What happened

A team from NASA’s Jet Propulsion Laboratory proposed a more precise way to track Earth’s center of mass. The center of mass is the point where all of Earth’s mass balances. It is not always the same as the center of Earth’s shape. Snow, rain, ocean water, ice, and air move with the seasons. These changes shift the center of mass by several millimeters relative to the geometric center. Earth is not gaining or losing its total mass. The NASA report describes the method and findings in a study published in Geophysical Journal International. Read NASA’s report here.

Background: why does the center move?

Imagine Earth as a hard blue ball. Its shape would have one clear middle. Real Earth is more complicated. Water and ice move across the surface. Air also becomes heavier or lighter in different places. These shifts change where Earth’s mass balances.

Scientists have tried to measure this point for decades. Two international estimates, made in 2017 and 2023, differed by 0.27 inches, or 7 millimeters. That difference was almost as large as the movement itself. The result showed how difficult the measurement is. Researchers are measuring a moving target while using ground stations and satellites as references.

Why it matters

Satellites naturally orbit Earth’s center of mass. Small errors in that reference can affect satellite navigation and height measurements. They also matter for mapping, global shipping, and precision agriculture. The change is too small for people to feel. It matters because modern systems depend on very accurate positions.

What has been confirmed

The new method combines several kinds of tracking. It adds GPS measurements and orbital data from several low Earth orbit satellites. It also considers how the weight of water and ice slightly deforms Earth’s crust. The method works with laser observations of LAGEOS 1 and 2, satellites covered with reflective prisms. Ground stations in more than 20 countries track those satellites.

The study also linked the movement to seasonal patterns. Snow in North America and Eurasia reaches its maximum in March. It shifts the center of mass about 3 millimeters toward the North Pole. In April, rainwater in the Amazon basin reaches 2,400 gigatons, or 2,400 billion metric tons. That shifts the center 2.2 millimeters toward South America. Southeast Asian monsoon water peaks at 600 gigatons in November. From August through October, rain and melting ice add water to the oceans. The large Pacific Ocean has the strongest effect. Cold, dense winter air also changes the balance.

These calculations agree with observations from GRACE-FO. The mission has two satellites. They detect monthly changes in Earth’s pull, mainly caused by moving water above and below the ground.

What remains uncertain

The new estimate says the yearly movement is about half the size scientists thought eight years ago. That does not make the new number a final answer. Different satellites, ground stations, and Earth models can produce different estimates. Researchers still need to reduce uncertainty around each part of the movement.

What to watch next

NASA says the GRACE-C mission is targeting a launch in late 2028. It would extend the GRACE record, which already covers nearly 25 years. Longer observations can test the seasonal pattern more carefully. Better reference systems could support more reliable maps, navigation, and height measurements. The story is not about Earth wobbling wildly. It is about measuring a tiny shift that modern technology needs to understand.