<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>