About The Hebrew Calendar

<BODY BGCOLOR="#FFFFFF" LINK="#000000" VLINK="#000000" ALINK="#000000"> <CENTER><TABLE><TR><TD WIDTH=20% VALIGN=TOP BGCOLOR="#151b7e" ROWSPAN=4> <FONT COLOR="#FFFFFF"> <A NAME="top"> <P> <B><a href="http://betmish.org/text/hebrewdate/index.html"><FONT COLOR="#FFFFFF">Text-Only Version</FONT></a></B> <P> <BIG><B><a href="http://betmish.org/index.html"><FONT COLOR="#FFFFFF">Home</FONT></a></B></BIG> <P> <BIG><B><a href="http://betmish.org/whatsnew.html"><FONT COLOR="#FFFFFF">What's New</FONT></A></B></BIG> <P> <BIG><B><a href="http://betmish.org/calendar.html"><FONT COLOR="#FFFFFF">Coming Events</FONT></A></B></BIG> <P> <BIG><B><a href="http://betmish.org/drashot.html"><FONT COLOR="#FFFFFF">D'rashot</FONT></A></B></BIG> <P> <BIG><B><a href="http://betmish.org/do.html"><FONT COLOR="#FFFFFF">What We Do</FONT></A></B></BIG> <P> <BIG><B><a href="http://betmish.org/who.html"><FONT COLOR="#FFFFFF">Who We Are</FONT></A></B></BIG> <P> <BIG><B><a href="http://betmish.org/contact.html"><FONT COLOR="#FFFFFF">Contact Us</FONT></A></B></BIG> <P> <BIG><B><a href="http://betmish.org/archives.html"><FONT COLOR="#FFFFFF">Archives</FONT></A></B></BIG> <P> <BIG><B><a href="http://betmish.org/links.html"><FONT COLOR="#FFFFFF">Links</FONT></A></B></BIG> <P> <BIG><B><a href="http://betmish.org/member.html"><FONT COLOR="#FFFFFF">Membership</FONT></A></B></BIG> <P> <BIG><B><a href="http://betmish.org/credits.html"><FONT COLOR="#FFFFFF">Credits</FONT></A></B></BIG> </FONT></TD> <TD ALIGN=CENTER VALIGN=TOP COLSPAN=2><table BORDER=0 CELLSPACING=0 CELLPADDING=0 WIDTH="80%" bgColor=#ffffff> <tr><td> <CENTER>    <table border="0" align="center"> <tr> <td align="center"> <a href="http://betmish.org/index.html" target="_top"><IMG border=0 src="http://betmish.org/images/banner2.gif" align=center height="104" width="512" hspace=5 alt="Bet Mishpachah banner"></a> </td> <td align="left"> <font size="-5"><a href="http://betmish.org/child/descriptions/banner.html">Banner<br>Description</a></font> </td> </tr> <tr> <td align="center"> <p><font size="-1"><i>Washington, DC's Egalitarian Synagogue</i><br> <i>Embracing a Diversity of Sexual and Gender Identities</i></font> </p> <p> <b><font face="Arial"><font size="+1">Rabbi Bob Saks<br> </font></font></b><br> <font face="Arial"><font size="-1"><i>Founded in 1975 by Members of the GLBT Community<font color="white" size="-4"> </font></i></font></font></p> </td> <td align="left"> <font size="-4" color="white"> </font> </td> </tr> </table> </center> <center> <H1>About the Hebrew Calendar</H1> </center> <p> The civil (or Julian) calendar is based on the cycle of the Earth around the sun. The length of this cycle, the solar year, is very close to 365 1/4 days. <p> The Hebrew calendar on the other hand is based on the cycle of the moon around the Earth. The length of this cycle, the lunar month, is about 29 1/2 days. Twelve lunar months make therefore about 354 days, which is 11 1/4 days shorter than the solar year. <p> In biblical times, the arrival of the new month was determined by watching the phase of the moon. However in modern times a fixed calendar is used in which the length of the months alternates between 29 and 30 days. Here are the names of the months in the Hebrew calendar: <p> <table width=100% border=0> <tr><td valign=top>  <table border=0> <tr> <td><font size=+1>1.</font></td><td><font size=+1>Tishrei</font></td> <td align=right><img src="http://betmish.org/images/tishrei.gif" width=84 height=38 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>2.</font></td><td><font size=+1>Cheshvan</font></td> <td align=right><img src="http://betmish.org/images/cheshvan.gif" width=77 height=38 hspace=20></td> <td><font size=+1>29/30 days</font></td> </tr><tr> <td><font size=+1>3.</font></td><td><font size=+1>Kislev</font></td> <td align=right><img src="http://betmish.org/images/kislev.gif" width=78 height=40 hspace=20></td> <td><font size=+1>29/30 days</font></td> </tr><tr> <td><font size=+1>4.</font></td><td><font size=+1>Tevet</font></td> <td align=right><img src="http://betmish.org/images/tevet.gif" width=75 height=39 hspace=20></td> <td><font size=+1>29 days</font></td> </tr><tr> <td><font size=+1>5.</font></td><td><font size=+1>Shvat</font></td> <td align=right><img src="http://betmish.org/images/shvat.gif" width=76 height=40 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>6.</font></td><td><font size=+1>Adar</font></td> <td align=right><img src="http://betmish.org/images/adar.gif" width=65 height=39 hspace=20></td> <td><font size=+1>29 days</font></td> </tr> </table> </td><td valign=top>  <table border=0> <tr> <td><font size=+1>7.</font></td><td><font size=+1>Nisan</font></td> <td align=right><img src="http://betmish.org/images/nisan.gif" width=63 height=40 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>8.</font></td><td><font size=+1>Iyyar</font></td> <td align=right><img src="http://betmish.org/images/iyyar.gif" width=65 height=35 hspace=20></td> <td><font size=+1>29 days</font></td> </tr><tr> <td><font size=+1>9.</font></td><td><font size=+1>Sivan</font></td> <td align=right><img src="http://betmish.org/images/sivan.gif" width=57 height=38 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>10.</font></td><td><font size=+1>Tamuz</font></td> <td align=right><img src="http://betmish.org/images/tamuz.gif" width=78 height=40 hspace=20></td> <td><font size=+1>29 days</font></td> </tr><tr> <td><font size=+1>11.</font></td><td><font size=+1>Av</font></td> <td align=right><img src="http://betmish.org/images/av.gif" width=47 height=39 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>12.</font></td><td><font size=+1>Elul</font></td> <td align=right><img src="http://betmish.org/images/elul.gif" width=77 height=39 hspace=20></td> <td><font size=+1>29 days</font></td> </tr> </table> </td></tr> </table> <p> The difference of 11 1/4 days between 12 lunar months and one solar year accumulates in three years to more than a month. If no adjustments are made, a summer month like <i>Av</i> or <i>Elul</i> could shift to the winter. <p> Because the Jewish holidays are closely related to the seasons (for example, the Torah commands to celebrate <i>Pesach</i> in the spring), an adjustment to the calendar must be made every few years. Every two or three years one extra month is added to a year. Such a year is called a <i>leap year</i> and it has two months of <i>Adar</i>. <p> Here are the months in a leap year: <p> <table width=100% border=0> <tr><td valign=top>  <table border=0> <tr> <td><font size=+1>1.</font></td><td><font size=+1>Tishrei</font></td> <td align=right><img src="http://betmish.org/images/tishrei.gif" width=84 height=38 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>2.</font></td><td><font size=+1>Cheshvan</font></td> <td align=right><img src="http://betmish.org/images/cheshvan.gif" width=77 height=38 hspace=20></td> <td><font size=+1>29/30 days</font></td> </tr><tr> <td><font size=+1>3.</font></td><td><font size=+1>Kislev</font></td> <td align=right><img src="http://betmish.org/images/kislev.gif" width=78 height=40 hspace=20></td> <td><font size=+1>29/30 days</font></td> </tr><tr> <td><font size=+1>4.</font></td><td><font size=+1>Tevet</font></td> <td align=right><img src="http://betmish.org/images/tevet.gif" width=75 height=39 hspace=20></td> <td><font size=+1>29 days</font></td> </tr><tr> <td><font size=+1>5.</font></td><td><font size=+1>Shvat</font></td> <td align=right><img src="http://betmish.org/images/shvat.gif" width=76 height=40 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>6.</font></td><td><font size=+1>Adar 1</font></td> <td align=right><img src="http://betmish.org/images/adar1.gif" width=103 height=39 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>7.</font></td><td><font size=+1>Adar 2</font></td> <td align=right><img src="http://betmish.org/images/adar2.gif" width=100 height=39 hspace=20></td> <td><font size=+1>29 days</font></td> </tr> </table> </td><td valign=top>  <table border=0> <tr> <td><font size=+1>8.</font></td><td><font size=+1>Nisan</font></td> <td align=right><img src="http://betmish.org/images/nisan.gif" width=63 height=40 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>9.</font></td><td><font size=+1>Iyyar</font></td> <td align=right><img src="http://betmish.org/images/iyyar.gif" width=65 height=35 hspace=20></td> <td><font size=+1>29 days</font></td> </tr><tr> <td><font size=+1>10.</font></td><td><font size=+1>Sivan</font></td> <td align=right><img src="http://betmish.org/images/sivan.gif" width=57 height=38 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>11.</font></td><td><font size=+1>Tamuz</font></td> <td align=right><img src="http://betmish.org/images/tamuz.gif" width=78 height=40 hspace=20></td> <td><font size=+1>29 days</font></td> </tr><tr> <td><font size=+1>12.</font></td><td><font size=+1>Av</font></td> <td align=right><img src="http://betmish.org/images/av.gif" width=47 height=39 hspace=20></td> <td><font size=+1>30 days</font></td> </tr><tr> <td><font size=+1>13.</font></td><td><font size=+1>Elul</font></td> <td align=right><img src="http://betmish.org/images/elul.gif" width=77 height=39 hspace=20></td> <td><font size=+1>29 days</font></td> </tr> </table> </td></tr> </table> <p> Holidays that occur in the month of <i>Adar</i> (such as <i>Purim</i>) are celebrated in <i>Adar 2</i> in a leap year. The same rule is applied to birthdays, anniversaries and other personal events. <p> The following 19 year cycle determines when a year is a leap year: <ol> <li>Regular. <li>Regular. <li>Leap. <li>Regular. <li>Regular. <li>Leap. <li>Regular. <li>Leap. <li>Regular. <li>Regular. <li>Leap. <li>Regular. <li>Regular. <li>Leap. <li>Regular. <li>Regular. <li>Leap. <li>Regular. <li>Leap. </ul> In addition to the leap year cycle, the length of each year is slightly adjusted to meet a number of constraints called <i>Dechiot</i>. These small adjustments are made by selecting the length of the two months of <i>Cheshvan</i> and <i>Kislev</i> to be 29 or 30 days. There are four possible combinations, but only three are actually used: <p> <table border=2 cellpadding=8 CELLSPACING = 3> <tr> <td align=center><b>year kind</b></td> <td align=center><b>Cheshvan</b></td> <td align=center><b>Kislev</b></td> <td align=center><b>length of regular year</b></td> </tr><tr> <td align=center><i>chasera</i><br>("incomplete")</td> <td align=center>29 days</td> <td align=center>29 days</td> <td align=center>353</td> </tr><tr> <td align=center><i>kesidra</i><br>("in order")</td> <td align=center>29 days</td> <td align=center>30 days</td> <td align=center>354</td> </tr><tr> <td align=center><i>----</i></td> <td align=center>30 days</td> <td align=center>29 days</td> <td align=center><i>----</i></td> </tr><tr> <td align=center><i>shleima</i><br>("complete")</td> <td align=center>30 days</td> <td align=center>30 days</td> <td align=center>355</td> </tr> </table> <p> The four constraints (<i>Dechiot</i>) that determine the exact year length have to do with the exact timing of the holidays in relation to the phase of the moon and with relations to the day of the week. <p> <b><u><i>Dechia</i> 1 - <i>Molad Zaken</i></u></b> <p> The moon goes in orbit around the Earth. Every month, there is one instance in which the moon is exactly between the Earth and the sun and the Earth faces the dark side of the moon. This instance is called <i>Molad</i> ("birth" of a new moon) and it marks the beginning of a new month. The <i>Molad</i> of the first month of the year, <i>Tishrei</i>, marks the Jewish New Year or <i>Rosh HaShana</i>. <p> Since the Earth is facing the dark side of the moon, the moon becomes visible later that day or the next day. <p> In a year when the <i>Molad</i> of <i>Tishrei</i> occurs after 12:00 noon, <i>Rosh HaShana</i> is postponed until the next day because the moon would not become visible until the next day. This is done by adding one day to the previous year. <p> <b><u><i>Dechia</i> 2 - Sunday, Wednsday, Friday</u></b> <p> If the <i>Molad</i> of <i>Tishrei</i> falls on Sunday, Wednesday or Friday, <i>Rosh HaShana</i> is postponed by one day to Monday, Thursday or Saturday, respectively. <p> The reason is that if <i>Rosh HaShana</i> is on Wednesday or Friday, then <i>Yom Kippur</i> would occur on Friday or Sunday. That would make <i>Yom Kippur</i> adjacent to <i>Shabbat</i> (Saturday) and there would be two consecutive days in which it is forbidden to do any kind of work including the preparation of food. <p> If <i>Rosh HaShana</i> is on Sunday, <i>Hoshana Raba</i> would fall on Saturday and that would prevent the custom of 7 <i>Hakafot</i>. <p> <b><u><i>Dechia</i> 3 - <i>Molad</i> of Regular Year on Tuesday</u></b> <p> If the <i>Molad</i> of <i>Tishrei</i> of a regular year with 12 months occurs on Tuesday morning, <i>Rosh HaShana</i> would occur on Tuesday. However, this would cause a problem with <i>Rosh HaShana</i> of the following year. <p> To see why, remember that the length of a regular year can be 353, 354 or 355 days. If <i>Rosh HaShana</i> occurs on Tuesday, we can determine the day of <i>Rosh HaShana</i> of the following year by adding (<i>days-in-year modulu 7</i>) days to Tueday (<i>number modulu 7</i> is the remainder resulting from dividing the number by 7). <p> <table border=2 cellpadding=8 CELLSPACING = 3> <tr> <td align=center><b>year kind</b></td> <td align=center><b>days in year</b></td> <td align=center><b>modulu 7</b></td> <td align=center><b>next Rosh HaShana</b></td> </tr><tr> <td align=center><i>chasera</i><br>("incomplete")</td> <td align=center>353</td> <td align=center>3</td> <td align=left>Tuesday + 3 = Friday</td> </tr><tr> <td align=center><i>kesidra</i><br>("in order")</td> <td align=center>354</td> <td align=center>4</td> <td align=left>Tuesday + 4 = Saturday</td> </tr><tr> <td align=center><i>shleima</i><br>("complete")</td> <td align=center>355</td> <td align=center>5</td> <td align=left>Tuesday + 5 = Sunday</td> </tr> </table> <p> We can see from this table that if this year has 353 or 355 days, the next <i>Rosh HaShana</i> falls on Friday or Sunday, which contradicts constraint number 2. <p> Therefore this year must have 354 days and the next <i>Rosh HaShana</i> will fall on Saturday. However, the accurate length of a lunar month is 29 days, 12 hours, 44 minutes and 3 1/3 seconds. The accurate length of a lunar year (12 lunar months) is therefore 354 days, 8 hours, 48 minutes and 40 seconds. This means that if the <i>Molad</i> of this year occurs on Tuesday 6 AM, the <i>Molad</i> of the following year will occur on Saturday 2:48:40 PM, and <i>Rosh HaShana</i> will have to be postponed to Sunday according to constraint number 1 and then postponed again to Monday according to constraint number 2. In order to do that, this year will have to be 356 days long, which is not possible. <p> The conclusion of the above logic is constraint number 3 which states that if a <i>Molad</i> of a regular year occurs after Tuesday 3:22 AM, <i>Rosh HaShana</i> is postponed to Thursday. <p> <b><u><i>Dechia</i> 4 - <i>Molad</i> of Leap Year on Thursday</u></b> <p> This is a similar situation to the one described in contraint number 4. Here the rule is that if a <i>Molad</i> of a leap year occurs after Thursday 12:00 noon, the next <i>Rosh HaShana</i> is postponed from Monday to Tuesday. <p> <hr> All the 4 constraints (<i>Dechiot</i>) can be pre-calculated and represented in tables of short cycles. Hebrew calendar software programs use relatively simple table lookup to determine the exact length of a given year and the date of <i>Rosh HaShana</i>. All the other dates and holidays are calculated by counting up from <i>Rosh HaShana</i>. <p> <a href="today.html" target=_top> Today's Hebrew Date</a> is a JavaScript implementation of a Hebrew calendar program. <p> <font size=-1> Reference: <i>Understanding the Jewish Calendar</i> by Rabbi Nathan Bushwick. Moznaim Publishing Corporation, 1989. </font> <P> <hr> <p> <center> <a href="about.html">About the Hebrew Calendar</a> | <a href="ashkseph.html">Ashkenazic and Sephardic</a> <BR><a href="candle.html">Candle-Lighting Times</a> | <a href="alefbet.html">Hebrew Alphabet</a> | <a href="numvalues.html">Numerical Values</a> </center> <CENTER><P>About the Hebrew Calendar and Numerical Values are courtesy of <a href="http://www.rtlsoft.com" target="_top">Right to Left Software</a>. <P>Hebrew Alphabet and Ashkenazic and Sephardic Jews are courtesy of <a href="http://www.jewfaq.org" target="_top">Judaism 101</a></CENTER> <P><HR> <p align="center"><strong><FONT face=Arial><FONT color=#000000><small><small>For a description of any of the graphics on this page, click on the D next to the graphic. <BR>For more information, contact our office at <a href="mailto:office@betmishpachah.org?SUBJECT=Request for Information from Web Site">office@betmishpachah.org</a><br> Send comments about this site to <a href="mailto:webweaver@betmishpachah.org?SUBJECT=Comments About Web Site">webweaver@betmishpachah.org</a>.<br> Copyright 1996 - 2008 Bet Mishpachah. </small></small></STRONG></FONT></FONT><br> <CENTER><FONT face=Arial><font color="#999999"><strong><small><small><P>This page last modified on Sunday, May 18, 2003 at 11:35 PM EDT.</small></small></STRONG></FONT></FONT> </CENTER>