Will The Earth Collide With The Sun? (No)

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.

And so, thanks to angular momentum, we have much more important things to worry about than the Earth falling into the Sun. Will the Earth collide with the Sun? No.

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7 Responses to Will The Earth Collide With The Sun? (No)

  1. Allen says:

    I was just wondering about this topic, and I ran into your site. I think it’s a great explanation however – the problem that I have with your story is the use of the word negligible. Isn’t it that over time all these negligible interactions add up, and would slow Earth down to a speed that would put us eventually in a collision course with the Sun? This is probably after the sun has already exploded, or however its life will end – but hypothetically speaking – would Earth not end up colliding with the Sun? Would collisions with asteroids over time decrease its momentum?

    • Well, it’s negligible because the some processes cause Earth to lose or gain momentum, and it balances out. Any imbalance in these processes are not noticeable on cosmic timescales. And, by the time the Sun runs out of fuel and turns into a red giant, the idea of “colliding” with the Sun becomes irrelevant, because the sun will be so large that it will engulf Mercury and Venus, and potentially Earth as well. In general, from what we understand, large scale migration of planets (changes in orbit) are over. Earth’s orbit is stable and even a collision with a huge asteroid would not take away enough momentum to make a noticeable change (although we’d be very unhappy here on the surface!).

  2. Ari Cooper Davis says:

    Thank you so much for this. I was aware that the earth is not spiralling into the sun but couldn’t explain to myself why – thanks for doing that for me!

  3. damien sonnney says:

    I think the question can be more precise :
    Is there cosmic wind that can make earth surf towards the sun the same way hot jupiters are formed?
    I think the answer is no, and the sun will eat the earth at the end of its life before anything happens to its orbit, but I’m not sure

    • Well, there isn’t necessarily a cosmic wind, but there is a solar wind–particles flying outwards from the Sun at very fast speeds. While this can have dangerous effects on satellites, it doesn’t form any significant drag on Earth, so particle winds cannot slow Earth down enough to have an effect on its orbit. But it is true that the Sun, in around 5 billion years, will swell and grow so much that it will roast and maybe even swallow up Earth. It’s a long time away, so I wouldn’t change your retirement plans.

      Hot Jupiters are an interesting phenomenon…we actually don’t yet understand how they are formed. Leading theories are that they form out farther and migrate in, but not from a cosmic wind. Rather, we think that interactions with the disk of material around the forming star can make the planet move inwards–essentially, the Jupiter-sized planet can take nearby dust (rocks) and fling them outwards, and in doing so, move a little inwards. There’s also been some recent articles about hot jupiter formation and how they might effect Earth-sized planet formation, if you’re interested, such as this one: http://www.cosmosmagazine.com/news/3404/new-hot-jupiters-rewrite-planetary-theory

  4. Thank you sir, your explanation was very helpful. But i have a confusion- like the example you gave of the spiral wishing well coin funnels, why don’t the earth collide to the sun like the coin to the hole??? Isn’t it true that the sun exerts a gravitational pull on earth??? So obviously the angular momentum of the earth will experience an external interference leading it to change its direction towards the sun. Therefore it must collide into the sun one day… Please reply and clear my dought.
    hopefully waiting for the reply.

    • I’m sorry that my example was a little confusing. It’s the Sun’s gravity that actually keeps Earth going around in its orbit. Perhaps my first bucket example is better:

      When you swing a bucket around in a circle, your act of pulling on it is what makes the bucket swing around. Your pulling gives the bucket a fast enough speed and enough angular momentum to move in a circle and not fall down. The Earth is like the bucket and the Sun is like you–its gravity is what does the pulling, and makes the Earth swing around in its orbit. In order to fall into the Sun, Earth would have to slow down, but that doesn’t happen because there’s nothing to make it slow down. Gravity is the only major driver of Earth’s motion (and it is what keeps Earth going around so fast), so unless something really massive comes along, Earth will continue to stay in this orbit.

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