Juliann Ellis, Age 11, Of Des Moines, Ia., for her question:
How close is Mercury to the earth
Mercury is the smallest planet of the solar system and close enough to us to shine like a bright star in the sky. But we rarely see it. Mercury is the closest planet to the parent sun. It never appears far from the sun, and as a rule it is out dazzled by the radiant star of the solar system.
The nine planets revo1ve around and around the sun, and mercury orbits around the inside traffic lane. The earth, from which we view our brother planets, orbits the third track from the sun. We orbit at an average speed of 182 miles a second, but mercury orbits at almost twice this speed. Its average orbital speed is 36 miles a second. It is the smallest planet, but it circles the sun faster than any of its bigger brothers.
Mercury takes only 88 earth days to complete its trip around the sun and makes more than four yearly orbits while the earth is making only one. The earth travels at about half mercury's speed, and the distance around our orbit is almost three times farther. Mercury on the inside traffic lane is always catching up to us and passing us.
The distance between the two planets changes every month. They are closest at inferior conjunction, when mercury passes hetween us and the sun. At this point we are separated from mercury by about 59 million miles. But the little speeder dashes onward without a pause, and meantime the earth also continues its orbiting. The distance between them is now increasing some 60,000 miles with every hour.
In about two months mercury is at superior conjunction on the opposite side of the sun. The two planets are then farthest apart, which is about 129 million miles. From this point the little speeder comes around the sun and catches up with us again. The two planets return to the same positions every 116 days.
There are variations in the traffic schedules because the orbits of the two planets are tipped at different angles. Also, their orbits are ellipses. The average distance of mercury from the sun is 36 million miles and ours is 93 million miles. But Mercury's distance from the sun varies by 25 million miles and ours by 3 million miles.
Through this month mercury is speeding to overtake us, and tonight we are about 92 million miles apart. On Sept. 2, the little planet will pass between us and the sun, and the two planets will be separated by only 59 million miles.
On each orbit Mercury seems to swing westward, then eastward of the sun. On its westward path we see it as a morning star at sunrise. On its eastward path it becomes an evening star at sunset. It is never more than 27 degrees from the rim of the brilliant sun. This sky distance is about 50 times the width of the full moon, and most of the time mercury is hidden from us in the glaring light of the sun.