Mercury is the closest planet to the sun and the second smallest in the solar system. It has an extremely elliptical orbit, rivaling that of Pluto. Mercury’s farthest distance from the sun (at aphelion) is 43,400,000 miles (69,800,000 kilometers), but at closest approach (perihelion), it is only 28,600,000 miles (46,000,000 kilometers) away—Earth, in comparison, orbits at three times that distance from the sun. Like Venus, Mercury shows phases.

Mercur/s statistics
Equatorial diameter:
3,031 miles (4,878 kilometers)
Mass: 0.056 (Earth = 1)
Mean relative density:
5.42 (water = 1)
Surface gravity:
0.386 (Earth = 1)
Escape velocity: 2.6 miles
(4.2 kilometers)
per second
Distance from sun:
farthest: 43,400,000 miles (69,800,000 kilometers)
nearest: 28,600,000 miles (46,000,000 kilometers) mean: 35,980,000 miles
(57,900,000 kilometers)
Closest approach to Earth: 57,000,000 miles (91,700,000 kilometers)
Orbital period:
87.97 earth-days
Rotational period:
59 earth-days

Observing Mercury

Because it is so close to the sun, the planet is difficult to see with the naked eye, appearing like a twinkling light near the horizon as a morning or evening star. Mercury is usually best seen in the Northern Hemisphere in the west at twilight in May, or just before sunrise in the east in November. These are the times at which Mercury reaches maximum elongation—the point farthest from the sun at the extreme east or west position in its orbit. But even at maximum elongation, the planet is never more than 28° from the sun. If favorably placed, Mercury can be seen during daytime through a telescope with an aperture of at least 3 inches (75 millimeters).
Mercury can also be observed when it crosses the sun’s disk, when it is said to be in transit. Transits are relatively rare because Mercury’s orbit is inclined appreciably (7°) to the ecliptic. They occur every 3 to 13 years, always in May (near perihelion) and November (near aphelion). November transits occur twice as frequently as those in May, although the May transits are longer, lasting almost nine hours. The first prediction of a transit of Mercury across the sun was made by Johannes Kepler for November 7,1631. It was then observed by Pierre Gassendi in Milan. On Earth, an observer sees Mercury start crossing the sun’s disk 43 seconds later than the actual event because the sun’s gravitational field bends the light rays reflected from the planet. It was because of this fact that predictions of the transit were found to be several seconds late. This apparent discrepancy puzzled astronomers until 1915, when Albert Einstein proposed his general theory of relativity, in which he explained the effect of gravity on light rays.

Mariner 10 was launched on a double space probe on November 3,1973, to view Venus and Mercury. Mariner had to be programmed to pass near Venus to use its gravitational force to accelerate the probe into an elliptical orbit around the sun. Mercury’s orbit around the sun takes about 88 earth-days, during which time it rotates on its axis one-and-a-half times. Its “day” (from sunrise to sunrise) lasts about 176 earth-days, during which it makes three axial rotations and orbits twice around the sun. In the 180 days it took Manner 10 to orbit once around the sun, Mercury had orbited twice.

Mercury’s rotation

the planet’s period of rotation. It was found to be 59 earth-days—19 days shorter than previously believed. During this time, it travels two-thirds of its orbit around the sun. All of the planet thus receives sunlight at some time; before this discovery, astronomers had for 75 years thought that Mercury had a captive rotation and always presented the same face to the sun, with one side of the planet in perpetual darkness. This was supposed to be due to the planet’s proximity to the huge mass of the sun with its high gravitational attraction. It is now known that Mercury’s “day” (from one sunrise to the next) lasts about 180 earth-days—a product of its comparatively slow rate of rotation and relatively swift orbit.

Mariner 10s discoveries

In 1974, the United States launched Mariner 10 on a space probe to observe Mercury and Venus. The close-up photographs showed Mercury’s surface for the first time, revealing it to be similar to the lunar landscape with a heavily cratered crust and lava-flooded plains. The argument as to whether these features were caused by meteoric bombardment or by volcanic activity remains unresolved.
The main impact feature is the Caloris Basin, or Caloris Planitia—similar in appearance to the moon’s large maria. The Caloris Basin is so called because it is near a “hot pole” (one of two) in a region of the highest temperatures, around 650° F. (345° C). It is 800 miles (1,287 kilometers) in diameter and is ringed by smooth mountains that rise to about 6,600 feet (2,000 meters). On the opposite side of the planet from Caloris is a region of chaotic terrain, possibly formed by shock waves that resulted from the same impact that created the Caloris Basin. Ray craters are also present; the largest—more than 25 miles (40 kilometers) in diameter—is the Kuiper Crater. These craters have bright streaks formed of surface deposits that radiate from the center to the walls of the crater. Other features include scarps (dorsa), possibly created as Mercury’s core cooled and contracted, causing the silicate-rich crust to wrinkle.
One surprising discovery of Mariner 10 was that Mercury has an atmosphere, albeit a very tenuous one. A deep atmosphere would not be expected because Mercury has a low surface gravity—only about one-third that of Earth. Any atmospheric gases would long since have escaped into space. Mercury’s “atmosphere” consists of gases, mostly helium, trapped from the passing solar wind by the planet’s magnetic field. There are also minute traces of neon and argon, which could cause auroral displays in Mercury’s night skies.
Because it has so little atmosphere, Mercury experiences extremes of temperature. Night temperatures can fall as low as —279° F. ( — 173° C), and day temperatures at perihelion can rise to more than 801° F. (427° O—hot enough to melt tin and lead.
The discovery that Mercury has a magnetic field also came as a surprise. But the strength of this field is only about one per cent that of Earth. Mercury also has a magnetosphere, a region of magnetic influence in space that deflects the solar wind, causing it to curve around the planet. There are two magnetic poles, one at each end of the rotational axis.

The surface of Mercury, photographed by Mariner 10 on March 29,1974, shows a similarity to that of the moon. The distribution of the craters resembles generally the pattern of those on the lunar surface. They tend to appear in pairs, groups, or lines, and some have central peaks. This view shows Mercury’s northern hemisphere and was taken 48,350 miles (77,800 kilometers) from the planet. The “tear” along the horizon was caused by some data being lost in the composition of the photograph.