Mercury is the smallest and innermost major planet (Figure 9.1); from the perspective of Earth-bound observers, it never appears more than 28° from the Sun. It is locked in a state of spin-orbit coupling: its rotation period (58.646 Earth days) is exactly two-thirds of its year (87.969 Earth days). Mercury is named after the Greco-Roman messenger to the gods, perhaps because of its rapid apparent day-to-day movement with respect to the Sun. Compared to the other major planets, Mercury has the most eccentric orbit (e = 0.2056), the highest inclination relative to the ecliptic (7.007°), and the smallest axial tilt (0.01°). It has a weak magnetic field (~1.1 percent as strong as Earth’s) and no natural satellites.