diff options
Diffstat (limited to 'doc/kstars/precession.docbook')
| -rw-r--r-- | doc/kstars/precession.docbook | 54 |
1 files changed, 54 insertions, 0 deletions
diff --git a/doc/kstars/precession.docbook b/doc/kstars/precession.docbook new file mode 100644 index 00000000..2e092263 --- /dev/null +++ b/doc/kstars/precession.docbook @@ -0,0 +1,54 @@ +<sect1 id="ai-precession"> +<sect1info> +<author> +<firstname>Jason</firstname> +<surname>Harris</surname> +</author> +</sect1info> +<title>Precession</title> +<indexterm><primary>Precession</primary> +</indexterm> +<para> +<firstterm>Precession</firstterm> is the gradual change in the direction of the +Earth's spin axis. The spin axis traces a cone, completing a full circuit in +26,000 years. If you have ever spun a top or a dreidel, the +<quote>wobbling</quote> rotation of the top as it spins is precession. +</para><para> +Because the direction of the Earth's spin axis changes, so does the location of +the <link linkend="ai-cpoles">Celestial Poles</link>. +</para><para> +The reason for the Earth's precession is complicated. The Earth is not a +perfect sphere, it is a bit flattened, meaning the +<link linkend="ai-greatcircle">Great Circle</link> of the equator is longer +than a <quote>meridonal</quote> great circle that +passes through the poles. Also, the Moon and Sun lie outside the Earth's +Equatorial plane. As a result, the gravitational pull of the Moon and Sun on +the oblate Earth induces a slight <emphasis>torque</emphasis> in addition to a +linear force. This torque on the spinning body of the Earth leads to the +precessional motion. +</para> +<tip> +<para>Exercise:</para> +<para> +Precession is easiest to see by observing the <link +linkend="ai-cpoles">Celestial Pole</link>. To find the pole, first switch to +Equatorial Coordinates in the <guilabel>Configure &kstars;</guilabel> window, and +then hold down the <keycap>Up arrow</keycap> key until the display stops +scrolling. The declination displayed in the center of the +<guilabel>Info Panel</guilabel> should be +90 degrees, and the bright star +Polaris should be nearly at the center of the screen. Try slewing with the left +and right arrow keys. Notice that the sky appears to rotate around the Pole. +</para><para> +We will now demonstrate Precession by changing the Date to a very remote year, +and observing that the location of the Celestial Pole is no longer near Polaris. +Open the <guilabel>Set Time</guilabel> window +(<keycombo action="simul">&Ctrl;<keycap>S</keycap></keycombo>), and set the date +to the year 8000 (currently, &kstars; cannot handle dates much more remote than +this, but this date is sufficient for our purposes). Notice that the sky +display is now centered at a point between the constellations Cygnus and +Cepheus. Verify that this is actually the pole by slewing left and right: the +sky rotates about this point; in the year 8000, the North celestial pole will no +longer be near Polaris. +</para> +</tip> +</sect1> |