Venus

Venus is known as the Earth's "twin" because the two planets are so similar in size. The diameter of Venus is about 7,520 miles (12,100 kilometers), approximately 400 miles (644 kilometers) smaller than that of the Earth. No other planet comes nearer to the Earth than Venus. At its closest approach, it is about 23.7 million miles (38.2 million kilometers) away.

As seen from the Earth, Venus is brighter than any other planet or even any star. At certain times of the year, Venus is the first planet or star that can be seen in the western sky in the evening. At other times, it is the last planet or star that can be seen in the eastern sky in the morning. When Venus is near its brightest point, it can be seen in daylight.

Orbit

Venus is closer to the sun than any other planet except Mercury. Its mean (average) distance from the sun is about 67.2 million miles (108.2 million kilometers), compared with about 93 million miles (150 million kilometers) for the Earth and about 36 million miles (57.9 million kilometers) for Mercury.

Venus travels around the sun in a nearly circular orbit. The planet's distance from the sun varies from about 67.7 million miles (108.9 million kilometers) at its farthest point to about 66.8 million miles (107.5 million kilometers) at its closest point. The orbits of all the other planets are more elliptical (oval-shaped). Venus takes about 225 Earth days, or about 71/2 months, to go around the sun once, compared with 365 days, or one year, for the Earth.

Phases

When viewed through a telescope, Venus can be seen going through "changes" in shape and size. These apparent changes are called phases, and they resemble those of the moon. They result from different parts of Venus's sunlit areas being visible from the Earth at different times.

As Venus and the Earth travel around the sun, Venus can be seen near the opposite side of the sun about every 584 days. At this point, almost all its sunlit area is visible. As Venus moves around the sun toward the Earth, its sunlit area appears to decrease and its size seems to increase. After about 221 days, only half the planet is visible. After another 71 days, Venus nears the same side of the sun as the Earth, and only a thin sunlit area can be seen.

Rotation

As Venus travels around the sun, it rotates very slowly on its axis, an imaginary line drawn through its center. Venus's axis is not perpendicular (at an angle of 90¡) to the planet's path around the sun. The axis tilts at an angle of approximately 178¡ from the perpendicular position. Unlike the Earth, Venus does not rotate in the same direction in which it travels around the sun. Rather, Venus rotates in the retrograde (opposite) direction and spins around once every 243 Earth days.

Surface and Atmosphere

Although Venus is called the Earth's "twin," its surface conditions appear to be very different from those of the Earth. Geologists have had difficulty learning about the surface of Venus because the planet is always surrounded by thick clouds of sulfuric acid. They have used radar, radio astronomy equipment, and space probes to "explore" Venus.

Temperature

The temperature of the uppermost layer of Venus's clouds averages about 55 degrees F (13 degrees C). However, the temperature of the planet's surface is about 870 degrees F (465 degrees C), higher than that of any other planet and hotter than most ovens.

The plants and animals that live on the Earth could not live on the surface of Venus, because of the high temperature. Astronomers do not know whether any form of life exists on Venus, but they doubt that it does.

Mass and Density

The mass of Venus is about four-fifths that of the Earth. The force of gravity on Venus is slightly less than on the Earth. For this reason, an object weighing 100 pounds on the Earth would weigh about 88 pounds on Venus. Venus is also slightly less dense than the Earth. A portion of Venus would weigh a little less than an equal-sized portion of the Earth.

Flights to Venus

Venus was the first planet to be observed by a passing spacecraft. The unmanned U.S. spacecraft Mariner 2 passed within 21,600 miles (34,760 kilometers) of Venus on Dec. 14, 1962, after traveling through space for more than 31/2 months. It measured various conditions on and near Venus. For example, instruments carried by the spacecraft measured the high temperatures of the planet.