Stupid Question ™
May 16, 2002
By John Ruch
Q: Why are there two tides every day?
A: You probably know that tides have something to do with the Moon’s gravity yanking water upward. But when high tide is happening on the side of Earth nearest the Moon, another high tide is bulging out on the opposite side of the planet—away from the Moon. (Meanwhile, low tides occur on the points in between).
Broadly speaking, a tide is the stretching that happens to a body that’s under another body’s gravitational attraction. In principle this happens between all bodies in the universe, but the effect is only significant with large masses at close distances. That’s why the Moon exerts much stronger tides on Earth than the Sun does; it’s a lot smaller than the Sun, but a lot closer.
Tides affect Earth’s entire surface, which can bulge as much as 14 inches (not noticeable over such a large area). But in common terms, tides specifically are the bulges in ocean water which move around the planet as tidal waves, causing high tides when they meet the shore or low tides when they suck water back out to sea.
(Depending on size, depth and geography, some bodies of water have miniscule tides or none at all; the Gulf of Mexico has only one per day.)
Tides on Earth are indeed influenced by the Moon. That’s why you get two high tides every 25 hours (or usually, though not always, twice every day), coincident with the Moon’s revolution around Earth.
But it’s a mistake to imagine Earth sitting there while the Moon flies by, dragging the ocean with it. In fact, Earth and the Moon are locked together into a single gravitational system, both orbiting a common center of gravity and causing tides on each other as they go.
The Earth’s center of mass is the center of the planet. But the Earth-and-Moon system’s center of mass shifts 3,000 miles toward the Moon, to a point 1,000 miles below the Earth’s surface on the Moon side.
Both the center of Earth and the center of the Moon orbit this point—the nexus of their combined gravitational forces.
Looking at this system, we see the Moon indeed creates a tidal bulge on Earth’s near surface. We also see that the center of Earth is orbiting the system’s center of gravity, and like all orbiting bodies is in free-fall. This means it is under the Moon’s gravitational pull, but has such strong angular velocity it keeps “falling” in a circular path nearly forever.
This centripetal (“center-seeking”) force is balanced, as Newton pointed out, by a centrifugal (“center-fleeing”) force that pushes outward—in this case, creating a tidal bulge that approximately equals that of the Moon on the other side.