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+<sect1 id="ai-julianday">
+<sect1info>
+<author>
+<firstname>John</firstname>
+<surname>Cirillo</surname>
+</author>
+</sect1info>
+<title>Julian Day</title>
+<indexterm><primary>Julian Day</primary>
+</indexterm>
+<para>
+Julian Days are a way of reckoning the current date by a simple count of
+the number of days that have passed since some remote, arbitrary date.  This
+number of days is called the <firstterm>Julian Day</firstterm>,
+abbreviated as <abbrev>JD</abbrev>.  The starting point, <abbrev>JD=0</abbrev>,
+is January 1, 4713 BC (or -4712 January 1, since there was no year '0').  Julian
+Days are very useful because they make it easy to determine the number of days
+between two events by simply subtracting their Julian Day numbers.
+Such a calculation is difficult for the standard (Gregorian) calendar, because
+days are grouped into months, which contain a variable number of days, and
+there is the added complication of <link linkend="ai-leapyear">Leap
+Years</link>.
+</para><para>
+Converting from the standard (Gregorian) calendar to Julian Days and vice versa
+is best left to a special program written to do this, such as the &kstars;
+<link linkend="tool-calculator">Astrocalculator</link>.  However, for those
+interested, here is a simple example of a Gregorian to Julian day converter:
+</para><para>
+<abbrev>JD</abbrev> = <abbrev>D</abbrev> - 32075 + 1461*( <abbrev>Y</abbrev> +
+4800 + ( <abbrev>M</abbrev> - 14 ) / 12 ) / 4 + 367*( <abbrev>M</abbrev> - 2 -
+( <abbrev>M</abbrev> - 14 ) / 12 * 12 ) / 12 - 3*( ( <abbrev>Y</abbrev> + 4900 +
+( <abbrev>M</abbrev> - 14 ) / 12 ) / 100 ) / 4
+</para><para>
+where <abbrev>D</abbrev> is the day (1-31), <abbrev>M</abbrev> is the Month
+(1-12), and <abbrev>Y</abbrev> is the year (1801-2099).  Note that this formula
+only works for dates between 1801 and 2099.  More remote dates require a more
+complicated transformation.
+</para><para>
+An example Julian Day is: <abbrev>JD</abbrev> 2440588, which corresponds to
+1 Jan, 1970.
+</para><para>
+Julian Days can also be used to tell time; the time of day is expressed as a
+fraction of a full day, with 12:00 noon (not midnight) as the zero point.  So,
+3:00 pm on 1 Jan 1970 is <abbrev>JD</abbrev> 2440588.125 (since 3:00 pm is 3
+hours since noon, and 3/24 = 0.125 day).  Note that the Julian Day is always
+determined from <link linkend="ai-utime">Universal Time</link>, not Local Time.
+</para><para>
+Astronomers use certain Julian Day values as important reference points, called
+<firstterm>Epochs</firstterm>.  One widely-used epoch is called J2000; it is the
+Julian Day for 1 Jan, 2000 at 12:00 noon = <abbrev>JD</abbrev> 2451545.0.
+</para><para>
+Much more information on Julian Days is available on the internet.  A good
+starting point is the <ulink
+url="http://aa.usno.navy.mil/faq/docs/JD_Formula.html">U.S. Naval
+Observatory</ulink>.  If that site is not available when you read this, try
+searching for <quote>Julian Day</quote> with your favorite search engine.
+</para>
+</sect1>