1 files changed, 65 insertions, 0 deletions
diff --git a/doc/kstars/parallax.docbook b/doc/kstars/parallax.docbook
new file mode 100644
index 00000000..688dd334
--- /dev/null
+++ b/doc/kstars/parallax.docbook
@@ -0,0 +1,65 @@
+<sect1 id="ai-parallax">
+<sect1info>
+<author>
+<firstname>James</firstname> <surname>Lindenschmidt</surname>
+</author>
+</sect1info>
+<title>Parallax</title>
+<indexterm><primary>Parallax</primary></indexterm>
+<indexterm><primary>Astronomical Unit</primary><see>Parallax</see></indexterm>
+<indexterm><primary>Parsec</primary><see>Parallax</see></indexterm>
+ <para>
+ <firstterm>Parallax</firstterm> is the apparent change of an observed
+ object's position caused by a shift in the observer's position. As an
+ example, hold your hand in front of you at arm's length, and observe
+ an object on the other side of the room behind your hand. Now tilt
+ your head to your right shoulder, and your hand will appear on the
+ left side of the distant object. Tilt your head to your left
+ shoulder, and your hand will appear to shift to the right side of the
+ distant object.
+ </para>
+ <para>
+ Because the Earth is in orbit around the Sun, we observe the sky from
+ a constantly moving position in space.  Therefore, we should expect
+ to see an <firstterm>annual parallax</firstterm> effect, in which the
+ positions of nearby objects appear to <quote>wobble</quote> back and forth in
+ response to our motion around the Sun.  This does in fact happen, but
+ the distances to even the nearest stars are so great that you need to
+ make careful observations with a telescope to detect
+ it<footnote><para>The ancient Greek astronomers knew about parallax;
+ because they could not observe an annual parallax in the positions of
+ stars, they concluded that the Earth could not be in motion around
+ the Sun.  What they did not realize was that the stars are millions of
+ times further away than the Sun, so the parallax effect is impossible
+ to see with the unaided eye.</para></footnote>.
+ </para>
+ <para>
+ Modern telescopes allow astronomers to use the annual parallax to
+ measure the distance to nearby stars, using triangulation.  The
+ astronomer carefully measures the position of the star on two dates,
+ spaced six months apart.  The nearer the star is to the Sun, the
+larger
+ the apparent shift in its position will be between the two dates.
+ </para>
+ <para>
+ Over the six-month period, the Earth has moved through half its orbit
+ around the Sun; in this time its position has changed by 2
+ <firstterm>Astronomical Units</firstterm> (abbreviated AU; 1 AU is
+ the distance from the Earth to the Sun, or about 150 million
+ kilometers).  This sounds like a really long distance, but even the
+ nearest star to the Sun (alpha Centauri) is about 40
+ <emphasis>trillion</emphasis> kilometers away.  Therefore, the annual
+ parallax is very small, typically smaller than one
+ <firstterm>arcsecond</firstterm>, which is only 1/3600 of one degree.
+ A convenient distance unit for nearby stars is the
+ <firstterm>parsec</firstterm>, which is short for "parallax
+ arcsecond".  One parsec is the distance a star would have if its
+ observed parallax angle was one arcsecond.  It is equal to 3.26
+ light-years, or 31 trillion kilometers<footnote><para>Astronomers
+ like this unit so much that they now use <quote>kiloparsecs</quote> to measure
+ galaxy-scale distances, and <quote>Megaparsecs</quote> to measure intergalactic
+ distances, even though these distances are much too large to have an
+ actual, observable parallax.  Other methods are required to determine
+ these distances</para></footnote>.
+ </para>
+</sect1>