Generated by Codex with GPT 5.6 Sol High

The moon feels permanent because its changes are almost invisible on a human timescale. Phil Plait’s Scientific American column looks much farther ahead and finds that the moon’s future is not a simple matter of drifting steadily away. Its fate depends first on the tidal exchange between Earth and the moon, then on the loss of Earth’s oceans, and finally on the aging Sun.

How the moon learned to retreat

The broad origin story begins several tens of millions of years after Earth formed, when a planet-size body struck the young world and threw material into orbit. That debris coalesced into the moon. At first the moon may have been only about 20,000 kilometers away, compared with an average distance of roughly 380,000 kilometers today. It would have appeared about 10 degrees wide in the sky—approximately the width of a fist held at arm’s length.

At such close range, Earth’s gravity raised enormous tidal bulges on the moon. Because the young moon was spinning quickly, those bulges were carried slightly out of alignment with Earth. Earth’s pull dragged them back, slowing the moon’s rotation. Some of that lost rotational energy entered the moon’s orbit and pushed it outward. Eventually its rotation matched its orbital period, producing the tidal locking that keeps nearly the same lunar face pointed toward Earth.

The exchange also works in the other direction. The moon raises tides on Earth, and Earth’s rotation carries the tidal bulges ahead of the moon. The moon pulls backward on them, gradually slowing Earth’s spin while gaining orbital energy. Earth’s day now lengthens by about two milliseconds per century, and the moon recedes by about four centimeters per year.

One might therefore expect the process to end with mutual tidal locking: Earth’s rotation would eventually match the moon’s orbit, leaving the moon fixed above one side of the planet. That outcome would take many billions of years, however, and the system will change radically before it can get there.

The Sun interrupts the tidal clock

Earth’s oceans absorb much of the energy in the lunar tide. Their motion and friction are central to slowing the planet’s rotation and driving the moon outward. But the Sun gradually grows brighter as helium accumulates in its core. In about a billion years, give or take, Earth should become hot enough for its oceans to boil away. Without them, tidal friction will weaken, the moon’s recession will slow and mutual locking will be delayed beyond the time the Sun has left in its present state.

Roughly six billion to seven billion years from now, the Sun will exhaust the hydrogen in its core and expand into a red giant. Whether it grows large enough to engulf Earth remains debated, but both Earth and the moon will in any case be scorched. The Sun will later shed its outer layers, lose about half its mass and leave behind a white dwarf that cools over billions of years.

Even that is not necessarily the end of the moon’s orbital story. The Sun raises tides on Earth too; at present their effect is about half as strong as the moon’s. How the remnant Sun’s tides would act on the Earth-moon system is complicated and not fully understood, especially because the Sun’s mass loss will weaken its influence. Over tens of billions of years, however, white-dwarf tides might destabilize the moon’s orbit. The moon could escape Earth altogether or spiral inward and collide with it.

The clean answer is therefore that there is no clean answer. The familiar moon will keep receding for now, but the oceans and Sun that help govern its motion will not remain as they are. A satellite that looks fixed in the night sky is part of a system evolving across several radically different eras, and its ultimate fate—escape, collision or something more stable—lies too far ahead and depends on physics too uncertain to call.