David Dean II, age 11, of Clarksville, Texas, for his question:
How does the earth revolve around the sun?
The motions of the heavenly bodies often are not what they seem, so young space agers keep their minds open until they learn the facts. However, many star gazers of the past believed what they wanted to believe. They were fooled into thinking that the sun revolves around the earth.
The heavenly bodies appear to rise in the east, parade over the sky and disappear out of sight below the western horizon. This is because the earth rotates on its axis towards the east. It also revolves around the sun in an easterly direction. In fact, this is the favorite direction of our entire Solar System. The sun rotates and all the planets revolve towards the east. Nobody knows how or why this easterly spin got started.
The earth's orbit around the sun is almost, though not quite, a circle. Its average distance from the sun is about 93 million miles. But its perihelion, its shortest distance, is one and a half million miles closer. Its aphelion, the longest distance, is one and a half million miles farther away. We pass the aphelion in July and the perihelion in January. It seems odd, but in mid winter we are three million miles closer to the sun than we are in July. But this is another question.
The earth revolves slightly faster when it is closest to the sun. Its average speed is 18.47 miles a second and it completes each yearly orbit in 365 1/4 days. Three of our calendar years have 365 days. The fourth gathers up the dangling quarters and we get a leap year of 366 days.. But actually, during each revolution around its orbit, the earth rotates 365 complete turns on its axis, plus one quarter turn.
The earth cannot change any of these patterns. It must obey certain complicated laws and forces of the universe. One is the mighty force of gravity that exerts a pulling attraction between the sun and the earth. The orbital speed counteracts this pull, or the earth would be pulled into the sun. Actually, the earth is falling constantly towards the sun. But its orbital speed keeps it from falling towards the sun in a straight, downward plunge. It bends the path of the plunge in a curved orbit. Instead of falling straight into the sun, the earth revolves around and around its orbit at an average speed of 66,600 miles an hour. The merry yearly spin around the orbit is eastward and the merry daily spin around the axis is eastward.
The exact orbit of the earth is held in place by a balance of cosmic forces. If the earth were more massive, its gravity would be stronger. To balance this extra force, its orbit would have to be wider and we would be farther from the sun. If the earth were less massive, its orbit would be closer to the sun. As it is, an average distance of 93 million miles is just right to keep things safely balanced.