<sect2 id="calc-julian">
<title>Julian Day module</title>
<indexterm><primary>Tools</primary>
<secondary>Astrocalculator</secondary>
<tertiary>Julian Day module</tertiary>
</indexterm>

<screenshot>
<screeninfo>The Julian Day calculator module </screeninfo>
<mediaobject>
  <imageobject>
    <imagedata fileref="calc-julian.png" format="PNG"/>
  </imageobject>
  <textobject>
    <phrase>Julian Day</phrase>
  </textobject>
</mediaobject>
</screenshot>

<para>This module converts between the calendar date, the <link linkend="ai-julianday">Julian Day</link> and the <firstterm>Modified Julian Day</firstterm>. The Modified Julian Day is simply equal to the Julian Day - 2,400,000.5. </para><para>To use the module, select which of the three dates will be the input, and then fill in its value. Then press the <guibutton>Compute</guibutton> button, and the corresponding values for the other two date systems will be displayed. </para>

<tip>
<para>Exercise:</para>
<para>What calendar date does MJD = 0.0 correspond to? </para>
</tip>

</sect2>
