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diff --git a/doc/kstars/julianday.docbook b/doc/kstars/julianday.docbook new file mode 100644 index 00000000..03ae53c3 --- /dev/null +++ b/doc/kstars/julianday.docbook @@ -0,0 +1,58 @@ +<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> |