# How can the Moon make a high tide on both sides of Earth?

The bulge facing the Moon is easy to picture. The one facing away only makes sense when Earth is allowed to move too.

## The Moon pulls the nearest water

For the first pass, pretend Earth is fixed in space. The Moon's gravity is strongest at the water closest to it, so that water stretches toward the Moon.

As the Moon circles Earth, the bulge follows. The shortcut explains only the near side.

## Earth wobbles around an off-center point

Earth and the Moon both orbit their barycenter: their shared center of mass. It sits inside Earth, offset from the planet's center, so Earth's center travels around a small circle.

This wobble takes a month. It is part of the shared Earth–Moon orbit, not Earth's daily spin on its axis.

## Put the pull and the wobble together

On the web page, a slider moves from the fixed-Earth shortcut to the shared orbit. The second bulge appears as Earth begins to move around the barycenter.

**The sentence to remember:** near water falls toward the Moon faster than Earth. Far water falls toward the Moon slower than Earth.

From a frame turning with the Earth–Moon system, NOAA describes the same balance as unequal lunar gravity plus the centrifugal effect of the monthly shared orbit. This is separate from the centrifugal effect of Earth's daily spin.

## The numbers behind the picture

These are mean values. The web page's drawing compresses the enormous gap so Earth, the barycenter, and the Moon can share one page.

| Measure | Mean value |
| --- | --- |
| Earth center to Moon center | 384,400 km |
| Barycenter to Moon center | 379,729 km |
| Earth center to barycenter | 4,671 km, about 1,700 km below the surface |
| Earth diameter | 12,742 km |
| Moon diameter | 3,475 km |
| Speed of Earth's center around the barycenter | 12.4 m/s (44.8 km/h) |
| Moon's mean orbital speed relative to Earth | 1.022 km/s |
| One shared orbit, relative to the distant stars | 27.3217 days |

The barycenter follows from r = d × Mₘ ÷ (Mₑ + Mₘ), and Earth's speed from v = 2πr ÷ T. Over the elliptical orbit, the Moon's distance varies from 363,300 to 405,500 km and its speed from 0.970 to 1.082 km/s.

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Sources: [NOAA, The Astronomical Tide-Producing Forces](https://www.tidesandcurrents.noaa.gov/restles3) and the [NASA Moon Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html). This first model assumes a smooth, water-covered Earth. Continents, depth, and friction are why real local tides are more complicated.

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