How does the Moon cause ocean tides?

The Moon's gravity pulls on every part of Earth, but the pull is stronger on the side facing the Moon and weaker on the far side. This difference is called the tidal force. It stretches Earth's water into bulges on the near side and the far side, and these bulges are high tides. As Earth rotates, coastlines pass through both bulges, so most shores see two high tides and two low tides a day. [1] [2]

A bulge facing the Moon is intuitive, but a second bulge on the opposite side seems odd. NOAA's education material explains that the tidal force is a differential force. It is found by subtracting the Moon's average pull on the whole Earth from its pull at each location. Near the Moon the leftover force points toward the Moon. On the far side the Moon's pull is weaker than average, so the leftover force points away from the Moon. The result is a stretching and squashing of Earth that produces two bulges. [1]

The University of Hawaii's Exploring Our Fluid Earth likewise attributes the far-side bulge to the difference in the Moon's gravitational force across Earth. The land also bulges, and so does the Moon, but less than the ocean does. [3]

The sources do not describe the far-side bulge in quite the same way. NOAA's Ocean Service tutorial says gravity and inertia act in opposition: on the near side gravity exceeds inertia, while on the far side inertia exceeds the weaker gravity and the water tries to keep moving in a straight line. The National Tidal and Sea Level Facility instead frames tides as arising from the rotation of the Earth-Moon system about its common centre of mass, where gravitational attraction balances the forces needed to hold the Moon in orbit. Away from that point the two are out of balance, and the imbalance is the tide-generating force. NASA and the NOAA NESDIS page describe the effect in terms of differences in the Moon's pull. These accounts are presented as explanations of the same phenomenon, though they use different language, and the results here do not say how specialists rank them. [4] [5] [2]

Earth rotates within this water layer, so a given coastline moves through both bulges. One high tide to the next takes a little over 12 hours. NASA adds that the bulges do not line up exactly with the Moon. Earth's spin is much faster than the Moon's orbit and carries the bulges slightly ahead of it. [2]

The bulge model is a simplification. The National Tidal and Sea Level Facility notes that such an ovoid of water does not exist in nature. Continents block the water, and ocean depth and the shape of the coast also matter. That is why the gap between high and low tide is small in some places and large in others. The Sun produces a similar, separate effect, called solar tides. The Moon dominates because it is much closer, even though the Sun is far larger. [1] [5] [6]