Why are there two tides a day? Most of us know the Moon plays a key role, but the real story is far more cosmic… and complex!

In this fascinating video from TED-Ed, we travel from ancient India, where Alexander the Great’s army was struck by a tidal bore, to the 17th century, when Isaac Newton first explained tides using gravity. As the video reveals, tides are caused not just by the Moon’s gravitational pull, but also by the subtle orbital dance between Earth, the Moon, and the Sun.

The Moon tugs on Earth’s oceans, creating a bulge on the side facing it. But thanks to Earth and the Moon orbiting a shared center of mass, a second bulge forms on the opposite side. As Earth spins, we pass through both bulges each day, hence two high tides and two low tides.

The Sun’s gravity adds another layer, amplifying tides during full moons (spring tides) and diminishing them during quarter moons (neap tides). Terrain also matters: narrow inlets can amplify tidal forces.

Beyond Earth, tides shape alien worlds too, from hidden oceans under Europa’s icy crust to volcanic eruptions on Io.

It’s a cosmic tug-of-war that quite literally moves the oceans.