kstars/doc/julianday.docbook

0001 <sect1 id="ai-julianday">
0002 <sect1info>
0003 <author>
0004 <firstname>John</firstname>
0005 <surname>Cirillo</surname>
0006 </author>
0007 </sect1info>
0008 <title>Julian Day</title>
0009 <indexterm><primary>Julian Day</primary>
0010 </indexterm>
0011 <para>
0012 Julian Days are a way of reckoning the current date by a simple count of
0013 the number of days that have passed since some remote, arbitrary date.  This
0014 number of days is called the <firstterm>Julian Day</firstterm>,
0015 abbreviated as <abbrev>JD</abbrev>.  The starting point, <abbrev>JD=0</abbrev>,
0016 is January 1, 4713 BC (or -4712 January 1, since there was no year '0').  Julian
0017 Days are very useful because they make it easy to determine the number of days
0018 between two events by simply subtracting their Julian Day numbers.
0019 Such a calculation is difficult for the standard (Gregorian) calendar, because
0020 days are grouped into months, which contain a variable number of days, and
0021 there is the added complication of <link linkend="ai-leapyear">Leap
0022 Years</link>.
0023 </para><para>
0024 Converting from the standard (Gregorian) calendar to Julian Days and vice versa
0025 is best left to a special program written to do this, such as the &kstars;
0026 <link linkend="tool-calculator">Astrocalculator</link>.  However, for those
0027 interested, here is a simple example of a Gregorian to Julian day converter:
0028 </para><para>
0029 <abbrev>JD</abbrev> = <abbrev>D</abbrev> - 32075 + 1461*( <abbrev>Y</abbrev> +
0030 4800 + ( <abbrev>M</abbrev> - 14 ) / 12 ) / 4 + 367*( <abbrev>M</abbrev> - 2 -
0031 ( <abbrev>M</abbrev> - 14 ) / 12 * 12 ) / 12 - 3*( ( <abbrev>Y</abbrev> + 4900 +
0032 ( <abbrev>M</abbrev> - 14 ) / 12 ) / 100 ) / 4
0033 </para><para>
0034 where <abbrev>D</abbrev> is the day (1-31), <abbrev>M</abbrev> is the Month
0035 (1-12), and <abbrev>Y</abbrev> is the year (1801-2099).  Note that this formula
0036 only works for dates between 1801 and 2099.  More remote dates require a more
0037 complicated transformation.
0038 </para><para>
0039 An example Julian Day is: <abbrev>JD</abbrev> 2440588, which corresponds to
0040 1 Jan, 1970.
0041 </para><para>
0042 Julian Days can also be used to tell time; the time of day is expressed as a
0043 fraction of a full day, with 12:00 noon (not midnight) as the zero point.  So,
0044 3:00 pm on 1 Jan 1970 is <abbrev>JD</abbrev> 2440588.125 (since 3:00 pm is 3
0045 hours since noon, and 3/24 = 0.125 day).  Note that the Julian Day is always
0046 determined from <link linkend="ai-utime">Universal Time</link>, not Local Time.
0047 </para><para>
0048 Astronomers use certain Julian Day values as important reference points, called
0049 <link linkend="ai-epoch">Epochs</link>.  One widely-used epoch is called J2000; it is the
0050 Julian Day for 1 Jan, 2000 at 12:00 noon = <abbrev>JD</abbrev> 2451545.0.
0051 </para><para>
0052 Much more information on Julian Days is available on the internet.  A good
0053 starting point is the <ulink
0054 url="http://aa.usno.navy.mil/faq/docs/JD_Formula.html">U.S. Naval
0055 Observatory</ulink>.  If that site is not available when you read this, try
0056 searching for <quote>Julian Day</quote> with your favorite search engine.
0057 </para>
0058 </sect1>