II.

Orbit and Rotation

Mercury orbits the Sun every 87.97 Earth days at an average distance of approximately 58 million km (about 36 million mi), or 0.3871 astronomical unit (AU). An AU is equal to the average distance between the Earth and the Sun, or about 150 million km (93 million mi). However, Mercury’s orbit is highly elliptical and ranges from 46 million km (28,580,000 mi/0.3075 AU) at its nearest point to the Sun (perihelion) to 69.8 million km (43,380,000 mi/0.4667 AU) at its farthest point (aphelion). As a result, sunlight is over 2.3 times stronger at perihelion than at aphelion—during a single orbit Mercury receives as much as 11 times the intensity of sunlight that Earth does to a minimum of about 4.5 times. Mercury’s orbital velocity is also about 46 percent faster at perihelion than at aphelion. The planet’s orbit is tilted 7 degrees to the plane in which Earth orbits around the Sun.

The point in Mercury’s orbit at which the planet is closest to the Sun (perihelion) moves a tiny amount every orbit, too much to be accounted for solely by the gravitational influence of other planets. The observation of these changes in Mercury’s perihelion was one of the first confirmations of Einstein’s general theory of relativity, which predicted such variation due to the curvature of space caused by the enormous mass of the Sun.

Like Earth and most other planets, Mercury turns counterclockwise (west to east) when seen from its north pole. Mercury’s axis is almost perfectly vertical, unlike Earth’s axis, which is tilted 23.5 degrees. Radar observations of Mercury show that it rotates only once every 58.65 days, two-thirds of its period of revolution around the Sun. As a result, only three rotations of the planet occur during every two of its years. This relationship is called a 3:2 spin-orbit resonance. It is thought to be the result of differences in the pull of the Sun’s gravity on Mercury as the planet moves nearer and farther away in its orbit, an effect called solid body tidal forces.

The 3:2 spin-orbit resonance means that a solar day on Mercury (the time when the Sun next passes the noon point in the sky) is very different from the planet’s actual period of rotation (called a sidereal day). In fact, a complete solar day on Mercury lasts 175.84 Earth days, or two of Mercury’s years, and a night and a day at the equator each last one Mercurian year (87.97 Earth days). The Sun’s movement across the daytime sky from east to west would look very strange to a human observer, however. The planet’s eccentric orbit, changing orbital velocity, and slow rotation combine to make the Sun appear to stop and reverse direction before returning to a westward path. This effect occurs when Mercury is closest to the Sun and the planet’s orbital velocity becomes faster than its rotational speed around its axis.The Sun’s apparent size would also change during an orbit, from over 3 times to about 2 times its average size (about 0.5 degree of arc) when seen from Earth.