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 <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>