The Moon’s Poles Have “Wandered” Over Billions of Years

Until 1959, humans had only seen one side of the Moon. The Moon is tidally locked with Earth, and so we can only see one side from the Earth’s surface. It took the soviet Luna 3 spacecraft to capture a blurry image for humans to get their first glimpse of the lunar far side. Because of this, many people imagine that the Moon has always been this way. But as a recent study shows, that isn’t quite true.

Although the Moon is tidally locked with Earth, it isn’t physically locked to Earth. It is still a freely moving body that rotates on its axis. The Moon always shows the same side to Earth because its period of axial rotation and its orbital period is the same. But even this isn’t an exact match. The Moon’s rotation is essentially constant, but its orbit isn’t exactly circular. So the Moon moves along at a bit faster or slower rate depending on where it is in its orbit. This makes the moon appear to wobble back and forth slightly. There’s also the fact that the Moon’s rotational axis is tilted slightly relative to its orbital plane around the Earth, and the orbit itself is tilted slightly relative to the Earth’s equator. All of this together gives the Moon a small but complex wobbly dance as seen from Earth, known as libration. So over the course of a few years, we actually see slightly more than half the lunar surface, though this effect is too small to notice in our daily lives.

The appearance of the Moon through one synodic period. Note that in addition to rocking back and forth (libration) and side-to-side (nutation), the Moon appears to swell and shrink in size. Wikimedia Commons graphic in the public Domain.

But because the Moon is a freely moving body, its rotational axis can shift from other things as well. For example, geological activity such as the drift of continents, and the freezing or melting of polar caps cause shifts in our rotational axis. It even shifts the length of our days slightly. But the Moon isn’t really geologically active, and it doesn’t have weather patterns to freeze or melt polar caps. But it has been bombarded by asteroids over the years, and that brings us to this latest study.

The team looked at high-resolution gravity maps of the Moon taken by NASA’s Gravity Recovery and Interior Laboratory (GRAIL). These maps give us a good idea of the distribution of mass within the Moon, since the more mass you have in a given area, the higher the gravity. They also used a detailed map of lunar craters captured by the Lunar Reconnaissance Orbiter (LRO) and its Lunar Orbiter Laser Altimeter (LOLA). They then used computer models to “remove” the craters’ age layer by age layer.

The wander of the Moon’s rotational axis, seen in black. Credit: Smith, et al.

Since a crater shifts the mass distribution slightly, it also shifts the Moon’s axis of rotation slightly. The team basically rewound the clock for about 5,200 crater impacts spanning more than 4 billion years of history. They found that the Moon’s orbital axis has shifted by about 10 degrees in that time. That might seem like much, but it’s pretty impressive to track the Moon’s rotation over billions of years. In the future, the team would like to include more data, including effects such as early volcanic eruptions.

Reference: Smith, David E., et al. “The Contribution of Small Impact Craters to Lunar Polar Wander.” The Planetary Science Journal 3.9 (2022): 217.

Brian Koberlein

Brian Koberlein is an astrophysicist and science writer with the National Radio Astronomy Observatory. He writes about astronomy and astrophysics on his blog. You can follow him on YouTube, and on Twitter @BrianKoberlein.

Recent Posts

Colliding Neutron Stars can Generate Long Gamma-ray Bursts

Gamma-Ray Bursts (GRBs) are the most energetic recurring events in the Universe. Only the Big…

3 hours ago

“Early Dark Energy” Could Explain the Crisis in Cosmology

A new study considers how the presence of Early Dark Energy could help resolve one…

4 hours ago

How Artificial Intelligence Can Find the Source of Gamma-Ray Bursts

Gamma-ray bursts come in two main flavors, short and long. While astronomers believe that they…

5 hours ago

The Geminids Will be Peaking on December 14th. They’re Usually the Most Active Meteor Shower Every Year

Meteor showers are a great way to share a love of astronomy with those who…

7 hours ago

A Star was Blocking a Galaxy, but Now it’s Moved Enough That Astronomers can Finally Examine What it Was Hiding

One of the biggest puzzles in astronomy, and one of the hardest ones to solve,…

9 hours ago

Will We Ever Go Back to Explore the Ice Giants? Yes, If We Keep the Missions Simple and Affordable

It's been over 35 years since a spacecraft visited Uranus and Neptune. That was Voyager…

1 day ago