A paper just published by a team of researchers answers the question “Why doesn’t zero longitude run through the Royal Observatory at Greenwich?"

The “Prime Meridian” that’s been running through the Royal Observatory at Greenwich, England since 1884 is now located 102 meters east of its historic spot. The research team comprised of geodesists, astronomers, and an engineer from the National Geospatial-Intelligence Agency, the U.S. Naval Observatory, the University of Virginia, and a Pennsylvania-based company known as Analytical Graphics Inc., investigated the cause of this apparent discrepancy.

The original location was decided by a delegation of international representatives meeting in Washington D.C., in 1884. They recommend that the world’s Prime Meridian (zero longitude) should pass through the Airy Transit Circle at the Royal Observatory in Greenwich, England. The original location was based on telescope observations used to measure the Earth’s rotation.

But the location where this meridian intersects the Earth’s surface changed in 1984, when new  technologies replaced traditional telescopes as a means to measure the Earth’s rotation. Geodesists focus on measuring the Earth’s size, shape, motion, and gravity, and precise positioning near the Earth’s surface. The authors conclude that a slight deflection in the natural direction of gravity at Greenwich is responsible for the offset, along with the way astronomical time was maintained over the 20th century.

The full paper is freely available in the Journal of Geodesy .