地球为什么是圆的?
<p>地球是太阳系从内到外的第三颗行星，也是太阳系中直径、质量和密度最大的类地行星。它也经常被称作世界。英语的地球<span word="Earth">Earth</span>一词来自于古英语及日耳曼语。地球已有44～46亿岁，有一颗天然卫星月球围绕着地球以27.32天的周期旋转，而地球以近24小时的周期自转并且以一年的周期绕太阳公转。迪士尼有同名纪录片。</p><p><span word="Why">Why</span> <span word="the">the</span> <span word="Earth">Earth</span> <span word="Is">Is</span> <span word="Round">Round</span></p><p>地球为什么是圆的？</p><p><span word="Our">Our</span> <span word="never">never</span>-<span word="thought">thought</span> <span word="to">to</span> <span word="ask">ask</span> <span word="question">question</span> <span word="for">for</span> <span word="today">today</span> <span word="is">is</span>: <span word="Why">Why</span> <span word="is">is</span> <span word="the">the</span> <span word="earth">earth</span> <span word="round">round</span>? <span word="Why">Why</span> <span word="not">not</span> <span word="a">a</span> <span word="cube">cube</span>? <span word="A">A</span> <span word="cube">cube</span>? <span word="No">No</span>,<span word="that">that</span>'<span word="s">s</span> <span word="silly">silly</span>. <span word="Is">Is</span> <span word="it">it</span>? <span word="Why">Why</span>? <span word="Because">Because</span>…,<span word="because">because</span> <span word="really">really</span> <span word="big">big</span> <span word="things">things</span> <span word="like">like</span> <span word="to">to</span> <span word="be">be</span> <span word="round">round</span> <span word="for">for</span> <span word="some">some</span> <span word="reason">reason</span>. <span word="Exactly">Exactly</span> <span word="right">right</span>.<span word="And">And</span> <span word="that">that</span> <span word="reason">reason</span> <span word="is">is</span> <span word="gravity">gravity</span>.</p><p>今天，我们“从未想过的问题”是：地球为什么是圆的？为什么不是方形的？方形？那太可笑了。是吗？为什么？因为……因为非常大的东西通常都是圆的。完全正确。这个原因就是重力。</p><p><span word="Think">Think</span> <span word="of">of</span> <span word="it">it</span> <span word="this">this</span> <span word="way">way</span>. <span word="Suppose">Suppose</span> <span word="the">the</span> <span word="earth">earth</span> <span word="were">were</span> <span word="a">a</span> <span word="cube">cube</span>. <span word="Then">Then</span> <span word="some">some</span> <span word="points">points</span> <span word="on">on</span> <span word="its">its</span> <span word="surface">surface</span> <span word="would">would</span> <span word="be">be</span> <span word="farther">farther</span> <span word="away">away</span> <span word="from">from</span> <span word="the">the</span> <span word="center">center</span> <span word="than">than</span> <span word="others">others</span>. <span word="The">The</span> <span word="corners">corners</span>, <span word="for">for</span> <span word="example">example</span>, <span word="would">would</span> <span word="be">be</span> <span word="much">much</span> <span word="farther">farther</span> <span word="out">out</span> <span word="than">than</span> <span word="the">the</span> <span word="rest">rest</span>.</p><p>我们这样想，假设地球是一个立方体。而地球表面的某些端点离中间或其它端点比较远。比如，立方体的各个角离得很远。</p><p><span word="Since">Since</span> <span word="gravity">gravity</span> <span word="works">works</span> <span word="equally">equally</span> <span word="on">on</span> <span word="everything">everything</span>, <span word="there">there</span> <span word="would">would</span> <span word="be">be</span> <span word="a">a</span> <span word="tendency">tendency</span> <span word="to">to</span> <span word="pull">pull</span> <span word="those">those</span> <span word="uneven">uneven</span> <span word="places">places</span> <span word="down">down</span>. <span word="The">The</span> <span word="surface">surface</span> <span word="would">would</span> <span word="even">even</span> <span word="itself">itself</span> <span word="out">out</span>. <span word="The">The</span> <span word="only">only</span> <span word="geometrical">geometrical</span> <span word="object">object</span> <span word="in">in</span> <span word="which">which</span> <span word="every">every</span> <span word="point">point</span> <span word="on">on</span> <span word="the">the</span> <span word="surface">surface</span> <span word="is">is</span> <span word="the">the</span> <span word="same">same</span> <span word="distance">distance</span> <span word="from">from</span> <span word="the">the</span> <span word="center">center</span> <span word="is">is</span> <span word="a">a</span> <span word="sphere">sphere</span>.</p>
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