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137 lines
6.2 KiB
HTML
137 lines
6.2 KiB
HTML
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<!-- Mirrored from www.mathsisfun.com/leap-years.html by HTTrack Website Copier/3.x [XR&CO'2014], Sat, 29 Oct 2022 00:57:43 GMT -->
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<meta http-equiv="content-type" content="text/html; charset=utf-8">
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<title>Leap Years</title>
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<article id="content" role="main"><!-- #BeginEditable "Body" -->
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<h1 align="center">Leap Years</h1>
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<p class="center"> <i> ..., 2008, 2012, 2016, <b>2020</b>, 2024, 2028, ... etc</i></p>
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<h2>Leap Year</h2>
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<ul>
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<div class="bigul">
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<li>A normal year has 365 days.
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<br /> </li>
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<li>A <b>Leap Year</b> has 366 days (the extra day is the 29th of February). </li>
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</div>
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</ul>
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<h2>How to know if it is a Leap Year:</h2>
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<table width="100%" border="0">
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<tr>
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<td><img src="images/style/yes.svg" height="46" alt="yes" /></td>
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<td colspan="3">Leap Years are any year that can be <b>exactly divided by 4</b> (such as 2016, 2020, 2024, etc)</td>
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</tr>
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<tr>
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<td> </td>
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<td><img src="images/style/no.svg" height="46" alt="not" /></td>
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<td colspan="2">except if it can be <b>exactly divided by 100</b>, then it <b>isn't</b> (such as 2100, 2200, etc)</td>
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</tr>
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<tr>
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<td> </td>
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<td> </td>
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<td><img src="images/style/yes.svg" height="46" alt="yes" /></td>
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<td><i><b>except if</b></i> it can be <b>exactly divided by 400</b>, then it <b>is</b> (such as 2000, 2400)</td>
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</tr>
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</table>
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<p>Try it here:</p>
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<script src="measure/images/leap-year.js" type="text/javascript"></script>
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<script type="text/javascript">leapyearMain();</script>
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<h2>Why?</h2>
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<div class="center80">
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<p align="center">Because the Earth rotates about <b>365.242375</b> times a year ...</p>
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<p align="center">... but a normal year is <b>365</b> days, ... </p>
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<p align="center">... so something has to be done to "catch up" the extra <b>0.242375</b> days a year. </p>
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</div>
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<p> </p>
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<p class="so">So every 4th year we add an extra day (the 29th of February), which makes <b>365.25</b> days a year. This is fairly close, but is wrong by about 1 day every 100 years. </p>
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<ul> </ul>
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<p class="so">So every 100 years we <b>don't</b> have a leap year, and that gets us <b>365.24</b> days per year (1 day less in 100 year = -0.01 days per year). Closer, but still not accurate enough!</p>
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<p class="so">So another rule says that every 400 years <b>is</b> a leap year again. This gets us <b>365.2425</b> days per year (1 day regained every 400 years = 0.0025 days per year), which is close enough to 365.242375 not to matter much.</p>
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<h2>So, Which Are and Which Aren't?</h2>
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<p>So 2000 and 2400 <b>are </b> leap years but 1800, 1900, 2100, 2200 and 2300 <b>are not</b>.</p>
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<p>Apart from that, <b>every year divisible by 4</b> (2012, 2016, 2020, 2024, etc.) is a leap year.</p>
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<div class="simple"></div>
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<p class="center"><span class="large">How far away each year is from the average</span>
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<br /> <img src="images/leap-year-graph.gif" width="601" height="328" alt="leap year graph" /> </p>
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<p><i>Example: look just before 2100, the worst year is 1.2 days ahead, but because 2100 is <b>not</b> a leap year they all get adjusted back by 1 day.</i></p>
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<p>So this keeps us pretty close, and any other adjustments can be done way in the future (when the Earth may be rotating a little slower, anyway!)</p>
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<p> </p>
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<h2>Only since 1582 (Gregorian Calendar)</h2>
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<p>These leap year rules were introduced in <b>1582</b> by the <b>Gregorian Calendar</b>, named after Pope Gregory XIII.</p>
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<p>(It replaced the old <b>Julian Calendar</b> by Julius Caesar that only has one rule of a leap year every 4th year, and is now about 13 days behind our current date.)</p>
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<p> </p>
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<div class="questions">
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<script type="text/javascript">
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getQ(3654, 1976, 3683, 3684, 3685, 1025, 1026, 2581, 1975, 3675);
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</script> </div>
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<div class="related"> <a href="earth-orbit.html">Earth's Orbit and Daylight</a> <a href="time-zones-world.html">World Time Zones</a> <a href="sun-clock.html">Sun Clock</a> <a href="time.html">Time: AM/PM and 24 Hour Clock</a> <a href="time-add-subtract.html">Adding and Subtracting Time</a> </div>
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Copyright © 2019 MathsIsFun.com
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