Margery Ann Salm, age 11, of Springfield, Ors., for her question:
Why is the planet Venus so bright?
When it canes to brilliance, Venus has only two rivals in the sky. They are the sun and the moon. Our neighboring planet can outshine the brightest of the stars. But this happens only at certain tlme8. The brilliance of Venus comes from cloudy atmosphere, and it varies because Of the golden planet's orbit around the sun.
The filmy clouds above the earth reflect back more sunlight than the rocky ground. When our globe is swathed in masses of stormy clouds, it reflects more sunlight out into space. At such times it looks brighter When seen from the moon or from one of its sister planets. The reflecting power of a heavenly body is called its albedo, a scientific term coined from an older word meaning white.
Venus gets a quota of light from the sun. Its albedo gives us the percentage of this light which is bounced back into space. This reflected light is what We See when we look at the brilliant planet in the sky. Since some surfaces reflect more sunlight than others, the albedo tells astronomers a lot about the surface and the atmosphere of a planet.
The albedo Of Venus is 59%, which means that it reflects back more than half of the sunshine that falls upon it. This high albedo could only come from layers of dense clouds. Venus is bright because it is swathed in thick blankets of clouds. The moon, with no atmosphere, has an albedo of 7%, which means it reflects only l/l4 of the sunshine that falls upon its rocky face.
The orbit of Venus brings it nearer to the sun than we are. As it travels around, it is sometimes on the far side of the sun. It is then at a distance of 160 million miles and the entire side facing us is bathed in sunshine. This, however, is not the time when Venus is brightest.
It gets brighter as it travels around to pass between us and the sun. Meantime it goes through phases like the moon. When it is closest, at 26 million miles, the opposite side is facing the sun and its dark side faces the earth. The golden planet is brightest when it is in the crescent phase, a month before and a month after it passes between us and the sun.
At its brightest, Venus is i5 times brighter than Sirius, which outshines all the other stars in our sky. The second most brilliant planet is giant Jupiter. When the two planets are brightest, Venus is six times brighter than Jupiter. It is brilliant enough to be seen in the sky at high noon.