From the January 2018 issue

I saw a program that showed the Moon revolving around Earth, causing the oceans to swell on both sides of the planet. Shouldn’t the ocean opposite the Moon be shallower than the side facing it?

Michael Gamble New Berlin, Illinois
By | Published: January 3, 2018 | Last updated on May 18, 2023
The ocean nearest the Moon bulges outward in a high tide because of increased gravitational attraction. On the other side of Earth, the ocean bulges outward as the rest of the planet is pulled toward the Moon. Low tide occurs at the two points on Earth between the bulges.
Astronomy: Roen Kelly
Tides occur because of the uneven pull of the Moon’s gravity on different parts of Earth. Portions closer to the Moon are pulled more strongly than those farther away. As the Moon pulls on the portion of the planet nearest its location, the water deforms and bulges toward the Moon more easily than the seafloor beneath. On the far side of Earth, the water is “left behind” as the rest of the Earth feels a stronger attraction to the Moon, causing the water to pile up and bulge away from the planet. Earth’s rotation causes most locations to experience these two bulges each day, approximately 12 hours apart.

The Sun also influences tides on Earth, though its effect is slightly less than half as strong as the Moon, due to its greater distance. During the Full and New Moon phases when Earth, the Moon, and the Sun are aligned, these gravitational interactions reinforce each other. This gives us the highest high tides and lowest low tides (called spring tides). During the First and Last Quarter Moon, the tidal forces from the Sun and Moon are working in different directions, leading to smaller tidal bulges (neap tides).

Tides are even more complex than this; not all places on Earth have two equal high and low tides per day (called semidiurnal tides). Some places, such as the Gulf of Mexico, only have one high and one low tide per day because the surrounding landmasses prevent the free flow of water throughout the globe. As a result, more complex patterns occur in particular regions.

April Russell 

 Visiting Professor, Siena College 
Loundonville, New York