Venus, Venus: ultraviolet imaging [Credit: NASA/JPL] Venus: ultraviolet imaging [Credit: NASA/JPL] second planet from the Sun and sixth in the solar system in size and mass. No planet approaches closer to Earth than Venus; at its nearest it is the closest large body to Earth other than the Moon. Because Venus’s orbit is nearer the Sun than Earth’s, the planet is always roughly in the same direction in the sky as the Sun and can be seen only in the hours near sunrise or sunset. When it is visible, it is the most brilliant planet in the sky. Venus is designated by the symbol ♀.

imaging radar: hemispheric view [Credit: NASA/JPL/California Institute of Technology] imaging radar: hemispheric view [Credit: NASA/JPL/California Institute of Technology] Venus was one of the five planets—along with Mercury, Mars, Jupiter, and Saturn—known in ancient times, and its motions were observed and studied for centuries prior to the invention of advanced astronomical instruments. Its appearances were recorded by the Babylonians, who equated it with the goddess Ishtar, about 3000 bce, and it also is mentioned prominently in the astronomical records of other ancient civilizations, including those of China, Central America, Egypt, and Greece. Like the planet Mercury, Venus was known in ancient Greece by two different names—Phosphorus (see Lucifer) when it appeared as a morning star and Hesperus when it appeared as an evening star. Its modern name comes from the Roman goddess of love and beauty (the Greek equivalent being Aphrodite), perhaps because of the planet’s luminous jewel-like appearance.

Venus [Credit: Copyright © 2004 AIMS Multimedia (www.aimsmultimedia.com)] Venus [Credit: Copyright © 2004 AIMS Multimedia (www.aimsmultimedia.com)] Venus has been called Earth’s twin because of the similarities in their masses, sizes, and densities and their similar relative locations in the solar system. Because they presumably formed in the solar nebula from the same kind of rocky planetary building blocks, they also likely have similar overall chemical compositions. Early telescopic observations of the planet revealed a perpetual veil of clouds, suggestive of a substantial atmosphere and leading to popular speculation that Venus was a warm, wet world, perhaps similar to Earth during its prehistoric age of swampy carboniferous forests and abundant life. Scientists now know, however, that Venus and Earth have evolved surface conditions that could hardly be more different. Venus is extremely hot, dry, and in other ways so forbidding that it is improbable that life as it is understood on Earth could have developed there. One of scientists’ major goals in studying Venus is to understand how its harsh conditions came about, which may hold important lessons about the causes of environmental change on Earth.

Basic astronomical data

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Viewed through a telescope, Venus presents a brilliant yellow-white, essentially featureless face to the observer. Its obscured appearance results from the surface of the planet being hidden from sight by a continuous and permanent cover of clouds. Features in the clouds are difficult to see in visible light. When observed at ultraviolet wavelengths, the clouds exhibit distinctive dark markings, with complex swirling patterns near the equator and global-scale bright and dark bands that are V-shaped and open toward the west. Because of the all-enveloping clouds, little was known about Venus’s surface, atmosphere, and evolution before the early 1960s, when the first radar observations were undertaken and spacecraft made the first flybys of the planet.

Venus orbits the Sun at a mean distance of 108 million km (67 million miles), which is about 0.7 times Earth’s distance from the Sun. It has the least eccentric orbit of any planet, with a deviation from a perfect circle of only about 1 part in 150. Consequently, its distances at perihelion and aphelion (i.e., when it is nearest and farthest from the Sun, respectively) vary little from the mean distance. The period of its orbit—that is, the length of the Venusian year—is 224.7 Earth days. As Venus and Earth revolve around the Sun, the distance between them varies from a minimum of about 42 million km (26 million miles) to a maximum of about 257 million km (160 million miles).

Because Venus’s orbit lies within Earth’s, the planet exhibits phases like those of the Moon when viewed from Earth. In fact, the discovery of these phases by the Italian scientist Galileo in 1610 was one of the most important in the history of astronomy. In Galileo’s day the prevailing model of the universe was based on the assertion by the Greek astronomer Ptolemy almost 15 centuries earlier that all celestial objects revolve around Earth (see Ptolemaic system). Observation of the phases of Venus was inconsistent with this view but was consistent with the Polish astronomer Nicolaus Copernicus’s idea that the solar system is centred on the Sun. Galileo’s observation of the phases of Venus provided the first direct observational evidence for Copernican theory.

The rotation of Venus on its axis is unusual in both its direction and its speed. The Sun and most of the planets in the solar system rotate in a counterclockwise direction when viewed from above their north poles; this direction is called direct, or prograde. Venus, however, rotates in the opposite, or retrograde, direction. Were it not for the planet’s clouds, an observer on Venus’s surface would see the Sun rise in the west and set in the east. Venus spins very slowly, taking about 243 Earth days to complete one rotation with respect to the stars—the length of its sidereal day. Venus’s spin and orbital periods are very nearly synchronized with Earth’s orbit such that, when the two planets are at their closest, Venus presents almost the same face toward Earth. The reasons for this are complex and have to do with the gravitational interactions of Venus, Earth, and the Sun, as well as the effects of Venus’s massive rotating atmosphere. Because Venus’s spin axis is tilted only about 3° toward the plane of its orbit, the planet does not have appreciable seasons. Also, Venus’s rotational period slowed by 6.5 minutes between two sets of measurements taken in 1990–92 and 2006–08. Astronomers as yet have no satisfactory explanation for Venus’s peculiar rotational characteristics. The idea cited most often is that, when Venus was forming from the accretion of planetary building blocks (planetesimals), one of the largest of these bodies collided with the proto-Venus in such a way as to tip it over and possibly slow its spin as well.

Venus’s mean radius is 6,051.8 km (3,760.4 miles), or about 95 percent of Earth’s at the Equator, while its mass is 4.87 × 10²⁴ kg, or 81.5 percent that of Earth. The similarities to Earth in size and mass produce a similarity in density—5.24 grams per cubic centimetre for Venus, compared with 5.52 for Earth. They also result in a comparable surface gravity—humans standing on Venus would possess nearly 90 percent of their weight on Earth. Venus is more nearly spherical than most planets. A planet’s rotation generally causes a bulging at the equator and a slight flattening at the poles, but Venus’s very slow spin allows it to maintain its highly spherical shape. For additional orbital and physical data, see the table.