Goodbye Moon?

“Some say the world will end in fire…” —Robert Frost

As we prepare for a spectacular eclipse on April 8, 2024, we can reflect on a cosmic question. Are we losing our moon? Although the moon seems constant in its orbit about 384,000 km from earth, it is moving slowly away. Each year the moon migrates about 4 cm (1.5 in.) farther away from earth. That’s about the rate at which our fingernails grow in a year. Although it’s not much, it all adds up over the vast time scale of the solar system. When the moon formed around 4.5 billion years ago, it was 22,500 km (14,000 mi) away instead of the current 400,000 km (250,000 mi). Imagine a moon 17 times larger in the sky, reflecting much more light on earth. The night sky on Earth would glow much brighter.

Earth and Moon are a linked system

The Earth and the Moon have circled the sun together from the early days of our solar system. Back when the Earth was still forming from hot, swirling plasma and was still a very gooey mass, a planetoid about the size of Mars smacked into Earth. This collision threw a huge glob of molten earth material into space. That glob, about a quarter of the size of Earth, hardened into the Moon. This explains why the Earth and the Moon have such similar compositions.

From the start, the Earth and the Moon bonded together through their mutual gravitational attraction. Barring cosmic collisions with asteroids, they will remain a coupled system even as the Moon continues to move farther away. So, no, we will not lose the Moon, though it will continue to edge away for a long, long time. Why will the Moon and the Earth remain like some human relationships, connected, though more distant?

The Reason Behind the Moon’s Retreat

The principle of angular momentum explains both the retreat of the Moon and the connected event of the gradual slowing of Earth’s rotation. This principle, stemming from Newton’s laws of motion, illustrates why the Moon’s orbit is expanding while the Earth’s day is lengthening.

Angular Momentum at Work

Visualize a figure skater spinning with arms extended. As they pull their arms in, they spin faster. This is angular momentum in action—keeping the energy in this rotating system constant. In the Earth-Moon system, the Moon’s gravitational pull tugs at Earth’s tidal bulges, slowing our planet’s rotation. Conversely, as Earth’s rotation slows, the Moon edges into a higher orbit, both actions preserving the system’s total angular momentum.

Slowing Rotation and Expanding Orbits

Earth’s day is lengthening by about 2 milliseconds per century, a gradual change that accumulates over millennia. Similarly, the Moon’s orbit expands, propelled by the energy transfer from Earth’s rotational slowdown. This dance of gravitational forces ensures the total angular momentum remains constant, even as the dancers move farther apart.

A Future in Gravitational Lock

 Today, the Moon always keeps the same side toward Earth because it rotates on its axis at the same speed it orbits the Earth. In billions of years, Earth’s rotation will slow to match the Moon’s orbit, creating a gravitational lock.

At that time, the Moon’s orbit will take 47 of our current days instead of 27 days. Earth’s rotation period will also be 47 days. One day on Earth will then be the same length of time as one month. When the Earth and the Moon reach this equilibrium, the same side of the Earth will always face the same side of the Moon. Then one day will equal one lunar month.

Our Fiery, Not Our Lonely, End

However, the Earth and the Moon will never achieve this equilibrium. Long before then, in about 5 billion years, the Sun will have consumed all its hydrogen fuel. Then it will become a Red Giant star. The Sun will swell up to 250 times its current size and engulf the inner planets, Mercury, Venus, and Earth, along with the Moon, in a blazing conclusion. Poet Robert Frost’s lyrical prophecy was right. The world will end in fire.