The Moon is drifting away from Earth at a rate of approximately 1.5 inches (3.8 centimeters) each year, according to research from Michigan State University. Scientists measure this distance by bouncing lasers off retroreflectors, which were placed on the Moon’s surface by astronauts during the Apollo missions. By calculating the time it takes for light to travel to the Moon and back, they can accurately assess both the distance and its gradual increase over time.
The average distance between the Earth and the Moon is about 239,000 miles (385,000 kilometers). However, this distance fluctuates monthly due to the Moon’s elliptical orbit, which can vary by around 12,400 miles (20,000 kilometers). These variations contribute to phenomena like supermoons, where the Moon appears larger in the sky.
Understanding the Dynamics of Tides
The reason for the Moon’s increasing distance lies in the gravitational interactions between the Earth and the Moon, particularly through tidal forces. The gravitational pull of the Moon is stronger on the side of Earth that faces it, leading to two bulges of water in the oceans. As the Earth rotates, these bulges shift, but they do not perfectly align with the Moon due to the planet’s motion.
As the tidal bulges exert a gravitational pull back on the Moon, they cause it to accelerate slightly. This forward pull acts like a boost, increasing the size of the Moon’s orbit over time. The gradual increase in distance means that, while the Moon gains momentum, the Earth’s rotation slows down slightly. This process results in the lengthening of a day on Earth by a minuscule amount, with the current increase averaging 0.00000001% per year.
Historical Context and Future Projections
Approximately 4.5 billion years ago, the Moon formed following a significant collision between the early Earth and a Mars-sized protoplanet. This event not only created the Moon but also placed it much closer to Earth than it is today. Fossil evidence indicates that around 70 million years ago, during the late Cretaceous period, days on Earth were only 23.5 hours long, supporting the notion that the Moon was once significantly nearer.
Looking ahead, the prospect of the Moon completely escaping Earth’s gravitational pull is unlikely. If we consider a timeframe of tens of billions of years, the Earth may eventually become tidally locked with the Moon, resulting in one side of Earth perpetually facing it. At that point, the Moon’s distance would stabilize. However, this scenario is complicated by the Sun’s future evolution. In about a billion years, the Sun is expected to become brighter, which would lead to the evaporation of Earth’s oceans and the absence of tidal forces necessary for the Moon’s further drift.
Ultimately, billions of years later, the Sun will expand into a red giant, possibly consuming the Earth and the Moon. While these cosmic events are far off, the enduring beauty of lunar phases, solar eclipses, and ocean tides will continue to captivate humanity for millions of years to come.
This informative perspective on the Moon’s gradual retreat from Earth comes from the insights of Stephen DiKerby, a postdoctoral researcher in Physics and Astronomy at Michigan State University. Understanding these cosmic dynamics not only sheds light on our celestial neighbor but also informs us about the long-term changes within our solar system.
