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Most of the world believed that Earth was the center of the universe for a really long time. Then a few scientists decided to take a closer look.

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Sources:
http://csep10.phys.utk.edu/astr161/lect/retrograde/aristotle.html
http://csep10.phys.utk.edu/astr161/lect/retrograde/copernican.html
http://io9.com/5464810/the-earth-revolves-around-the-sun--prove-it
http://www.universetoday.com/18097/the-earth-goes-around-the-sun/
http://earthobservatory.nasa.gov/Features/OrbitsHistory/
http://scienceblogs.com/startswithabang/2010/09/13/geocentrism-was-galileo-wrong/
http://www.realclearscience.com/blog/2014/08/technically_the_earth_does_not_orbit_the_sun.html
http://www.wired.com/2014/04/how-do-we-know-the-earth-orbits-the-sun/
http://www.universetoday.com/36487/difference-between-geocentric-and-heliocentric/
http://www.astro.umd.edu/~miller/teaching/honr229Xs11/lecture03.pdf
http://hyperphysics.phy-astr.gsu.edu/hbase/cm.html

You, me, every single thing on this planet -- we’re all zooming through space right now, orbiting the Sun at an average speed of about 107,000 kilometers an hour. And rotating while we’re at it. But the Sun, Moon, planets, and stars all look like they’re moving through the sky, right? So it’s easy to imagine that we’re staying put while they all move around us -- as long as you don’t look too closely.

Which is why the whole idea that the Earth-orbits-the-Sun wasn’t really accepted until a few hundred years ago -- and it took a lot of convincing. A lot of the geocentric model -- where Earth’s at the center of everything -- was based on ideas developed by ancient Greek philosophers Aristotle and Ptolemy -- basically, that the Earth stayed put at the center of everything, and anything we can see in the sky -- the Sun, Moon, planets, stars, everything -- orbited it in perfect circles.

But that didn’t explain why planets -- which at the time referred to anything that looked different from the background stars, including the Sun and Moon -- sometimes changed brightness, or occasionally looked like they were moving in a completely different direction. So the geocentric model tried to explain them using epicycles: imaginary points that were moving in imaginary circles, with planets orbiting those points. And it sort of worked, though eventually they had to draw more circles around the circles to explain some things, and have the planets move faster or slower at certain points in their orbits -- diagrams of the sky were kind of a mess.

There were at least a couple of Greek philosophers who disagreed, and the geocentric model was questioned by Muslim scholars since at least the tenth century. But for a long time, especially among Western civilizations, it was just accepted: everything revolved around Earth, even if that was hard to explain sometimes. At least, until Copernicus came along.

Nicolaus Copernicus was a well-known Renaissance thinker who spent the last few decades of his life developing a heliocentric model, where the Sun is the center of the universe, which he published just before he died. And, okay, we now know that the Sun isn’t the center of the Universe, but it was a lot closer to the truth. Copernicus realized that the math was a little simpler if you just had Mercury, Venus, Earth, Mars, Jupiter, and Saturn -- the only planets they knew about at the time -- orbiting the Sun, with the Moon orbiting Earth. He used epicycles too, but all the planets in his model kept the same speed throughout their orbits -- which is what you’d expect if that orbit is a circle.

But it was still hard to prove that the heliocentric model was more accurate -- for one thing, even if they made things simpler, those circular orbits in Copernicus’ universe were wrong, and it was just as bad as the geocentric model for predicting how the planets would move. Then, in the early 1600s, Johannes Kepler realized that orbits didn’t have to be circles -- and if he assumed they were ellipses, the math fit the motion of the planets, Sun, and Moon a lot better using the heliocentric model.

Around the same time, Galileo Galilei’s telescope made things even more convincing -- mostly thanks to Jupiter and Venus. When Galileo pointed his telescope at Jupiter, he was able to see its four biggest moons: Ganymede, Callisto, Io, and Europa. That was pretty concrete proof that not everything in the sky was orbiting Earth. Then he noticed something weird about Venus: it goes through phases just like our Moon does, from new to full and back, but it’s much bigger when it’s new than when it’s full. That made no sense in the geocentric model, which predicted that sometimes Venus would be closer to Earth when it was new, but sometimes it would be closer when it was full. In the heliocentric model, though, the Sun, Venus, and Earth lined up so that Venus would be closer to Earth when it was new.

Even with all this evidence, it took a long time for the heliocentric model to be accepted. The Church banned heliocentric works until 1758. These days, we know that the heliocentric model wasn’t exactly right, either. The Sun isn’t at the center of the universe, there’s a lot more out there, and if you want to get technical about it, both Earth and the Sun orbit the same point -- the center of mass of the solar system, which is just the center point of all the objects in it, based on their mass and how far they are from each other.

Since the Sun has 99.87 percent of all the mass in the solar system, the center of mass is pretty close to the center of the Sun. But it does move around a bit, depending on where the planets are in their orbits. But one thing that we know one hundred percent for sure is that the geocentric model is wrong. We’ve sent telescopes, probes, and people up there and seen it for ourselves.

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