为什么会有四年一次的闰日？
<p>2020年是一个闰年，2月29日这个闰日即将登场。为什么会有闰年和闰日呢？其实，这是一个历史遗留问题。而且，闰日也不是“四年等一回”那么简单。</p><p><span word="At">At</span> <span word="some">some</span> <span word="point">point</span> <span word="in">in</span> <span word="elementary">elementary</span> <span word="school">school</span>, <span word="your">your</span> <span word="science">science</span> <span word="teacher">teacher</span> <span word="probably">probably</span> <span word="explained">explained</span> <span word="to">to</span> <span word="you">you</span> <span word="that">that</span> <span word="there">there</span> <span word="are">are</span> 365 <span word="days">days</span> <span word="in">in</span> <span word="a">a</span> <span word="year">year</span> <span word="because">because</span> <span word="that">that</span>’<span word="s">s</span> <span word="how">how</span> <span word="long">long</span> <span word="it">it</span> <span word="takes">takes</span> <span word="for">for</span> <span word="Earth">Earth</span> <span word="to">to</span> <span word="complete">complete</span> <span word="one">one</span> <span word="full">full</span> <span word="rotation">rotation</span> <span word="around">around</span> <span word="the">the</span> <span word="sun">sun</span>. <span word="What">What</span> <span word="they">they</span> <span word="might">might</span> <span word="not">not</span> <span word="have">have</span> <span word="specified">specified</span>, <span word="however">however</span>, <span word="is">is</span> <span word="that">that</span> <span word="it">it</span>’<span word="s">s</span> <span word="not">not</span> <span word="exactly">exactly</span> 365 <span word="days">days</span>—<span word="it">it</span>’<span word="s">s</span> <span word="actually">actually</span> <span word="closer">closer</span> <span word="to">to</span> 365.2421 <span word="days">days</span>.</p><p>上小学时，你们的科学老师很可能会向你们解释一年有365天，因为这是地球绕太阳公转一圈的时间。不过，他们可能没有详细说明，地球公转一圈并不是精确的365天，实际上说是365.2421天更准确一些。</p>
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