What Will Happen To Ocean Tides When The Moon Moves Away From Earth?

The moon is drifting away from Earth at about 3.8 cm per year, weakening lunar tides over geologic time. After roughly 50 billion years, Earth would tidally lock to the moon, so lunar tides would freeze in place (solar tides would still occur). But this never plays out, because the Sun will swell into a red giant and engulf the Earth-Moon system around 7.6 billion years from now.

Our natural satellite – the moon – has been revolving around our home planet ever since it was first created nearly 4.51 billion years ago. The circular motion of the moon in its orbit not only gives us a glowing grey sphere that appears in our night sky, but also causes some fascinating phenomena on the surface of our planet.

One of them is tidal motion!

For those who don’t know about this, the tides that you witness in seas and oceans are all caused by the moon. This is due to the gravitational pull that the moon exerts on our planet (also called “tidal force”). This force causes Earth and all the water on its surface to ‘bulge out’ to the side that’s closest to the moon.

Is The Moon Moving Away From Earth?

Yes, and it has been ever since it formed. Lunar Laser Ranging measurements (using mirrors left on the moon by the Apollo astronauts) show that the moon is receding from Earth at about 3.8 cm (1.5 inches) per year.

Here’s why that happens: Earth spins on its axis much faster than the moon orbits Earth. That mismatch drags the tidal bulges in Earth’s oceans slightly ahead of the moon’s position. The leading bulge’s gravity gives the moon a tiny forward tug, handing it some of Earth’s rotational angular momentum each year. The moon climbs into a larger orbit, and counter-intuitively, its orbital speed actually decreases as it does (Kepler’s third law). Meanwhile, Earth’s spin slowly grinds down.

While 3.8 centimeters per year doesn’t sound like much, over a period of many years (we’re talking billions), this value accumulates and becomes very significant.

Let’s assume that the moon’s orbit gets so big that it leaves the vicinity of Earth completely… what will happen to our planet’s tides then?

What Will Happen To Earth’s Tides If The Moon Goes Away?

It’s not quite as straightforward as it sounds. The moon won’t escape Earth’s orbit; the recession slows down and eventually stops at an equilibrium called mutual tidal locking. At the current rate, scientists predict that in around 50 billion years, Earth will have slowed its rotation enough that a single Earth day equals one lunar orbit (about 47 of today’s days long), and the moon will stop moving away (Wikipedia, Tidal locking).

At that point, the moon would still raise a tidal bulge on Earth, but because the same face of Earth would always point at the moon, that bulge would be fixed in place. Coastlines would no longer see daily lunar tides sweeping in and out. Solar tides (which are about 46% as strong as lunar tides) would still continue, though, because Earth is nowhere near tidally locking to the sun.

In other words, in 50 billion years or so, lunar tides would freeze in place; only weaker solar tides would still ebb and flow.

Why We Don’t Need To Worry About The Moon Running Away

Although the loss of lunar tides would certainly be a terrible thing, we don’t really need to worry about that, as there are other things at play too.

The sun will exhaust its core hydrogen in roughly 5 billion years and begin its red giant phase. Current models (Schröder & Smith, 2008) predict it will swell enough to engulf both Earth and the moon around 7.6 billion years from now (Schröder & Smith, MNRAS). And long before that — within about a billion years from now — rising solar luminosity is expected to boil off Earth’s oceans and end surface life as we know it.

The sun keeps burning due to a process called nuclear fusion, which is perpetually occurring inside its core. Consider this: the sun burns through 600 million tons of hydrogen every second. That’s how incredibly extensive the fusion process inside the sun truly is. You can read more about it in detail here: What happens inside the sun?

As the nuclear fusion inside the sun’s core progresses, the sun gets nearly 10% brighter every billion years. This would, in due time, wreak havoc on our planet—boil Earth’s oceans, melt the ice caps that are left, strip away the atmosphere… you name it. The moon won’t be spared either.

In a nutshell, if the moon gets away from Earth, there will be no tides on the planet. But long before that happens, it’s far more likely that both of these celestial bodies will be gobbled up by the sun!