Yesterday, a young friend (perhaps a few young friends) posed this question to me: “When will the Earth collide with the Sun?” I will amend the question to “will the Earth collide with the Sun” so that I can provide the answer: no.
To understand why the answer is no, we must first understand a few concepts of physics, mainly that of angular momentum. If you remember, an object in motion (or at rest) will want to stay that way unless it interacts with something that will change its motion. For instance, when you are running really fast, you find it hard to slow down, because your body wants to stay in motion–to slow down to a halt, you create friction between your feet and the ground (as well as with the air), and you eventually come to a stop. This type of momentum is called linear momentum–the tendency for moving (or resting) objects to continue in that direction unless something else interferes. This is why, in baseball, the player running to first base always runs way past it…even though he/she would want to slow down once they step on the base, it takes a while before they can.
The same thing is true for angular movement, like spinning or circular motion. Think about when you spin a bucket around–when the bucket is spinning around, it’s hard to keep it from completing the circle, because it wants to continue moving. This is appropriately called angular momentum, and it’s the reason we don’t fall into the sun.
For anyone who has been to a museum (often ones with scientific backgrounds), you may remember the neat spiral wishing well coin funnels, where you drop a coin in and it spirals in towards the center. These are popular with science museums because they are perfect examples of the principle of angular momentum. What happens when you drop a coin in? It starts out going in a circle at the outer edge. According to angular momentum, unless something interferes with the coin, it should remain moving in a circle around the hole in the middle forever, staying at the same height and speed. But that doesn’t happen, because gravity and friction remove energy from the coin, slowing it down. When the coin slows, it loses angular momentum. Because the coin is slowing down and losing angular momentum, it begins to spiral into the hole in the center, and eventually, drops into the middle and comes to rest.
Now think of the Earth as the coin and the Sun as the hole in the center of the funnel. The Earth is going around the Sun in a circle (well, it travels in an ellipse, but it’s the same idea), and it has a speed in which it is traveling. Therefore, the Earth has angular momentum which keeps it traveling in the circle. Like the coin, if you wanted the Earth to collide with the Sun, it would have to slow down, and therefore lose angular momentum. While the interactions the Earth undergoes on a yearly basis are very complicated, suffice it to say that the net interactions are luckily negligible, and so we are not spiraling towards the sun.
Don’t believe me? If we were losing angular momentum, and were spiraling into the Sun, then our orbit around the sun would be shrinking, and our distance from the Sun would be shorter. While the Earth is getting warmer, this is not why (see: Climate Change for more information). What is noticeable, however, is that the Earth’s rotation is slowing down ever so slightly. Over time, the length of our day is getting slightly longer, and this has caused us to establish leap seconds to help compensate.