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0001 <sect1 id="ai-parallax">
0002 <sect1info>
0003 <author>
0004 <firstname>James</firstname> <surname>Lindenschmidt</surname>
0005 </author>
0006 </sect1info>
0007 <title>Parallax</title>
0008 <indexterm><primary>Parallax</primary></indexterm>
0009 <indexterm><primary>Astronomical Unit</primary><see>Parallax</see></indexterm>
0010 <indexterm><primary>Parsec</primary><see>Parallax</see></indexterm>
0011  <para>
0012  <firstterm>Parallax</firstterm> is the apparent change of an observed
0013  object's position caused by a shift in the observer's position. As an
0014  example, hold your hand in front of you at arm's length, and observe
0015  an object on the other side of the room behind your hand. Now tilt
0016  your head to your right shoulder, and your hand will appear on the
0017  left side of the distant object. Tilt your head to your left
0018  shoulder, and your hand will appear to shift to the right side of the
0019  distant object.
0020  </para>
0021  <para>
0022  Because the Earth is in orbit around the Sun, we observe the sky from
0023  a constantly moving position in space.  Therefore, we should expect
0024  to see an <firstterm>annual parallax</firstterm> effect, in which the
0025  positions of nearby objects appear to <quote>wobble</quote> back and forth in
0026  response to our motion around the Sun.  This does in fact happen, but
0027  the distances to even the nearest stars are so great that you need to
0028  make careful observations with a telescope to detect
0029  it<footnote><para>The ancient Greek astronomers knew about parallax;
0030  because they could not observe an annual parallax in the positions of
0031  stars, they concluded that the Earth could not be in motion around
0032  the Sun.  What they did not realize was that the stars are millions of
0033  times further away than the Sun, so the parallax effect is impossible
0034  to see with the unaided eye.</para></footnote>.
0035  </para>
0036  <para>
0037  Modern telescopes allow astronomers to use the annual parallax to
0038  measure the distance to nearby stars, using triangulation.  The
0039  astronomer carefully measures the position of the star on two dates,
0040  spaced six months apart.  The nearer the star is to the Sun, the
0041 larger
0042  the apparent shift in its position will be between the two dates.
0043  </para>
0044  <para>
0045  Over the six-month period, the Earth has moved through half its orbit
0046  around the Sun; in this time its position has changed by 2
0047  <firstterm>Astronomical Units</firstterm> (abbreviated AU; 1 AU is
0048  the distance from the Earth to the Sun, or about 150 million
0049  kilometers).  This sounds like a really long distance, but even the
0050  nearest star to the Sun (alpha Centauri) is about 40
0051  <emphasis>trillion</emphasis> kilometers away.  Therefore, the annual
0052  parallax is very small, typically smaller than one
0053  <firstterm>arcsecond</firstterm>, which is only 1/3600 of one degree.
0054  A convenient distance unit for nearby stars is the
0055  <firstterm>parsec</firstterm>, which is short for "parallax
0056  arcsecond".  One parsec is the distance a star would have if its
0057  observed parallax angle was one arcsecond.  It is equal to 3.26
0058  light-years, or 31 trillion kilometers<footnote><para>Astronomers
0059  like this unit so much that they now use <quote>kiloparsecs</quote> to measure
0060  galaxy-scale distances, and <quote>Megaparsecs</quote> to measure intergalactic
0061  distances, even though these distances are much too large to have an
0062  actual, observable parallax.  Other methods are required to determine
0063  these distances</para></footnote>.
0064  </para>
0065 </sect1>