The second planet from the Sun is Venus. After the Moon, Venus is the most brilliant natural object in the nighttime sky. It is the closest planet to Earth, and it is also the most similar to Earth in size, mass, volume, and density. These similarities suggest that the two planets may have had similar histories. Scientists are thus intrigued by the question of why Venus and Earth are now so different.
Venus was named after the ancient Roman goddess of love and beauty, but its conditions are anything but hospitable and inviting to humans. Unlike Earth, Venus is extremely hot and dry. The planet is always shrouded by a thick layer of clouds. Venus has a massive atmosphere, or surrounding layers of gases, composed mainly of carbon dioxide. This thick atmosphere traps heat, making Venus the hottest planet in the solar system.
The permanent blanket of clouds also makes it difficult to study the planet. Little was known about the surface and atmosphere until the 1960s, when astronomers made the first radar observations of Venus and unmanned spacecraft began visiting the planet. Venus’s orbit lies between the orbits of Mercury and Earth. Like Mercury, Venus has no known moon.
Scientists use radar to pierce the thick clouds shrouding Venus and “see” the surface below. An image generated by computer from radar data collected by the Magellan spacecraft shows the surface of the northern hemisphere. NASA/JPL/Caltech (NASA photo # PIA00271)
Venus is the third smallest planet in the solar system, after Mercury and Mars. It is a near twin of Earth in size, mass, and density. Venus’s diameter is about 7,521 miles (12,104 kilometers), compared with some 7,926 miles (12,756 kilometers) for Earth. Its mass is approximately 80 percent of Earth’s, and its density is about 95 percent of Earth’s. The surface gravity of the two planets is also of similar strength.
An infrared image taken by the Venus Express orbiter shows wind-blown clouds in the night sky above Venus. ESA/VIRTIS/INAF-IASF/Obs. de Paris-LEIA
Along with Mercury, Venus is an “inferior” planet, or one whose orbit is smaller than Earth’s. For this reason, Venus always appears in Earth’s sky in roughly the same direction as the Sun. At some times of the year the planet can be seen as a “morning star,” appearing in the hours before sunrise. At other times it can be seen as an “evening star” in the hours after sunset. Venus often can be seen in clear skies during daylight, if the observer knows exactly where to look.
Because Venus orbits closer to the Sun than Earth does, it exhibits phase changes as viewed from Earth. These phases are similar to those of the Moon and Mercury. Venus sometimes appears as a thin crescent and sometimes as a half or fuller disk. It passes through one cycle of phases about every 584 Earth days. The phases can be seen easily in high-power binoculars or a small telescope.
Transits of Venus are rare, but when they do happen, they occur in December and June. Transits of Venus generally follow a recurrence pattern of 8, 121, 8, and 105 years before starting over. Following the transits of Dec. 9, 1874, and Dec. 6, 1882, the world waited 121 years until June 8, 2004, for the next transit to occur. Dates for successive transits of Venus are June 6, 2012, and, after a 105-year interval, Dec. 11, 2117, and Dec. 8, 2125. Unlike a transit of Mercury, a transit of Venus can be watched without magnification through a suitable dark filter or as an image projected on a screen through a pinhole lens.
Observing the transits of Venus was of great importance to 18th- and 19th-century astronomers because careful timings of the events permitted accurate measurement of the distance between Venus and Earth. This distance in turn allowed calculation of the distance between Earth and the Sun, called the astronomical unit, as well as the distances to the Sun of all the other planets.
Venus rarely but regularly passes directly between Earth and the Sun. During this event, or transit, the planet appears to observers on Earth as a small black disk crossing the bright disk of the Sun. Two transits of Venus occur about every 125 years. The transits occur in pairs eight years apart.
Venus’s orbit is the most nearly circular of all the planets. It orbits the Sun at a mean distance of about 67 million miles (108 million kilometers). This is about 30 percent closer to the Sun than Earth’s orbit is. At its closest approach to Earth, Venus is about 26 million miles (42 million kilometers) away; at its farthest, Venus is some 160 million miles (257 million kilometers) away. Venus completes one orbital revolution about every 225 Earth days, which is the length of one year on Venus.
Venus’s rotation is unusual in a couple of ways. It spins about its axis very slowly, completing one rotation about every 243 Earth days. It is the only planet in the solar system that takes longer to rotate once about its axis than to travel once around the Sun. These two motions combine so that a day on Venus—the time it takes for the Sun to appear straight overhead, to set, and then to rise straight overhead again—lasts about 117 Earth days.
Because of its slow rotation, Venus is more nearly spherical than Earth and most other planets. The force from a planet’s rotation generally causes some bulging at the equator and flattening at the poles. These distortions are minimized on Venus.
Venus also rotates in retrograde motion, or the direction opposite that of most of the other planets and members of the solar system. Six of the eight planets rotate clockwise when viewed from above the northern pole, while only Venus and Uranus rotate counterclockwise. To an observer on Venus, the Sun would appear to rise in the west and set in the east (if one could see through the thick clouds). Venus’s spin axis is tilted only about 3 degrees relative to the plane of its orbit. This means that seasonal variations on the planet are probably very slight.
Unlike Earth and most of the other planets, Venus does not have a global magnetic field. This might result from its extremely slow rotation rate. Scientists think that a planet’s rotation helps drive the fluid motions in the planet’s core that generate a magnetic field.
