diff options
Diffstat (limited to 'doc/kstars/cpoles.docbook')
| -rw-r--r-- | doc/kstars/cpoles.docbook | 58 |
1 files changed, 58 insertions, 0 deletions
diff --git a/doc/kstars/cpoles.docbook b/doc/kstars/cpoles.docbook new file mode 100644 index 00000000..c5f28957 --- /dev/null +++ b/doc/kstars/cpoles.docbook @@ -0,0 +1,58 @@ +<sect1 id="ai-cpoles"> +<sect1info> +<author> +<firstname>Jason</firstname> +<surname>Harris</surname> +</author> +</sect1info> +<title>The Celestial Poles</title> +<indexterm><primary>Celestial Poles</primary> +<seealso>Equatorial Coordinates</seealso> +</indexterm> +<para> +The sky appears to drift overhead from east to west, completing a full circuit +around the sky in 24 (<link linkend="ai-sidereal">Sidereal</link>) hours. This +phenomenon is due to the spinning of the Earth on its axis. The Earth's +spin axis intersects the <link linkend="ai-csphere">Celestial Sphere</link> at +two points. These points are the <firstterm>Celestial Poles</firstterm>. As the +Earth spins; they remain fixed in the sky, and all other points seem to rotate +around them. The celestial poles are also the poles of the <link +linkend="equatorial">Equatorial Coordinate System</link>, meaning +they have <firstterm>Declinations</firstterm> of +90 degrees and -90 degrees +(for the North and South celestial poles, respectively). +</para><para> +The North Celestial Pole currently has nearly the same coordinates as +the bright star <firstterm>Polaris</firstterm> (which is Latin for <quote>Pole Star</quote>). +This makes Polaris useful for navigation: not only is it always above the North +point of the horizon, but its <link +linkend="horizontal">Altitude</link> angle is always (nearly) +equal to the observer's <link linkend="ai-geocoords">Geographic Latitude</link> +(however, Polaris can only be seen from locations in the Northern hemisphere). +</para><para> +The fact that Polaris is near the pole is purely a coincidence. In fact, +because of <link linkend="ai-precession">Precession</link>, Polaris is only near +the pole for a small fraction of the time. +</para> +<tip> +<para>Exercises:</para> +<para> +Use the <guilabel>Find Object</guilabel> window +(<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>) to locate +Polaris. Notice that its Declination is almost (but not exactly) +90 degrees. +Compare the Altitude reading when focused on Polaris to your location's +geographic latitude. They are always within one degree of each other. +They are not exactly the same because Polaris isn't exactly at the Pole. +(you can point exactly at the pole by switching to Equatorial +coordinates, and pressing the up-arrow key until the sky stops scrolling). +</para><para> +Use the <guilabel>Time Step</guilabel> spinbox in the toolbar to accelerate time + to a +step of 100 seconds. You can see the entire sky appears to rotate around +Polaris, while Polaris itself remains nearly stationary. +</para><para> +We said that the celestial pole is the pole of the Equatorial coordinate +system. What do you think is the pole of the horizontal (Altitude/Azimuth) +coordinate system? (The <link linkend="ai-zenith">Zenith</link>). +</para> +</tip> +</sect1> |