Marianne Mielke, age 11, of Winston Salm, N.C., for her question:
What causes Venus to have phases like the moon?
It was Galileo who first saw that Venus has phases like the moon. He figured out that this happens because Venus is closer than the earth is to the sun. The sister planets swing around and around their orbits, but Venus has the inside track: It is forever overtaking the earth, passing it and swooping around to the opposite side of the sun to start the heavenly game of tag all over again.
Half of Venus, of course, is always in daylight, while the other half is in the shadows of night. The dark side is invisible to us, and we see only where sunshine strikes the golden planet. When Venus is on the opposite side of the sun, we see the whole daylight side. This is its full moon phase.
When Venus comes between us and the sun, its dark side is facing us and our neighbor planet is invisible. When it is on the east or west of the sun, we see part of the day side and part of the night side. Venus passes through quarter and gibbous or half phases. Last January, the Sister planets were on opposite sides of the sun, and Venus was in full phase. Through the summer, it swung around to the east of the sun, and we saw it as the golden evening star. As it came nearer to us, its phases shrank from the gibbous to the quarter phase.
On Nov. 12, Venus passed directly between us and the sun; and since its back, or night side, was facing us, it became invisible. A new crescent appeared as Venus moved ahead of us to the west side of the sun. Now it appears in the role of the morning star. It is almost at the gibbous phase, and soon Venus will swing around back of the sun again showing more and Mort of its sunlit face as it goes. Next Aug. 30, the two planets will again be on opposite sides of the sun and 161 million miles apart. In Novcmber, the sister planets were only 25 million miles apart, which is closer than any other planet eyer comes to the earth.
You would expect Venus to be brightest during its full phase. But it is more than twice as bright when we see only the slim crescent phases. At such times, Venus is six times closer to us, and it shines six times brighter than giant Jupiter and 15 times brighter than Sirius, which is the brightest of all the stars. These brilliant phases occur about one month before and one month after the planet Venus becomes invisible.