Bill Harris, age 10, of Somerset, Ky., for his question:

Why is Venus called Earth's twin?

No two planets are exactly alike. But Venus and the earth probably are as much alike as two planets can be. They are almost twins in size Venus being a little smaller. The diameter of the earth at the equator is 7,927 miles, 227 miles greater than the diameter of Venus. Venus is a little lighter, tipping the scales at 82 percent the weight of the earth. In volume, Venus equals 92 percent the volume of the earth.

The orbits of the almost‑twin planets are closer than those of any other two planets. The average distance between these two orbits is 26 million miles. Venus gets a little, but not much more heat and light from the sun than does the earth because it is somewhat closer to the sun. Both planets are clothed in cloudy atmosphere, though only the earth has a moon.