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diff --git a/doc/kstars/utime.docbook b/doc/kstars/utime.docbook new file mode 100644 index 00000000..c8c47375 --- /dev/null +++ b/doc/kstars/utime.docbook @@ -0,0 +1,49 @@ +<sect1 id="ai-utime"> +<sect1info> +<author> +<firstname>Jason</firstname> +<surname>Harris</surname> +</author> +</sect1info> +<title>Universal Time</title> +<indexterm><primary>Universal Time</primary> +<seealso>Time Zones</seealso> +</indexterm> +<para> +The time on our clocks is essentially a measurement of the current position of +the Sun in the sky, which is different for places at different Longitudes +because the Earth is round (see <link linkend="ai-timezones">Time Zones</link>). +</para><para> +However, it is sometimes necessary to define a global time, one that is the same +for all places on Earth. One way to do this is to pick a place on the Earth, +and adopt the Local Time at that place as the <firstterm>Universal +Time</firstterm>, abbreviated <abbrev>UT</abbrev>. (The name is a bit of a +misnomer, since Universal Time has little to do with the Universe. It would +perhaps be better to think of it as <emphasis>global time</emphasis>). +</para><para> +The geographic location chosen to represent Universal Time is Greenwich, +England. The choice is arbitrary and historical. Universal Time became an +important concept when European ships began to sail the wide open seas, far from +any known landmarks. A navigator could reckon the ship's longitude by comparing +the Local Time (as measured from the Sun's position) to the time back at the +home port (as kept by an accurate clock on board the ship). Greenwich was home +to England's Royal Observatory, which was charged with keeping time +very accurately, so that ships in port could re-calibrate their clocks before +setting sail. +</para> +<tip> +<para>Exercise:</para> +<para> +Set the geographic location to <quote>Greenwich, England</quote> using the +<guilabel>Set Location</guilabel> window +(<keycombo action="simul">&Ctrl;<keycap>G</keycap></keycombo>). Note that the +Local Time (<abbrev>LT</abbrev>)and the Universal Time (<abbrev>UT</abbrev>) are +now the same. +</para><para> +Further Reading: The history behind the construction of the first clock +that was accurate and stable enough to be used on ships to keep Universal Time +is a fascinating tale, and one told expertly in the book +<quote>Longitude</quote>, by Dava Sobel. +</para> +</tip> +</sect1> |