<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>