The planet that orbits closest to the Sun is Mercury. It is also the smallest of the eight planets in the solar system. These features make Mercury difficult to view from Earth, as the small planet rises and sets within about two hours of the Sun. Observers on Earth can only ever see the planet during twilight, when the Sun is just below the horizon. Relatively little was known about Mercury until the Mariner 10 spacecraft visited it in 1974–75. It was more than 30 years before another spacecraft, Messenger, visited the planet.
Mercury’s orbit lies between the Sun and the orbit of Venus. Along with Venus, Earth, and Mars, Mercury is one of the inner planets nearest to the Sun. The inner planets are also known as the terrestrial, or Earth-like, planets. They are dense, rocky bodies that are much smaller than the solar system’s outer planets. Mercury has no known moons.
Mercury is the smallest planet in both mass and diameter. It is about 18 times less massive than Earth. With a diameter of about 3,032 miles (4,879 kilometers), Mercury is not quite two-fifths the size of Earth. It is only about a third larger than Earth’s Moon. In fact, two moons in the solar system—Jupiter’s moon Ganymede and Saturn’s moon Titan—are larger than Mercury. However, Mercury is larger than the dwarf planet Pluto. For some 75 years, when Pluto was classified as a planet, Mercury was considered the second-smallest planet.
Each view of the planet Mercury is a mosaic of many images taken by the Mariner 10 spacecraft during its first flyby of the planet in March 1974. Each view shows about half of the hemisphere that was in sunlight at the time. NASA/JPLNASA/JPL
Mercury is the densest planet in the solar system, followed by Earth (if one takes into account the planets’ internal compression because of gravity). Mercury is unusually dense because it is composed of a high percentage of metal. The metal is concentrated in a comparatively huge core, which accounts for nearly 75 percent of Mercury’s diameter.
Mercury can be seen from Earth without a telescope. It always appears close to the Sun (within about 28 angular degrees). For this reason, the planet can only be seen near the horizon. At certain times of the year it appears as a “morning star” just before sunrise, while at other times it appears as an “evening star” just after sunset.
Because Mercury’s orbit lies between Earth’s orbit and the Sun, Mercury displays phases like those of the Moon and the planet Venus. These phases can only be seen with the aid of a telescope. Mercury sometimes looks like a crescent to observers on Earth. At other times, when Mercury and Earth are in different positions, more of the sunlight reflected off Mercury can be seen from Earth. Mercury then appears as a half or fuller disk.
Five separate images were combined to show Mercury crossing in front of the Sun during a type of eclipse called a transit on Nov. 15, 1999. The false-color images were taken in ultraviolet light by the Transition Region and Coronal Explorer (TRACE) satellite in Earth’s orbit. The time between successive images was about seven minutes. NASA/TRACE/SMEX
About a dozen times each century, Mercury passes directly between Earth and the Sun. This event, called a transit, is a type of eclipse. During a transit of Mercury, the planet appears as a small black disk against the background of the bright Sun. Binoculars or a small telescope, safely equipped with a solar filter, are required to observe a transit, preferably by projection of the Sun’s image onto a white card. The next transit of Mercury is in 2016.
On average, Mercury orbits the Sun at a distance of nearly 36 million miles (58 million kilometers). Like all the planets, it travels around the Sun in an elliptical (oval-shaped) orbit. Mercury’s orbit is the most eccentric, or elongated, of all the planets. Its orbit is also the most tilted. The plane of Mercury’s orbit is tipped about 7 degrees relative to the ecliptic, or the plane of Earth’s orbit. Mercury completes one orbit around the Sun about every 88 Earth days. In other words, one year on Mercury lasts some 88 Earth days.
The planet was named after the ancient Roman god Mercury, the counterpart of the ancient Greek god Hermes. Like Hermes, the swift-footed messenger of the gods, the planet Mercury is known for the speed with which it moves across the sky. The planet circles the Sun at an average rate of about 30 miles (48 kilometers) per second, the fastest of the eight planets.
Numerous craters and impact basins mark the surface of Mercury. An image taken by a space probe called the Messenger during its first flyby in January 2008 shows part of the planet. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Although Mercury moves along its orbit very quickly, it spins slowly. It takes almost 59 Earth days to complete one rotation about its axis. Mercury rotates on its axis only three times for every two revolutions it makes around the Sun. The combination of a slow spin and a fast orbit leads to an unusual situation. A day on Mercury—the time it takes for the Sun to appear straight overhead, to set, and then to rise straight overhead again—lasts about 176 Earth days. So on Mercury a day is twice as long as a year.
Like the other seven planets, Mercury orbits the Sun in a counterclockwise direction—the same direction as the Sun’s rotation. The planets also orbit in nearly the same plane, so that their paths trace out a large disk around the Sun’s equator. The orbital planes of most of the eight planets are within about 3.5 degrees of the ecliptic plane, or the plane in which Earth orbits. As mentioned earlier, however, Mercury’s orbit is inclined about 7 degrees relative to the ecliptic plane.
In the early 1600s German astronomer Johannes Kepler discovered three major laws that govern the motions of the planets. The first law describes the shape of their orbits, which are not exactly circular but slightly oval, or elliptical. Mercury has the most eccentric orbit of the planets, while Venus and Neptune have the most circular orbits.
Kepler’s second law describes the velocities (speeds) of the planets in their orbits. It states that an imaginary line drawn from a planet to the Sun sweeps across equal areas in equal periods of time. This means that the planets move faster when their orbits bring them closer to the Sun and more slowly when they are farther away. It takes much more time for Uranus to orbit the Sun than Mercury, for instance. Kepler’s third law allows one to calculate a planet’s orbital period, or the time it takes the planet to complete one orbit around the Sun, if one knows its average distance from the Sun, and vice versa. The law states that the square of a planet’s orbital period is proportional to the cube of the planet’s average distance from the Sun.
This characteristic, combined with Mercury’s highly eccentric orbit, creates some strange effects. The planet’s distance from the Sun varies greatly as it travels along its orbit. The farthest Mercury gets from the Sun is about 43 million miles (70 million kilometers). At that point in the planet’s orbit, an observer on Mercury would see the Sun appear about twice as large as it does from Earth. The closest Mercury gets to the Sun is some 29 million miles (46 million kilometers). The Sun would at that point appear some three times as large as it does from Earth. Even more unusually, the Sun would not seem to move steadily across Mercury’s sky. Its apparent speed would change depending on the viewer’s location on the planet and on the planet’s distance from the Sun. The Sun would sometimes even appear to briefly reverse its course.
Mercury’s spin axis is very nearly perpendicular, or upright, relative to its plane of orbit. By comparison, Earth’s axis is tilted almost 24 degrees. This inclination is the main reason there are seasons on Earth. Because Mercury’s axis is not tilted, it does not have Earth-like seasons.
