“Why does the earth orbit the sun?” The question posed to two dozen students five years ago in my middle school science class seemed deceptively simple. Yet after almost a minute, only a few tentative hands rose, and every answer was wrong. Dead wrong. So why does the earth orbit the sun? Two words summarize the answer: velocity and gravity.
Every time someone drops an item on earth, it invariably strikes the ground as a result of gravity. As discussed in the last article, an item thrown extremely fast (23,000 mph or more), would escape earth’s gravity forever. But what would happen if an item were thrown a little slower — perhaps at 20,000 mph? It would not escape earth’s gravity, but it wouldn’t necessarily strike the ground either.
But how can that be? The answer can be found in almost every fair or amusement park in America: the spiral wishing well. One rolls a penny along the edge as fast as possible without it tipping over. The penny does not immediately fall into the hole, but rather goes around and around the funnel. On earth, of course, the friction of the well itself causes the penny to slow down, going closer and closer to the hole in the funnel before finally falling in. Fortunately for us, we are not destined to fall into the sun, as there is no friction in space to slow the earth down. We are simply going around the sun too quickly to go into the sun.
How then, did we end up traveling so quickly around the sun? The amount of energy required to hurl a two trillion pound rock at over 60,000 miles per hour is astounding. If the entire energy consumption of human civilization were used to move the earth, we would move at the lofty speed of one inch per year. The colossal amount of energy required came from the big bang itself. The force which created our universe hurled the interstellar cloud which became our solar system at massive speeds. Eventually, the cloud, which scientists estimate was several hundred light years across, began to collapse as the gravity of the denser parts pulled the rest of the cloud towards it. The sun eventually formed over the eons, and the particles of dust in the cloud formed the asteroids, planets, and comets which make up our solar system. All of these objects were still moving incredibly fast, and while some movement energy was lost during the collisions that formed our world and those around us, we still had enough left over to travel at the speeds we do today, a fact which spared us the oblivion the sun offers, should we ever fall into it (which isn’t likely to happen).
The sun itself is still moving as the cloud it came from did before. This has also been a life saver, protecting us from falling into the massive black hole in the center of our galaxy. Our existence today owes entirely to the energy which set the universe in motion: if we travelled just 15 percent slower and we would be as close to the sun as Venus, a world whose temperatures rise as high as 900 degrees Fahrenheit. The sun had a greater margin of error, but had it formed near the center of the galaxy, it likely would have detonated before earth even had the opportunity to form.
How fortunate we are.
Alexander Link is student at Tahoma High School and a self described math nerd.
