why do the planets orbit around the sun
Why do the planets rotate around the Sun? First, please note that "rotate" actually is used to describe an celestial body's spin, and "revolve" is used to describe its orbital motion. For example, the Earth completes one rotation about its axis about every 24 hours, but it completes one revolution around the Sun about every 365 days. Anyway, the basic reason why the planets revolve around, or, the Sun, is that the
of the Sun keeps them in their orbits. Just as the Moon orbits the Earth because of the pull of Earth's gravity, the Earth orbits the Sun because of the pull of the Sun's gravity. Why, then, does it travel in an around the Sun, rather than just getting pulled in all the way? This happens because the Earth has a velocity in the direction perpendicular to the force of the Sun's pull. If the Sun weren't there, the Earth would travel in a straight line. But the gravity of the Sun alters its course, causing it to travel around the Sun, in a shape very near to a circle. This is a little hard to visualize, so let me give you an example of how to visualize an object in orbit around the Earth, and it's analogous to what happens with the Earth and the Sun. Imagine Superman is standing on Mt. Everest holding a football. He throws it as hard as he can, which is incredibly hard because he's Superman. Just like if you threw a football, eventually it will fall back down and hit the ground. But because he threw it so hard, it goes past the horizon before it can fall. And because the Earth is curved, it just keeps on going, constantly "falling," but not hitting the ground because the ground curves away before it can. Eventually the football will come around and smack Superman in the back of the head, which of course won't hurt him at all because he's Superman.
That is how orbits work, but objects like spaceships and moons are much farther from the Earth than the football that Superman threw. (We're ignoring air resistance with the football example; actual spacecraft must be well above most of a planet's atmosphere, or air resistance will cause them to spiral downward and eventually crash into the planet's surface. ) This same situation can be applied to the Earth orbiting the Sun - except now Superman is standing on the Sun (which he can do because he's Superman) and he throws the Earth. The next question, then, is how did Earth get that velocity, since in real life there's no Superman throwing it. For that, you need to go way back to. This page was last updated on January 31, 2016. Every 23 hours, 56 minutes, and 4. 1 seconds, the Earth spins once around its axis. We usually call this a БdayБ and just round it out to 24 hours. However, Earth is not the only thing spinning as it careens through space. Almost every celestial object like stars and planets are spinning. WhatБs more interesting, almost everything within the solar system spins and orbits in the same direction. The planets are dancing a seemingly choreographed waltz through, but why? To answer the question we have to go back to the beginning, to the formation of a solar system like ours. Before a star and its planets exist, thereБs just a cloud of disorganized gas and small. This is often called a molecular cloud or Бstellar nursery. Б The Eagle and Orion Nebulae are some of the more famous stellar nurseries we have been able to observe with the Hubble Space Telescope.
These clouds are composed mostly of molecular hydrogen, which wouldnБt be able to congregate outside the dense molecular clouds. These clouds can be of any size not just massive structures like the Orion Nebula. Over time the energy of the molecules in the cloud pushing outward can be overcome by slower molecules collapsing together farther in. As long as there is sufficient mass in the molecular cloud, it continues collapsing in toward the center until it reaches a high enough mass to fuse hydrogen and become a new star. The spin that we see quite clearly now is related to the process of the molecular cloud collapsing. The original cloud was very, very large and made up of many individual molecules and small clumps of matter. On that scale, there is some small amount of rotation within the cloud. It could be caused by the gravity of nearby stellar objects, local differences in mass as the cloud churns, or even the impact of a distant supernova. The point is, most molecular clouds have at least a little rotation. As the cloud collapses to form a star, it has what physicists call angular momentum. This is the movement an object has as it rotates around a central point. In a large system like a molecular cloud, each particle has some angular momentum, and it all adds together across a very wide area. ThatБs a lot of momentum, and it is conserved as the cloud continues to collapse. But how does that get us to objects that spin and orbit? Imagine a figure skater spinning around with arms outstretched. That is a model of angular momentum just like a collapsing cloud of gas.
When the arms are drawn inward, the rotational velocity goes up because the total angular momentum is conserved unless there is some external force acting on it. There are such forces acting on the figure skater, but less so on a collapsing molecular cloud. So if a molecular cloud was maybe a light year across, then collapsed down to be just a fraction of that, it would be a huge change in size. Just like the figure skater pulling her arms in, the velocity must increase to conserve angular momentum and therefore form a spinning protostellar disc. It is from this orbiting matter that all the form, and of course, they are also spinning and orbiting in the same direction because of the conservation of angular momentum. There are two outliers in the solar system which seem to break the rules about conserving momentum Uranus and Venus. Uranus spins on an axis of almost 90-degrees (on its side). Venus meanwhile spins the opposite direction as and the other planets. In both cases there is strong evidence that these planets were struck by large objects at some point in the distant past. The impacts were large enough to overcome the angular momentum of the bodies, and give them a different spin. The tl;dr here is that almost everything in the solar system is spinning because the matter it is made of was always spinning in some small way. That kind of momentum doesnБt just go away. There is no force that makes planets rotate or orbit itБs just the energy from the formation of the solar system still being expended. Edit: Yes, leap year was an unnecessary tangent, so we removed that bit. б
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