scijinks.gov/tides/

Taylor, C. (2022, October 12). What are tides and how do they work? Maximum Weather Instruments. https://www.maximum-inc.com/how-do-tides-work/