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<br>   <table border="0" cellspacing="0" cellpadding="0"> <tr> <td bgcolor="#339999" valign="top" width="2"><img src="/images/tealtab_topleft1.gif" alt="" border="0"><br></td> <td bgcolor="#339999" valign="top" width="18"><img src="/images/tealtab_topleft2.gif" alt="" border="0"><br></td> <td bgcolor="#339999" align="left" valign="middle" height="20"><font color="#ffffff" size="+1" face="Helvetica"><b>Day</b></font></td> <td bgcolor="#339999" valign="top" align="left" width="14"><img src="/images/tealtab_topright.gif" alt="" border="0"><br></td> </tr> </table>   <table border="0" cellspacing="0" cellpadding="0"> <tr valign="top"> <td colspan="6"><img src="/images/gradient-teal.gif" alt="" height="2"></td> </tr> <tr> <td bgcolor="#339999"><img src="/images/spacer.gif" alt="" width="2" height="10"></td> </tr> <tr> <td bgcolor="#339999"><img src="/images/spacer.gif" alt="" width="2"></td> <td colspan="3">    </td> <td> <P> The day is (roughly) defined as the time required for the <a href="Earth.html">Earth</a> to complete a rotation. The length of the day is not a constant over long timescales (i.e., millions of years), but changes over the course of time as the Earth's rotation slows due to the <a href="http://scienceworld.wolfram.com/physics/TidalTorque.html" target="_blank">tidal torque</a> <img src="http://scienceworld.wolfram.com/images/crossrefs/physics.gif" width="12" height="12" alt="Eric Weisstein's World of Physics"> from the <a href="Moon.html">Moon</a>. The current deceleration is <IMG WIDTH="91" HEIGHT="33" ALIGN="MIDDLE" BORDER="0" SRC="dimg1.gif"> radians per second squared, <P> In civil calendars, the length of individual calendar days may be adjusted as a result of <a href="DaylightSavingTime.html">daylight saving time</a> (by adding or subtracting one hour out of the usual 24) or addition of a <a href="LeapSecond.html">leap second</a>. <P> <DIV ALIGN="CENTER"> <IMG WIDTH="410" HEIGHT="247" BORDER="0" SRC="dimg2.gif"> </DIV> <P> The plot above shows the duration of daylight (as defined by the interval between <a href="Sunrise.html">sunrise</a> and <a href="Sunset.html">sunset</a>) for Chicago in the year 1999. The length varies from a minimum of 9:20 on the <a href="WinterSolstice.html">winter solstice</a> of December 22, 1999 to a maximum of 15:02 on the <a href="SummerSolstice.html">summer solstice</a> of June 21, 1999. <P> <P> <IMG WIDTH="52" HEIGHT="13" ALIGN="LEFT" SRC="/images/entries/see_also.gif"><a href="DayoftheWeek.html">Day of the Week</a>, <a href="DaylightSavingTime.html">Daylight Saving Time</a>, <a href="Hour.html">Hour</a>, <a href="LeapSecond.html">Leap Second</a>, <a href="Month.html">Month</a>, <a href="SiderealDay.html">Sidereal Day</a>, <a href="SolarDay.html">Solar Day</a>, <a href="SummerSolstice.html">Summer Solstice</a>, <a href="Sunrise.html">Sunrise</a>, <a href="Sunset.html">Sunset</a>, <a href="WinterSolstice.html">Winter Solstice</a> <P> <P> <br clear=all> <img src="/images/entries/underline.gif" width="300" height="3" alt=""><br><br> <img src="/images/entries/references.gif" width="67" height="13" alt="References"> <div class="references"> <P> <P> Hollon, B. "An Introduction to Calendars." <TT><A NAME="tex2html2" HREF="http://www.12x30.net/intro.html">http://www.12x30.net/intro.html</A></TT>. <P> <P> </div> <P> <P> <p> <BR> <br> <div class="copyright"> <i>© 1996-2007 Eric W. Weisstein</i> <p> </div> </td> </tr> <tr> <td colspan="6"><img src="/images/gradient-teal.gif" alt="" height="2"></td> </tr> </table> </body> </html>