Dave Austin, age 13, of Haggerstown, Md., for his question:
WHY IS VENUS CALLED 'EARTH'S TWIN?'
Venus is known as the earth's "twin" because the two planets are so similar in size. Venus is just 360 miles smaller in diameter than earth.
Venus appears to be brighter than any other planet or even any star, as soon from the earth. 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. It is often called the Evening Star.
Ancient astronomers called the object that appeared in the morning Phosphorus, and the object that appeared in the evening Hesperus. Later they realized these objects were the same planet. They named Venus in honor of the Roman goddess of love and beauty.
Venus travels around the sun in a nearly circular orbit. It takes about 225 earth days, or about seven and a half months, to go around the sun once, compared with 365 days, or one year, for the earth.
Venus' orbit around the sun is closer to the sun than is earth's orbit. Only the planet Mercury orbits the sun closer than does Venus.
The average temperature on Venus is over 800 degrees Fahrenheit, higher than that of any other planet. Temperatures of areas facing the sun may rise above 980 degrees while the dark side of the planet is rarely cooler than 600 degrees.
The temperatures on Venus are much higher than had originally been predicted. Most astronomers thought the thick clouds surrounding Venus reflected much of the sun's energy. If the clouds did this, the surface temperature would be similar to those on the earth.
Instead, much of the sun's energy seems to filter through the clouds and be trapped by the thick atmosphere.
By letting the sun's energy filter into Venus' atmosphere, but preventing it from escaping, the clouds work in the same manner as a greenhouse. For this reason, astronomers call Venus' heating method the greenhouse effect.
Venus was the first planet to be observed by a passing spacecraft. The unmanned United States spacecraft Mariner II passed within 21,600 miles of Venus on Dec. 14, 1962 after taveling through space for three and a ha if months.
Mariner II made various measurements of conditions on and near Venus. Instruments in the spacecraft, for example, measured the high surface temperatures of the planet.
The United States and Russia have made a number of additional explorations of Venus.
Although Venus is called the earth's "twin," its surface conditions appear to be entirely different from those of the earth. The surface of Venus is very hot and dry. Intense heat and rain containing strong acids are believed to have worn away part of the surface, making it rather smooth and flat.
Photographs and radar measurements incidate that other parts of the surface are rough and covered with large sharp edge rocks.