Michael Hansing, age 11, of Clearfield, Utah, for his question:
Which is the smallest planet?
We expect the baby to stay close to Mamma. When the baby of the planet family is closest to Father Sol. The little fellow is Mercury, only 36 million miles from the sun. The earth is roughly 93 million miles from the sun, which makes us almost three times farther away than little Mercury, the baby of the planet family.
Mercury is so small that its surface gravity is only one quarter that of the earth. This means that if on earth you weigh 100 pounds, on Mercury you would weigh only 25 pounds. With the strong muscles you developed for earth gravity, you would be able to walk along with giant strides and loop over trees with case ‑ if there ware trees on Mercury.
Actually, we know for sure there are no trees on Mercury. The temperatures are so extreme that there could not possibly be life as we know it. One side of the little planet is hot enough to melt tin and the other side is hundreds of degrees below zero.
What's more, Mercury has no alternating day and night. One side, the day side is always facing the sun, the opposite is always in the shadows of night. Hence, the same side is always scathing hot and the other side always bitterly cold.
Our day and night are caused by the earth rotating on its axis. As the big globe turns, first one side, then another side face the sun and get bathed in daylight. Would you guess that little Mercury does not revolve on its axis? You would be wrong. For it does revolve, but in a very tricky manner.
We all know that the :noon revolves once on its axis. Yet it keeps the same face turned towards the earth. This is because one revelation is equal to the time it takes to make one trip around the earth. Mercury also turns once on its axis while it is making one trip around its orbit.
You can demonstrate this trick for yourself with a floor lamp. Place the lamp in the middle of the floor where you have space to walk all the way around it. The lamp will be the sun and you will play the role of Mercury. Your orbit will be a circle not far from the lamp.
Stand facing the lamp and start to move clockwise around your orbit. You will have to shuffle sideways, because you must always keep your face towards the lamp. Stop when you are a quarter, half and three quarters of the distance around. You will notice that you face first one wall, then another, then another. In one trip around your orbit, you will make one complete turn on your axis.
In size, little Mercury is only about one and a half times the size of our moon. Its diameter is 3,100 miles. It whizzes around its orbit at 30 miles a second and it makes one round trip in 88 earth days.