Right in the Middle

NASA’s Scientific Visualization Studio The Earth’s center of mass drifts by millimeters over the years. Where is Earth’s center of mass? The answer seems trivially obvious. It’s right in the middle of the planet. Ah, but that kind of thinking won’t get you a paper in the Geophysical Journal International.1 It’s also not a trivial question. Knowing Earth’s center of mass down to the order of millimeters reveals some interesting facts about our planet. To measure Earth’s center of mass accurately, we must look to space. Satellites don’t just orbit Earth; they orbit the exact center of Earth’s mass. Any small shift in that center of mass means there is a shift in orbital trajectories. All you have to do is measure the orbits precisely. Back in the 1980s and 1990s, this was done using satellites known as LAGEOS. These spherical satellites were covered in reflective panels so that ground-based laser observatories could measure their positions over time. These allowed us to determine the…