The Earth is spherical in shape.

Thanks to Edl for accepting. I'll be presenting three arguments this round in support of a round earth. Should I deem it necessary, I may present additional arguments in the next round. I'll jump right in. Photographic Evidence This is no doubt a much-belabored point with this particular topic, but nonetheless, it is an important argument. The idea is straightforward -- there exists sufficient photographic evidence of a round earth to warrant belief in it. Importantly, most images of the entire Earth from space are computer generated images; manned satellites are too close to the Earth to capture this sort of image. However, orbiting satellites are far enough from the planet to capture large enough sections of the earth to clearly see curvature. 1. NASA - Images 1, 2, 4, 13, 15. [1] 2. ROSCOSMOS (Russian Space Agency) - Several Images [2] 3. CMS (Chinese Space Agency) - Image 1. Unsurprisingly, the majority of images from space are provided by NASA as, regardless of the shape of the planet, it is the most active space agency in history. The common response to this argument is in the vein of "all of those images are fake". This is undoubtedly *possible*, but we will analyze how likely this is. Consider that at least three governments have images of the Earth from space, all of which demonstrate clear curvature. (These were sourced.) The probability that all of these are faked is the probability that the U.S. is lying to us, times the likelihood that the Chinese government is lying to us, times the likelihood that the Russian government is lying to us. The only reasonable scenario in which all of these governments would agree to lie to us is one in which there is an international conspiracy. But this doesn't make sense for the following reasons: (a) There is no benefit to spreading this lie. The governments involved do not spend great amounts of money (relative to the total budget) on these programs, so it clearly isn't some ploy to steal taxpayer dollars. Additionally, the habits of the "average human" would not change greatly if they thought the Earth was one shape or the other, so there is clearly no controlling benefit here. The only explanation would be that these governments are maintaining this conspiracy, simply for fun. And this conclusion is obviously unreasonable -- no government in history has ever wasted millions of dollars to delude its citizens in the name of "lulz". (b) There are political disincentives for this conspiracy. It's easy to imagine how, if Russia were to suddenly publish proof that the Earth was flat (or any non-round shape), it would erode American trust in the government to a significant and impactful degree. In recent months, Russia has made it clear that it's interested in messing with American democratic processes, and what better way to do this than to prove that the government has been lying about this *literally massive* thing for so long. If Russia could do this, it clearly would. As would China. As would the U.S. But none of these countries have done this, leading one to believe that there isn't an international conspiracy concerning the shape of the planet. Unless my opponent can reasonably explain political motivations for keeping this secret, it is difficult to regard the photographic evidence as anything but fact. Gravity I'm particularly attracted to this argument because I believe it's the strongest argument for the spherical shape of the planet. We know the following things to be true: 1. There exists some force which pulls (or pushes) physical objects toward the planet. 2. There exists some force which pushes (or pulls) any object to another. [4] We will call this force gravity. We also know a few more things are true: 1. The earth is at least 3 billion years old. [5] 2. The earth is mostly composed of disconnected solids/liquids. (Dirt, rocks, molten liquids, sheeted/fractured beds.) Putting all of these things together we can reason that the Earth should already have settled into a fairly low energy state, as far as its shape is concerned. This low energy shape is necessarily spheroid. I'll explain. When you have disconnected solids and liquids, they will attempt to settle into a configuration in which the entire system has as little potential energy as possible. Imagine pouring sand into an empty box such that it creates a mound. If then gently shake this box back and forth, the sand will become fairly level. Even if you left the box alone, after a long enough time it would become level, as the particles themselves have a vibration. The sand is now in a state of low potential energy. Now, consider that the entire earth is a sandbox in the vast emptiness of space. Regardless of the initial configuration of the sand, it will still attempt to enter a state of low potential energy. However, there is no longer a downward force on the sand (it's no longer on a planet). Instead, the only strong gravitational force the sand experiences are with the other sand particles. The particles will all be drawn toward the "point of average mass", or the point within the mass of particles which describes the average location of all the particles. This can be more easily seen by considering a 2-dimensional geometry problem. Consider any closed geometric shape and calculate its center of mass. This will be the center of gravitation and all points along the edge of the shape will attempt to minimize their distance to the center of gravitation, while maintaining the area of the shape. For example, picture a square, with the center of mass marked at O (at the center of the circle), a point A on a vertex and a point B directly vertical from the CoM. This shape does a pretty poor job of minimizing the distance of its edges to the O. Point A is about 1.4 times further from O than is point B. Now imagine a circle. This shape is much better at minimizing the distance from the center of mass. Any point A is as close to O as any point B. Thus, we can see that in 2-dimesions, the shape which is in the lowest energy state is a circle. It is trivial to see that this is true for a sphere in 3-dimensions. In summary, gravity, the composition of the planet and the age of the planet all work together to necessitate that the earth is spherical in shape. After 4 billion years, our particle slushy of a planet would have largely settled into a low energy state. There are some interesting responses to this argument which I have heard in the past, but I'm going to respond to them as they come; attempting to preempt all of them would be a waste of characters. Local Changes in Daylight While this isn't my favorite argument for a round earth, it is compelling -- and more importantly, it's easy to test if you have a group of international friends. If it is noon in America and you FaceTime a friend in China, it will be dark there. This immediately disqualifies a flat Earth from being possible. On a flat earth, if it is light in one place, it should be light in all places -- as light does not significantly bend in short distances. (Except in very severe gravitational fields, which don't exist near Earth.) What about other shapes? If we lived on, say, a cubic planet, one still might expect darkness in China during a bright America. However, if this were the case, we would expect large swaths of the planet to suddenly go dark as the entire face rotated out of the sun's rays. We can experimentally verify (with enough people) that the light hitting the earth at any time makes the shape referenced here: [6] Note that the transition from dark to light is smooth at any given point (except near the poles). This implies that the shape of the Earth is smooth, eliminating the possibility of any shapes with straight edges. Finally, note that the amount of lightness and darkness are approximately equal. This eliminates the possibility of a significant ellipse. This leaves us with only one remaining possible shape -- a sphere. Summary To summarize: 1. Photographic evidence sufficiently proves a round earth. There are no reasonable justifications for the international conspiracy required for all the photographic evidence to be fake. 2. Gravity necessitates a spherical Earth. No other shape could physically form other than a sphere. 3. A spherical earth is the only model which adequately explains the day/night cycles experienced on Earth. I look forward to my opponent's response; I know he's a big fan of gravity. Sources: 1. https://www.nasa.gov... 2. http://en.roscosmos.ru... 3. http://en.cmse.gov.cn... 4. http://ffden-2.phys.uaf.edu... 5. http://www.talkorigins.org... 6. http://i209.photobucket.com...