For centuries, scientists have studied Mercury’s motion, paying particular attention to its perihelion, or the closest point to the Sun during its orbit. Observations long ago revealed that the perihelion shifts over time, called precession. Although the gravitational tugs of other planets account for most of Mercury’s precession, they don’t account for all of it.
By Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Md.
Editor: Rob Garner